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Jaramillo AM, Vladar EK, Holguin F, Dickey BF, Evans CM. Emerging cell and molecular targets for treating mucus hypersecretion in asthma. Allergol Int 2024; 73:375-381. [PMID: 38692992 DOI: 10.1016/j.alit.2024.04.002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2024] [Accepted: 04/03/2024] [Indexed: 05/03/2024] Open
Abstract
Mucus provides a protective barrier that is crucial for host defense in the lungs. However, excessive or abnormal mucus can have pathophysiological consequences in many pulmonary diseases, including asthma. Patients with asthma are treated with agents that relax airway smooth muscle and reduce airway inflammation, but responses are often inadequate. In part, this is due to the inability of existing therapeutic agents to directly target mucus. Accordingly, there is a critical need to better understand how mucus hypersecretion and airway plugging are affected by the epithelial cells that synthesize, secrete, and transport mucus components. This review highlights recent advances in the biology of mucin glycoproteins with a specific focus on MUC5AC and MUC5B, the chief macromolecular components of airway mucus. An improved mechanistic understanding of key steps in mucin production and secretion will help reveal novel potential therapeutic strategies.
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Affiliation(s)
- Ana M Jaramillo
- Division of Pulmonary Sciences and Critical Care Medicine, University of Colorado School of Medicine, Aurora, CO, USA
| | - Eszter K Vladar
- Division of Pulmonary Sciences and Critical Care Medicine, University of Colorado School of Medicine, Aurora, CO, USA
| | - Fernando Holguin
- Division of Pulmonary Sciences and Critical Care Medicine, University of Colorado School of Medicine, Aurora, CO, USA
| | - Burton F Dickey
- Department of Pulmonary Medicine, Anderson Cancer Center, University of Texas M.D., Houston, TX, USA
| | - Christopher M Evans
- Division of Pulmonary Sciences and Critical Care Medicine, University of Colorado School of Medicine, Aurora, CO, USA.
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2
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Ilani T, Reznik N, Yeshaya N, Feldman T, Vilela P, Lansky Z, Javitt G, Shemesh M, Brenner O, Elkis Y, Varsano N, Jaramillo AM, Evans CM, Fass D. The disulfide catalyst QSOX1 maintains the colon mucosal barrier by regulating Golgi glycosyltransferases. EMBO J 2023; 42:e111869. [PMID: 36245281 PMCID: PMC9841341 DOI: 10.15252/embj.2022111869] [Citation(s) in RCA: 10] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/22/2022] [Revised: 09/25/2022] [Accepted: 09/29/2022] [Indexed: 01/27/2023] Open
Abstract
Mucus is made of enormous mucin glycoproteins that polymerize by disulfide crosslinking in the Golgi apparatus. QSOX1 is a catalyst of disulfide bond formation localized to the Golgi. Both QSOX1 and mucins are highly expressed in goblet cells of mucosal tissues, leading to the hypothesis that QSOX1 catalyzes disulfide-mediated mucin polymerization. We found that knockout mice lacking QSOX1 had impaired mucus barrier function due to production of defective mucus. However, an investigation on the molecular level revealed normal disulfide-mediated polymerization of mucins and related glycoproteins. Instead, we detected a drastic decrease in sialic acid in the gut mucus glycome of the QSOX1 knockout mice, leading to the discovery that QSOX1 forms regulatory disulfides in Golgi glycosyltransferases. Sialylation defects in the colon are known to cause colitis in humans. Here we show that QSOX1 redox control of sialylation is essential for maintaining mucosal function.
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Affiliation(s)
- Tal Ilani
- Department of Chemical and Structural Biology, Weizmann Institute of Science, Rehovot, Israel
| | - Nava Reznik
- Department of Chemical and Structural Biology, Weizmann Institute of Science, Rehovot, Israel
| | - Noa Yeshaya
- Department of Chemical and Structural Biology, Weizmann Institute of Science, Rehovot, Israel
| | - Tal Feldman
- Department of Chemical and Structural Biology, Weizmann Institute of Science, Rehovot, Israel
| | - Patrick Vilela
- Department of Chemical and Structural Biology, Weizmann Institute of Science, Rehovot, Israel
| | - Zipora Lansky
- Department of Chemical and Structural Biology, Weizmann Institute of Science, Rehovot, Israel
| | - Gabriel Javitt
- Department of Chemical and Structural Biology, Weizmann Institute of Science, Rehovot, Israel
| | - Michal Shemesh
- Life Sciences Core Facilities, Weizmann Institute of Science, Rehovot, Israel
| | - Ori Brenner
- Department of Veterinary Resources, Weizmann Institute of Science, Rehovot, Israel
| | | | - Neta Varsano
- Department of Chemical Research Support, Weizmann Institute of Science, Rehovot, Israel
| | - Ana M Jaramillo
- Department of Immunology and Microbiology, School of Medicine, University of Colorado, Aurora, CO, USA
| | - Christopher M Evans
- Department of Immunology and Microbiology, School of Medicine, University of Colorado, Aurora, CO, USA.,Department of Medicine, School of Medicine, University of Colorado, Aurora, CO, USA
| | - Deborah Fass
- Department of Chemical and Structural Biology, Weizmann Institute of Science, Rehovot, Israel
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3
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Hill DB, Button B, Rubinstein M, Boucher RC. Physiology and pathophysiology of human airway mucus. Physiol Rev 2022; 102:1757-1836. [PMID: 35001665 PMCID: PMC9665957 DOI: 10.1152/physrev.00004.2021] [Citation(s) in RCA: 77] [Impact Index Per Article: 38.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2021] [Revised: 12/13/2021] [Accepted: 12/19/2021] [Indexed: 01/27/2023] Open
Abstract
The mucus clearance system is the dominant mechanical host defense system of the human lung. Mucus is cleared from the lung by cilia and airflow, including both two-phase gas-liquid pumping and cough-dependent mechanisms, and mucus transport rates are heavily dependent on mucus concentration. Importantly, mucus transport rates are accurately predicted by the gel-on-brush model of the mucociliary apparatus from the relative osmotic moduli of the mucus and periciliary-glycocalyceal (PCL-G) layers. The fluid available to hydrate mucus is generated by transepithelial fluid transport. Feedback interactions between mucus concentrations and cilia beating, via purinergic signaling, coordinate Na+ absorptive vs Cl- secretory rates to maintain mucus hydration in health. In disease, mucus becomes hyperconcentrated (dehydrated). Multiple mechanisms derange the ion transport pathways that normally hydrate mucus in muco-obstructive lung diseases, e.g., cystic fibrosis (CF), chronic obstructive pulmonary disease (COPD), non-CF bronchiectasis (NCFB), and primary ciliary dyskinesia (PCD). A key step in muco-obstructive disease pathogenesis is the osmotic compression of the mucus layer onto the airway surface with the formation of adherent mucus plaques and plugs, particularly in distal airways. Mucus plaques create locally hypoxic conditions and produce airflow obstruction, inflammation, infection, and, ultimately, airway wall damage. Therapies to clear adherent mucus with hydrating and mucolytic agents are rational, and strategies to develop these agents are reviewed.
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Affiliation(s)
- David B Hill
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
- Joint Department of Biomedical Engineering, The University of North Carolina and North Carolina State University, Chapel Hill, North Carolina
| | - Brian Button
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
| | - Michael Rubinstein
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
- Department of Mechanical Engineering and Materials Science, Biomedical Engineering, Physics, and Chemistry, Duke University, Durham, North Carolina
| | - Richard C Boucher
- Marsico Lung Institute, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
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4
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Helical self-assembly of a mucin segment suggests an evolutionary origin for von Willebrand factor tubules. Proc Natl Acad Sci U S A 2022; 119:e2116790119. [PMID: 35377815 PMCID: PMC9169620 DOI: 10.1073/pnas.2116790119] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Abstract
Extracellular proteins with mechanical functions often require specialized assembly processes to form covalent oligomers. Progress in tissue bioengineering and repair will benefit from an understanding of how to harness and manipulate these processes. Here, we show that a particular supramolecular assembly mode was pre-encoded in the ancient domain organization common to gel-forming mucins and von Willebrand factor, glycoproteins that are deceptively different due to their divergence for distinct mechanical tasks. This finding highlights symmetry principles and building blocks retooled in nature to construct polymers with wide-ranging properties. These building blocks and knowledge of their self-assembly can be used to design new polymeric structures. The glycoprotein von Willebrand factor (VWF) contributes to hemostasis by stanching injuries in blood vessel walls. A distinctive feature of VWF is its assembly into long, helical tubules in endothelial cells prior to secretion. When VWF is released into the bloodstream, these tubules unfurl to release linear polymers that bind subendothelial collagen at wound sites, recruit platelets, and initiate the clotting cascade. VWF evolved from gel-forming mucins, the polymeric glycoproteins that coat and protect exposed epithelia. Despite the divergent function of VWF in blood vessel repair, sequence conservation and shared domain organization imply that VWF retained key aspects of the mucin bioassembly mechanism. Here, we show using cryo-electron microscopy that the ability to form tubules, a property hitherto thought to have arisen as a VWF adaptation to the vasculature, is a feature of the amino-terminal region of mucin. This segment of the human intestinal gel-forming mucin (MUC2) was found to self-assemble into tubules with a striking resemblance to those of VWF itself. To facilitate a comparison, we determined the residue-resolution structure of tubules formed by the homologous segment of VWF. The structures of the MUC2 and VWF tubules revealed the flexible joints and the intermolecular interactions required for tubule formation. Steric constraints in full-length MUC2 suggest that linear filaments, a previously observed supramolecular assembly form, are more likely than tubules to be the physiological mucin storage intermediate. Nevertheless, MUC2 tubules indicate a possible evolutionary origin for VWF tubules and elucidate design principles present in mucins and VWF.
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5
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Javitt G, Khmelnitsky L, Albert L, Bigman LS, Elad N, Morgenstern D, Ilani T, Levy Y, Diskin R, Fass D. Assembly Mechanism of Mucin and von Willebrand Factor Polymers. Cell 2020; 183:717-729.e16. [PMID: 33031746 PMCID: PMC7599080 DOI: 10.1016/j.cell.2020.09.021] [Citation(s) in RCA: 71] [Impact Index Per Article: 17.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2020] [Revised: 06/25/2020] [Accepted: 09/08/2020] [Indexed: 12/11/2022]
Abstract
The respiratory and intestinal tracts are exposed to physical and biological hazards accompanying the intake of air and food. Likewise, the vasculature is threatened by inflammation and trauma. Mucin glycoproteins and the related von Willebrand factor guard the vulnerable cell layers in these diverse systems. Colon mucins additionally house and feed the gut microbiome. Here, we present an integrated structural analysis of the intestinal mucin MUC2. Our findings reveal the shared mechanism by which complex macromolecules responsible for blood clotting, mucociliary clearance, and the intestinal mucosal barrier form protective polymers and hydrogels. Specifically, cryo-electron microscopy and crystal structures show how disulfide-rich bridges and pH-tunable interfaces control successive assembly steps in the endoplasmic reticulum and Golgi apparatus. Remarkably, a densely O-glycosylated mucin domain performs an organizational role in MUC2. The mucin assembly mechanism and its adaptation for hemostasis provide the foundation for rational manipulation of barrier function and coagulation.
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Affiliation(s)
- Gabriel Javitt
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Lev Khmelnitsky
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Lis Albert
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Lavi Shlomo Bigman
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Nadav Elad
- Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - David Morgenstern
- De Botton Institute for Protein Profiling, Nancy and Stephen Grand Israel National Center for Personalized Medicine, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Tal Ilani
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Yaakov Levy
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Ron Diskin
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Deborah Fass
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel.
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6
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Ridley C, Lockhart-Cairns MP, Collins RF, Jowitt TA, Subramani DB, Kesimer M, Baldock C, Thornton DJ. The C-terminal dimerization domain of the respiratory mucin MUC5B functions in mucin stability and intracellular packaging before secretion. J Biol Chem 2019; 294:17105-17116. [PMID: 31570524 PMCID: PMC6851316 DOI: 10.1074/jbc.ra119.010771] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2019] [Revised: 09/20/2019] [Indexed: 12/19/2022] Open
Abstract
Mucin 5B (MUC5B) has an essential role in mucociliary clearance that protects the pulmonary airways. Accordingly, knowledge of MUC5B structure and its interactions with itself and other proteins is critical to better understand airway mucus biology and improve the management of lung diseases such as asthma, cystic fibrosis, and chronic obstructive pulmonary disease (COPD). The role of an N-terminal multimerization domain in the supramolecular organization of MUC5B has been previously described, but less is known about its C-terminal dimerization domain. Here, using cryogenic electron microscopy (cryo-EM) and small-angle X-ray scattering (SAXS) analyses of recombinant disulfide-linked dimeric MUC5B dimerization domain we identified an asymmetric, elongated twisted structure, with a double globular base. We found that the dimerization domain is more resistant to disruption than the multimerization domain suggesting the twisted structure of the dimerization domain confers additional stability to MUC5B polymers. Size-exclusion chromatography-multiangle light scattering (SEC-MALS), SPR-based biophysical analyses and microscale thermophoresis of the dimerization domain disclosed no further assembly, but did reveal reversible, calcium-dependent interactions between the dimerization and multimerization domains that were most active at acidic pH, suggesting that these domains have a role in MUC5B intragranular organization. In summary, our results suggest a role for the C-terminal dimerization domain of MUC5B in compaction of mucin chains during granular packaging via interactions with the N-terminal multimerization domain. Our findings further suggest that the less stable multimerization domain provides a potential target for mucin depolymerization to remove mucus plugs in COPD and other lung pathologies.
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Affiliation(s)
- Caroline Ridley
- Wellcome Trust Centre for Cell-Matrix Research, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom.,Division of Infection Immunity and Respiratory Medicine, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom.,School of Biological Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - Michael P Lockhart-Cairns
- Wellcome Trust Centre for Cell-Matrix Research, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom.,School of Biological Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom.,Division of Cell-Matrix Biology and Regenerative Medicine, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - Richard F Collins
- School of Biological Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - Thomas A Jowitt
- Wellcome Trust Centre for Cell-Matrix Research, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom.,School of Biological Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - Durai B Subramani
- Marsico Lung Institute/Cystic Fibrosis Research Center, University of North Carolina School of Medicine, Chapel Hill, North Carolina 27599-7362
| | - Mehmet Kesimer
- Marsico Lung Institute/Cystic Fibrosis Research Center, University of North Carolina School of Medicine, Chapel Hill, North Carolina 27599-7362
| | - Clair Baldock
- Wellcome Trust Centre for Cell-Matrix Research, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom .,School of Biological Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom.,Division of Cell-Matrix Biology and Regenerative Medicine, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - David J Thornton
- Wellcome Trust Centre for Cell-Matrix Research, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom .,Division of Infection Immunity and Respiratory Medicine, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom.,School of Biological Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom.,Lydia Becker Institute for Immunology and Inflammation, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
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7
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Javitt G, Calvo MLG, Albert L, Reznik N, Ilani T, Diskin R, Fass D. Intestinal Gel-Forming Mucins Polymerize by Disulfide-Mediated Dimerization of D3 Domains. J Mol Biol 2019; 431:3740-3752. [PMID: 31310764 PMCID: PMC6739602 DOI: 10.1016/j.jmb.2019.07.018] [Citation(s) in RCA: 25] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2019] [Revised: 07/03/2019] [Accepted: 07/09/2019] [Indexed: 01/06/2023]
Abstract
The mucin 2 glycoprotein assembles into a complex hydrogel that protects intestinal epithelia and houses the gut microbiome. A major step in mucin 2 assembly is further multimerization of preformed mucin dimers, thought to produce a honeycomb-like arrangement upon hydrogel expansion. Important open questions are how multiple mucin 2 dimers become covalently linked to one another and how mucin 2 multimerization compares with analogous processes in related polymers such as respiratory tract mucins and the hemostasis protein von Willebrand factor. Here we report the x-ray crystal structure of the mucin 2 multimerization module, found to form a dimer linked by two intersubunit disulfide bonds. The dimer structure calls into question the current model for intestinal mucin assembly, which proposes disulfide-mediated trimerization of the same module. Key residues making interactions across the dimer interface are highly conserved in intestinal mucin orthologs, supporting the physiological relevance of the observed quaternary structure. With knowledge of the interface residues, it can be demonstrated that many of these amino acids are also present in other mucins and in von Willebrand factor, further indicating that the stable dimer arrangement reported herein is likely to be shared across this functionally broad protein family. The mucin 2 module structure thus reveals the manner by which both mucins and von Willebrand factor polymerize, drawing deep structural parallels between macromolecular assemblies critical to mucosal epithelia and the vasculature.
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Affiliation(s)
- Gabriel Javitt
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | | | - Lis Albert
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Nava Reznik
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Tal Ilani
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Ron Diskin
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Deborah Fass
- Department of Structural Biology, Weizmann Institute of Science, Rehovot 7610001, Israel.
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8
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Bonser LR, Erle DJ. Airway Mucus and Asthma: The Role of MUC5AC and MUC5B. J Clin Med 2017; 6:jcm6120112. [PMID: 29186064 PMCID: PMC5742801 DOI: 10.3390/jcm6120112] [Citation(s) in RCA: 196] [Impact Index Per Article: 28.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2017] [Revised: 11/25/2017] [Accepted: 11/27/2017] [Indexed: 11/16/2022] Open
Abstract
Asthma is characterized by mucus abnormalities. Airway epithelial hyperplasia and metaplasia result in changes in stored and secreted mucin and the production of a pathologic mucus gel. Mucus transport is impaired, culminating in mucus plugging and airway obstruction—a major cause of morbidity in asthma. The polymeric mucins MUC5AC and MUC5B are integral components of airway mucus. MUC5AC and MUC5B gene expression is altered in asthma, and recent work sheds light on their contribution to asthma pathogenesis. Herein, we review our current understanding of the role of MUC5AC and MUC5B in mucus dysfunction in asthma.
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Affiliation(s)
- Luke R Bonser
- Lung Biology Center, University of California San Francisco, San Francisco, CA 94143, USA.
| | - David J Erle
- Lung Biology Center, University of California San Francisco, San Francisco, CA 94143, USA.
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9
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Frenkel ES, Ribbeck K. Salivary mucins in host defense and disease prevention. J Oral Microbiol 2015; 7:29759. [PMID: 26701274 PMCID: PMC4689954 DOI: 10.3402/jom.v7.29759] [Citation(s) in RCA: 98] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2015] [Revised: 11/16/2015] [Accepted: 11/17/2015] [Indexed: 12/15/2022] Open
Abstract
Mucus forms a protective coating on wet epithelial surfaces throughout the body that houses the microbiota and plays a key role in host defense. Mucins, the primary structural components of mucus that creates its viscoelastic properties, are critical components of the gel layer that protect against invading pathogens. Altered mucin production has been implicated in diseases such as ulcerative colitis, asthma, and cystic fibrosis, which highlights the importance of mucins in maintaining homeostasis. Different types of mucins exist throughout the body in various locations such as the gastrointestinal tract, lungs, and female genital tract, but this review will focus on mucins in the oral cavity. Salivary mucin structure, localization within the oral cavity, and defense mechanisms will be discussed. These concepts will then be applied to present what is known about the protective function of mucins in oral diseases such as HIV/AIDS, oral candidiasis, and dental caries.
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Affiliation(s)
- Erica Shapiro Frenkel
- Biological Sciences in Dental Medicine, Harvard University, Cambridge, MA, USA.,Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA
| | - Katharina Ribbeck
- Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA;
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10
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Ridley C, Kouvatsos N, Raynal BD, Howard M, Collins RF, Desseyn JL, Jowitt TA, Baldock C, Davis CW, Hardingham TE, Thornton DJ. Assembly of the respiratory mucin MUC5B: a new model for a gel-forming mucin. J Biol Chem 2014; 289:16409-20. [PMID: 24778189 PMCID: PMC4047408 DOI: 10.1074/jbc.m114.566679] [Citation(s) in RCA: 70] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2014] [Revised: 04/17/2014] [Indexed: 12/17/2022] Open
Abstract
Mucins are essential components in mucus gels that form protective barriers at all epithelial surfaces, but much remains unknown about their assembly, intragranular organization, and post-secretion unfurling to form mucus. MUC5B is a major polymeric mucin expressed by respiratory epithelia, and we investigated the molecular mechanisms involved during its assembly. Studies of intact polymeric MUC5B revealed a single high affinity calcium-binding site, distinct from multiple low affinity sites on each MUC5B monomer. Self-diffusion studies with intact MUC5B showed that calcium binding at the protein site catalyzed reversible cross-links between MUC5B chains to form networks. The site of cross-linking was identified in the MUC5B D3-domain as it was specifically blocked by D3 peptide antibodies. Biophysical analysis and single particle EM of recombinant MUC5B N terminus (D1D2D'D3; NT5B) and subdomains (D1, D1-D2, D2-D'-D3, and D3) generated structural models of monomers and disulfide-linked dimers and suggested that MUC5B multimerizes by disulfide linkage between D3-domains to form linear polymer chains. Moreover, these analyses revealed reversible homotypic interactions of NT5B at low pH and in high calcium, between disulfide-linked NT5B dimers, but not monomers. These results enable a model of MUC5B to be derived, which predicts mechanisms of mucin intracellular assembly and storage, which may be common to the other major gel-forming polymeric mucins.
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Affiliation(s)
- Caroline Ridley
- From the Wellcome Trust Centre for Cell-Matrix Research and the Faculty of Life Sciences, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - Nikos Kouvatsos
- From the Wellcome Trust Centre for Cell-Matrix Research and the Faculty of Life Sciences, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - Bertrand D Raynal
- From the Wellcome Trust Centre for Cell-Matrix Research and the Faculty of Life Sciences, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - Marj Howard
- From the Wellcome Trust Centre for Cell-Matrix Research and the Faculty of Life Sciences, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - Richard F Collins
- the Faculty of Life Sciences, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - Jean-Luc Desseyn
- the INSERM U995, University of Lille, F-59045 Lille, France, and
| | - Thomas A Jowitt
- From the Wellcome Trust Centre for Cell-Matrix Research and the Faculty of Life Sciences, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - Clair Baldock
- From the Wellcome Trust Centre for Cell-Matrix Research and the Faculty of Life Sciences, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - C William Davis
- the Cystic Fibrosis/Pulmonary Research and Treatment Center, University of North Carolina, Chapel Hill, North Carolina 27517-7248
| | - Timothy E Hardingham
- From the Wellcome Trust Centre for Cell-Matrix Research and the Faculty of Life Sciences, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom
| | - David J Thornton
- From the Wellcome Trust Centre for Cell-Matrix Research and the Faculty of Life Sciences, The University of Manchester, Oxford Road, Manchester M13 9PT, United Kingdom,
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11
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Semaan SM, Wang X, Marshall AG, Sang QXA. Identification of Potential Glycoprotein Biomarkers in Estrogen Receptor Positive (ER+) and Negative (ER-) Human Breast Cancer Tissues by LC-LTQ/FT-ICR Mass Spectrometry. J Cancer 2012; 3:269-84. [PMID: 22773931 PMCID: PMC3390597 DOI: 10.7150/jca.4592] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2012] [Accepted: 06/20/2012] [Indexed: 01/02/2023] Open
Abstract
Breast cancer is the second most fatal cancer in American women. To increase the life expectancy of patients with breast cancer new diagnostic and prognostic biomarkers and drug targets must be identified. A change in the glycosylation on a glycoprotein often causes a change in the function of that glycoprotein; such a phenomenon is correlated with cancerous transformation. Thus, glycoproteins in human breast cancer estrogen receptor positive (ER+) tissues and those in the more advanced stage of breast cancer, estrogen receptor negative (ER-) tissues, were compared. Glycoproteins showing differences in glycosylation were examined by 2-dimensional gel electrophoresis with double staining (glyco- and total protein staining) and identified by reversed-phase nano-liquid chromatography coupled with a hybrid linear quadrupole ion trap/ Fourier transform ion cyclotron resonance mass spectrometer. Among the identified glycosylated proteins are alpha 1 acid glycoprotein, alpha-1-antitrypsin, calmodulin, and superoxide dismutase mitochondrial precursor that were further verified by Western blotting for both ER+ and ER- human breast tissues. Results show the presence of a possible glycosylation difference in alpha-1-antitrypsin, a potential tumor-derived biomarker for breast cancer progression, which was expressed highest in the ER- samples.
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Affiliation(s)
- Suzan M Semaan
- 1. Department of Chemistry and Biochemistry and Institute of Molecular Biophysics
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12
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Dang LT, Purvis AR, Huang RH, Westfield LA, Sadler JE. Phylogenetic and functional analysis of histidine residues essential for pH-dependent multimerization of von Willebrand factor. J Biol Chem 2011; 286:25763-9. [PMID: 21592973 DOI: 10.1074/jbc.m111.249151] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
von Willebrand factor (VWF) is a multimeric plasma protein that mediates platelet adhesion to sites of vascular injury. The hemostatic function of VWF depends upon the formation of disulfide-linked multimers, which requires the VWF propeptide (D1D2 domains) and adjacent D'D3 domains. VWF multimer assembly occurs in the trans-Golgi at pH ~ 6.2 but not at pH 7.4, which suggests that protonation of one or more His residues (pK(a) ~6.0) mediates the pH dependence of multimerization. Alignment of 30 vertebrate VWF sequences identified 13 highly conserved His residues in the D1D2D'D3 domains, and His-to-Ala mutagenesis identified His³⁹⁵ and His⁴⁶⁰ in the D2 domain as critical for VWF multimerization. Replacement of His³⁹⁵ with Lys or Arg prevented multimer assembly, suggesting that reversible protonation of this His residue is essential. In contrast, replacement of His⁴⁶⁰ with Lys or Arg preserved normal multimer assembly, whereas Leu, Met, and Gln did not, indicating that the function of His⁴⁶⁰ depends primarily upon the presence of a positive charge. These results suggest that pH sensing by evolutionarily conserved His residues facilitates the assembly and packaging of VWF multimers upon arrival in the trans-Golgi.
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Affiliation(s)
- Luke T Dang
- Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA
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13
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Schmidt TA, Plaas AH, Sandy JD. Disulfide-bonded multimers of proteoglycan 4 (PRG4) are present in normal synovial fluids. Biochim Biophys Acta Gen Subj 2009; 1790:375-84. [DOI: 10.1016/j.bbagen.2009.03.016] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2008] [Revised: 03/03/2009] [Accepted: 03/16/2009] [Indexed: 12/01/2022]
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14
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Proteomic analysis of polymeric salivary mucins: no evidence for MUC19 in human saliva. Biochem J 2008; 413:545-52. [PMID: 18426393 DOI: 10.1042/bj20080260] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
MUC5B is the predominant polymeric mucin in human saliva [Thornton, Khan, Mehrotra, Howard, Veerman, Packer and Sheehan (1999) Glycobiology 9, 293-302], where it contributes to oral cavity hydration and protection. More recently, the gene for another putative polymeric mucin, MUC19, has been shown to be expressed in human salivary glands [Chen, Zhao, Kalaslavadi, Hamati, Nehrke, Le, Ann and Wu (2004) Am. J. Respir. Cell Mol. Biol. 30, 155-165]. However, to date, the MUC19 mucin has not been isolated from human saliva. Our aim was therefore to purify and characterize the MUC19 glycoprotein from human saliva. Saliva was solubilized in 4 M guanidinium chloride and the high-density mucins were purified by density-gradient centrifugation. The presence of MUC19 was investigated using tandem MS of tryptic peptides derived from this mucin preparation. Using this approach, we found multiple MUC5B-derived tryptic peptides, but were unable to detect any putative MUC19 peptides. These results suggest that MUC19 is not a major component in human saliva. In contrast, using the same experimental approach, we identified Muc19 and Muc5b glycoproteins in horse saliva. Moreover, we also identified Muc19 from pig, cow and rat saliva; the saliva of cow and rat also contained Muc5b; however, due to the lack of pig Muc5b genomic sequence data, we were unable to identify Muc5b in pig saliva. Our results suggest that unlike human saliva, which contains MUC5B, cow, horse and rat saliva are a heterogeneous mixture of Muc5b and Muc19. The functional consequence of these species differences remains to be elucidated.
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15
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Gong H, Zhou J, Liao M, Hatta T, Harnnoi T, Umemiya R, Inoue N, Xuan X, Fujisaki K. Characterization of a carboxypeptidase inhibitor from the tick Haemaphysalis longicornis. JOURNAL OF INSECT PHYSIOLOGY 2007; 53:1079-87. [PMID: 17651749 DOI: 10.1016/j.jinsphys.2007.06.008] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/22/2007] [Revised: 06/01/2007] [Accepted: 06/05/2007] [Indexed: 05/16/2023]
Abstract
A carboxypeptidase inhibitor called HlTCI was isolated from Haemaphysalis longicornis in this study. The full-length cDNA of HlTCI contains an open reading frame (ORF) of 291bp, encoding 96 amino acid residues consisting of a predicted 19-residue signal peptide and a putative mature 77-residue protein. The expected mature protein is cysteine-rich and has 12 cysteine residues assumed to construct six disulfide bridges. The deduced peptide sequence shows 63.9% homology to the carboxypeptidase inhibitor from another ixodid tick, Rhipicephalus bursa. Reverse-transcription PCR (RT-PCR) indicated that HlTCI was specifically expressed in the ovary from partially engorged adult ticks. The recombinant protein of HlTCI (rHlTCI) with glutathione S-transferase (GST) was expressed in Escherichia coli strain BL21 (DE3) and purified by glutathione-Sepharose 4B beads. rHlTCI showed inhibitory activity against digestive metallocarboxypeptidases A and B, but the activity was affected by the increase of the temperature treatment. High concentrations of rHlTCI were shown to significantly accelerate fibrinolysis in vitro. This effect of rHlTCI on clot lysis suggests its promising potential for use in some thrombotic disorders.
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Affiliation(s)
- Haiyan Gong
- National Research Center for Protozoan Diseases, Obihiro University of Agriculture and Veterinary Medicine, Obihiro, Hokkaido, Japan
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16
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Perez-Vilar J, Olsen JC, Chua M, Boucher RC. pH-dependent intraluminal organization of mucin granules in live human mucous/goblet cells. J Biol Chem 2005; 280:16868-81. [PMID: 15718243 DOI: 10.1074/jbc.m413289200] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To study the mechanism of gel-forming mucin packaging within mucin granules, we generated human mucous/goblet cells stably expressing a recombinant MUC5AC domain fused to green fluorescent protein (GFP). The fusion protein, named SHGFP-MUC5AC/CK, accumulated in the granules together with native MUC5AC. Inhibition of protein synthesis or disorganization of the Golgi complex did not result in diminished intragranular SHGFP-MUC5AC/CK signals, consistent with long-term storage of the fusion protein. However, SHGFP-MUC5AC/CK was rapidly discharged from the granules upon incubation of the cells with ATP, an established mucin secretagogue. Several criteria indicated that SHGFP-MUC5AC/CK was not covalently linked to endogenous MUC5AC. Analysis of fluorescence recovery after photobleaching suggested that the intragranular SHGFP-MUC5AC/CK mobile fraction and mobility were significantly lower than in the endoplasmic reticulum lumen. Incubation of the cells with bafilomycin A1, a specific inhibitor of the vacuolar H+-ATPase, did not alter the fusion protein mobility, although it significantly increased (approximately 20%) the intragranular SHGFP-MUC5AC/CK mobile fraction. In addition, the granules in bafilomycin-incubated cells typically exhibited a heterogeneous intraluminal distribution of the fluorescent fusion protein. These results are consistent with a model of mucin granule intraluminal organization with two phases: a mobile phase in which secretory proteins diffuse as in the endoplasmic reticulum lumen but at a lower rate and an immobile phase or matrix in which proteins are immobilized by noncovalent pH-dependent interactions. An intraluminal acidic pH, maintained by the vacuolar H+-ATPase, is one of the critical factors for secretory protein binding to the immobile phase and also for its organization.
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Affiliation(s)
- Juan Perez-Vilar
- Cystic Fibrosis/Pulmonary Research and Treatment Center, School of Medicine, University of North Carolina, Chapel Hill, North Carolina 27599-7248, USA.
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17
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Perez-Vilar J, Boucher RC. Reevaluating gel-forming mucins' roles in cystic fibrosis lung disease. Free Radic Biol Med 2004; 37:1564-77. [PMID: 15477008 DOI: 10.1016/j.freeradbiomed.2004.07.027] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/04/2004] [Revised: 07/13/2004] [Accepted: 07/22/2004] [Indexed: 10/26/2022]
Abstract
The existence of mucus plugs, containing mucins, bacteria, and neutrophils, blocking the lower airways in the lung of cystic fibrosis (CF) patients has raised the possibility that production of "abnormal" mucins is a critical characteristic of this disease. The molecular nature, if any, of this abnormality is unknown. Recent studies suggest that CF lung disease progression is characterized by an early phase in which airway surface liquid (ASL) increased dehydration is accompanied by altered pH and levels of reduced glutathione (GSH). In a later phase, bacterial infection and neutrophil invasion lead to increased ASL of concentrations myeloperoxidase and hypochlorous acid (HOCl). Independent studies indicate that gel-forming mucins, the key components of airway mucus, form disulfide-linked polymers through a pH-dependent, likely self-catalyzed mechanism. In this article, we present the hypothesis that increased mucus concentration (dehydration) and altered pH, and levels of GSH, myeloperoxidase, and/or HOCl result in the extracellular formation of additional interchain bonds among airway mucins. These novel interactions would create an atypical mucin network with abnormal viscoelastic and adhesive properties.
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Affiliation(s)
- Juan Perez-Vilar
- The Cystic Fibrosis/Pulmonary Research and Treatment Center, School of Medicine, University of North Carolina at Chapel Hill, CB7248, 4011 Thurston-Bowles Bldg., Chapel Hill, NC 27599, USA.
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18
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Sheehan JK, Kirkham S, Howard M, Woodman P, Kutay S, Brazeau C, Buckley J, Thornton DJ. Identification of Molecular Intermediates in the Assembly Pathway of the MUC5AC Mucin. J Biol Chem 2004; 279:15698-705. [PMID: 14749330 DOI: 10.1074/jbc.m313241200] [Citation(s) in RCA: 82] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
MUC5AC mucins secreted by HT-29 cells in culture are oligomeric glycoproteins with characteristics similar to the MUC5AC mucins isolated from human airway sputum (Sheehan, J. K., Brazeau, C., Kutay, S., Pigeon, H., Kirkham, S., Howard, M., and Thornton, D. J. (2000) Biochem. J. 347, 37-44). Therefore we have used this cell line as a model system to investigate the biosynthesis of this major airway mucin. Initial experiments showed that the MUC5AC mucins isolated from the cells were liable to depolymerization depending on the conditions used for their solubilization. Prevention against reduction resulted in large oligomers associated with the cells, similar to those secreted into the medium. Using a combination of density gradient centrifugation and agarose gel electrophoresis coupled with probes specific for different forms of the mucin we identified five major intracellular populations of the MUC5AC polypeptide (unglycosylated monomer and dimer, GalNAc-substituted dimer, fully glycosylated dimer, and higher order oligomers). Pulse-chase studies were performed to follow the flow of radioactivity through these various intracellular forms into the mature oligomeric mucin secreted into the medium (a process taking approximately 2-4 h). The results show that the mucin polypeptide undergoes dimerization and then becomes substituted with GalNAc residues prior to glycan elaboration to produce a mature mucin dimer, which then undergoes multimerization. These data indicate that this oligomeric mucin follows a similar assembly to the von Willebrand factor glycoprotein to yield long linear disulfide-linked chains.
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Affiliation(s)
- John K Sheehan
- School of Biological Sciences, University of Manchester, Michael Smith Building, Manchester M13 9PT, United Kingdom.
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19
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Chen Y, Zhao YH, Kalaslavadi TB, Hamati E, Nehrke K, Le AD, Ann DK, Wu R. Genome-wide search and identification of a novel gel-forming mucin MUC19/Muc19 in glandular tissues. Am J Respir Cell Mol Biol 2004; 30:155-65. [PMID: 12882755 DOI: 10.1165/rcmb.2003-0103oc] [Citation(s) in RCA: 171] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
Gel-forming mucins are major contributors to the viscoelastic properties of mucus secretion. Currently, four gel-forming mucin genes have been identified: MUC2, MUC5AC, MUC5B, and MUC6. All these genes have five major cysteine-rich domains (four von Willebrand factor [vWF] C or D domains and one Cystine-knot [CT] domain) as their distinctive features, in contrast to other non-gel-forming type of mucins. The CT domain is believed to be involved in the initial mucin dimer formation and have very succinct relationship between different gel-forming mucins across different species. Because of gene duplication and evolutional modification, it is very likely that other gel-forming mucin genes exist. To search for new gel-forming mucin candidate genes, a "Hidden Markov Model"(HMM) was built from the common features of the CT domains of those gel-forming mucins. By using this model to screen all protein databases as well as the six-frame translated expression sequence tag and translated human genomic databases, we identified a locus located at the peri-centromere region of human chromosome 12 and the corresponding homologous region of mouse chromosome 15. We cloned the 3' end of this gene and its mouse homolog. We found one vWF C domain, one CT domain, and various mucin-like threonine/serine-rich repeats. Phylogenetic analysis indicated the close relationship between this gene and the submaxillary mucin from porcine and bovine. A polydispersed signal was observed on the Northern blot, which indicates very large mRNA size. Further analysis of the upstream genomic sequences generated from human and mouse genome projects revealed three additional vWF D domains and many mucin-like threonine/serine-rich repeats. The expression of this gene is restricted to the mucous cells of various glandular tissues, including sublingual gland, submandibular gland, and submucosal gland of the trachea. Based on the chronological convention, we have given the name MUC19 to the human ortholog and Muc19 to the mouse.
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Affiliation(s)
- Yin Chen
- Center for Comparative Respiratory Biology and Medicine, University of California at Davis, Davis, CA 95616, USA
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20
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Lidell ME, Johansson MEV, Mörgelin M, Asker N, Gum JR, Kim YS, Hansson GC. The recombinant C-terminus of the human MUC2 mucin forms dimers in Chinese-hamster ovary cells and heterodimers with full-length MUC2 in LS 174T cells. Biochem J 2003; 372:335-45. [PMID: 12597771 PMCID: PMC1223394 DOI: 10.1042/bj20030003] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2003] [Revised: 02/20/2003] [Accepted: 02/21/2003] [Indexed: 11/17/2022]
Abstract
The entire cDNA corresponding to the C-terminal cysteine-rich domain of the human MUC2 apomucin, after the serine- and threonine-rich tandem repeat, was expressed in Chinese-hamster ovary-K1 cells and in the human colon carcinoma cell line, LS 174T. The C-terminus was expressed as a fusion protein with the green fluorescent protein and mycTag sequences and the murine immunoglobulin kappa-chain signal sequence to direct the protein to the secretory pathway. Pulse-chase studies showed a rapid conversion of the C-terminal monomer into a dimer in both Chinese-hamster ovary-K1 and LS 174T cells. Disulphide-bond-stabilized dimers secreted into the media of both cell lines had a higher apparent molecular mass compared with the intracellular forms. The MUC2 C-terminus was purified from the spent culture medium and visualized by molecular electron microscopy. The dimer nature of the molecule was visible clearly and revealed that each monomer was attached to the other by a large globular domain. Gold-labelled antibodies against the mycTag or green fluorescent protein revealed that these were localized to the ends opposite to the parts responsible for the dimerization. The C-terminus expressed in LS 174T cells formed heterodimers with the full-length wild-type MUC2, but not with the MUC5AC mucin, normally expressed in LS 174T cells. The homodimers of the MUC2 C-termini were secreted continuously from the LS 174T cells, but no wild-type MUC2 secretion has been observed from these cells. This suggests that the information for sorting the MUC2 mucin into the regulated secretory pathway in cells having this ability is present in parts other than the C-terminus of MUC2.
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Affiliation(s)
- Martin E Lidell
- Department of Medical Biochemistry, Göteborg University, Medicinaregatan 9A, 413 90 Gothenburg, Sweden
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21
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Lidell ME, Johansson MEV, Hansson GC. An autocatalytic cleavage in the C terminus of the human MUC2 mucin occurs at the low pH of the late secretory pathway. J Biol Chem 2003; 278:13944-51. [PMID: 12582180 DOI: 10.1074/jbc.m210069200] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
During purification of a recombinant MUC2 C terminus expressed in CHO-K1 cells, the protein was partly cleaved when buffers with a pH of 6.0 were used. When buffers with higher pH values were used, less cleavage was found. Disulfide bonds held the two fragments generated together as these were only observed after reduction. Edman sequencing of the C-terminal 110-kDa fragment revealed that the cleavage had occurred at an Asp-Pro bond, a site described previously to generate the so-called "link peptide" after disulfide bond reduction. In vitro studies on the conditions for cleavage showed that it occurred in a time-dependent manner at a pH below 6.0. Furthermore, the reaction was not enzyme-mediated as it occurred in pure preparations of the MUC2 C terminus and was not inhibited by protease inhibitors. When expressed in the mucin producing cell line LS 174T, the C terminus was cleaved to a higher extent compared with the CHO-K1 cells. Neutralizing the secretory pathway with either NH(4)Cl or bafilomycin A1 inhibited this cleavage. Altogether, our results suggest that the cleavage is an autocatalytic reaction that occurs in the acidic environment of the late secretory pathway. Furthermore, the cleavage produced a new, reactive C terminus that has the potential to attach the mucin to itself or other molecules. Because a pH below 6 can be reached in the late secretory pathway and on mucosal surfaces, the cleavage and possible cross-linking are likely to be of biological importance.
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Affiliation(s)
- Martin E Lidell
- Department of Medical Biochemistry, Göteborg University, Medicinaregatan 9A, Sweden
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22
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Spiro RG. Protein glycosylation: nature, distribution, enzymatic formation, and disease implications of glycopeptide bonds. Glycobiology 2002; 12:43R-56R. [PMID: 12042244 DOI: 10.1093/glycob/12.4.43r] [Citation(s) in RCA: 966] [Impact Index Per Article: 43.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Formation of the sugar-amino acid linkage is a crucial event in the biosynthesis of the carbohydrate units of glycoproteins. It sets into motion a complex series of posttranslational enzymatic steps that lead to the formation of a host of protein-bound oligosaccharides with diverse biological functions. These reactions occur throughout the entire phylogenetic spectrum, ranging from archaea and eubacteria to eukaryotes. It is the aim of this review to describe the glycopeptide linkages that have been found to date and specify their presence on well-characterized glycoproteins. A survey is also made of the enzymes involved in the formation of the various glycopeptide bonds as well as the site of their intracellular action and their affinity for particular peptide domains is evaluated. This examination indicates that 13 different monosaccharides and 8 amino acids are involved in glycoprotein linkages leading to a total of at least 41 bonds, if the anomeric configurations, the phosphoglycosyl linkages, as well as the GPI (glycophosphatidylinositol) phosphoethanolamine bridge are also considered. These bonds represent the products of N- and O-glycosylation, C-mannosylation, phosphoglycation, and glypiation. Currently at least 16 enzymes involved in their formation have been identified and in many cases cloned. Their intracellular site of action varies and includes the endoplasmic reticulum, Golgi apparatus, cytosol, and nucleus. With the exception of the Asn-linked carbohydrate and the GPI anchor, which are transferred to the polypeptide en bloc, the sugar-amino acid linkages are formed by the enzymatic transfer of an activated monosaccharide directly to the protein. This review also deals briefly with glycosidases, which are involved in physiologically important cleavages of glycopeptide bonds in higher organisms, and with a number of human disease states in which defects in enzymatic transfer of saccharides to protein have been implicated.
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Affiliation(s)
- Robert G Spiro
- Department of Biological Chemistry, Harvard Medical School and the Joslin Diabetes Center, One Joslin Place, Boston, MA 02215, USA
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Wickström C, Carlstedt I. N-terminal cleavage of the salivary MUC5B mucin. Analogy with the Van Willebrand propolypeptide? J Biol Chem 2001; 276:47116-21. [PMID: 11602588 DOI: 10.1074/jbc.m106593200] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Sequence similarities between the oligomeric mucins (MUC2, MUC5AC, MUC5B) and the von Willebrand factor suggest that they may be assembled in a similar way. After oligomerization, a fragment corresponding to the D1 and D2 domains is released from the von Willebrand factor. This cleavage does not appear to occur in pig submaxillary mucin, the only mammalian mucin in which this cleavage has been examined thus far, but whether other oligomeric mucins undergo N terminus proteolysis is not known. Antibodies recognizing the D1, D2, D3, and the first Cys domains in MUC5B were established and used to investigate to what extent proteolytic cleavage occurs within the N-terminal part of salivary MUC5B. The antibodies against the D1 and D2 domains identified a polypeptide corresponding in size to a MUC5B fragment generated by cleavage within the D' domain analogously with the von Willebrand factor propolypeptide. The antibodies did not recognize the main mucin population, suggesting that the major part of salivary MUC5B is subjected to this cleavage. An antibody recognizing the D3 domain was used to reveal a second cleavage site in the "soluble" but not in the "insoluble" MUC5B fraction: the first structural difference observed between soluble and insoluble salivary MUC5B. The identification of these cleavage events shows that the N-terminal sites for MUC5B oligomerization are present in the D3 domain and/or in domains located C-terminal to this part of the molecule.
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Affiliation(s)
- C Wickström
- Mucosal Biology Group, Department of Cell and Molecular Biology, Lund University, S-22184 Lund, Sweden
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24
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Bell SL, Xu G, Forstner JF. Role of the cystine-knot motif at the C-terminus of rat mucin protein Muc2 in dimer formation and secretion. Biochem J 2001; 357:203-9. [PMID: 11415450 PMCID: PMC1221942 DOI: 10.1042/0264-6021:3570203] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
DNA constructs based on the 534-amino-acid C-terminus of rat mucin protein Muc2 (RMC), were transfected into COS cells and the resultant (35)S-labelled dimers and monomers were detected by SDS/PAGE of immunoprecipitates. The cystine-knot construct, encoding the C-terminal 115 amino acids, appeared in cell lysates as a 45 kDa dimer, but was not secreted. A construct, devoid of the cystine knot, failed to form dimers. Site-specific mutagenesis within the cystine knot was performed on a conserved unpaired cysteine (designated Cys-X), which has been implicated in some cystine-knot-containing growth factors as being important for intermolecular disulphide-bond formation. Dimerization of RMC was effectively abolished. Each cysteine (Cys-1-Cys-6) comprising the three intramolecular disulphide bonds of the cystine knot was then mutated. Dimer formation was impaired in each case, although much less so for the Cys-3 mutant than the others. Abnormal high-molecular-mass, disulphide-dependent aggregates formed with mutations Cys-1, Cys-2, Cys-4 and Cys-5(,) and were poorly secreted. It is concluded that the intact cystine-knot domain is essential for dimerization of the C-terminal domain of rat Muc2, and that residue Cys-X in the knot plays a key role. The structural integrity of the cystine knot, maintained by intramolecular bonds Cys-1-Cys-4, Cys-2-Cys-5 and Cys-3-Cys-6, also appears to be important for dimerization, probably by allowing correct positioning of the unpaired Cys-X residue for stable intermolecular cystine-bond formation.
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Affiliation(s)
- S L Bell
- Division of Structural Biology and Biochemistry, Research Institute, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8
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25
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Jones I, Lindberg C, Jakobsson S, Hellqvist A, Hellman U, Borg B, Olsson PE. Molecular cloning and characterization of spiggin. An androgen-regulated extraorganismal adhesive with structural similarities to von Willebrand Factor-related proteins. J Biol Chem 2001; 276:17857-63. [PMID: 11279178 DOI: 10.1074/jbc.m101142200] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
One of the most definitive examples of a vertebrate extraorganismal structural protein can be found in three-spined sticklebacks (Gasterosteus aculeatus). In the breeding male the kidney hypertrophies and synthesizes an adhesive protein called "spiggin," which is secreted into the urinary bladder from where it is employed as a structural thread for nest building. This paper describes the first molecular characterization of spiggin and demonstrates that this adhesive is a protein complex assembled from a potential of three distinct subunits (alpha, beta, and gamma). These subunits arise by alternative splicing, and 11-ketoandrogens induce their expression in stickleback kidneys. Analysis of the predicted amino acid sequence of each subunit reveals a modular organization whose structural elements display a similarity to the multimerization domains found within von Willebrand Factor-related proteins. These results implicate that spiggin utilizes a conserved multimerization mechanism for the formation of a viscous agglutinate from its constituent subunits in the urinary bladders of male sticklebacks. This novel extraorganismal structural protein is therefore ideally suited to its function as an adhesive thread.
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Affiliation(s)
- I Jones
- Department of Cell and Molecular Biology, Unit of Physiology, Umeå University, SE-901 87 Umeå, Sweden
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26
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Yeo SY, Little MH, Yamada T, Miyashita T, Halloran MC, Kuwada JY, Huh TL, Okamoto H. Overexpression of a slit homologue impairs convergent extension of the mesoderm and causes cyclopia in embryonic zebrafish. Dev Biol 2001; 230:1-17. [PMID: 11161558 DOI: 10.1006/dbio.2000.0105] [Citation(s) in RCA: 77] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Slit is expressed in the midline of the central nervous system both in vertebrates and invertebrates. In Drosophila, it is the midline repellent acting as a ligand for the Roundabout (Robo) protein, the repulsive receptor which is expressed on the growth cones of the commissural neurons. We have isolated cDNA fragments of the zebrafish slit2 and slit3 homologues and found that both genes start to be expressed by the midgastrula stage well before the axonogenesis begins in the nervous system, both in the axial mesoderm, and slit2 in the anterior margin of the neural plate and slit3 in the polster at the anterior end of the prechordal mesoderm. Later, expression of slit2 mRNA is detected mainly in midline structures such as the floor plate cells and the hypochord, and in the anterior margins of the neural plates in the zebrafish embryo, while slit3 expression is observed in the anterior margin of the prechordal plate, the floorplate cells in the hindbrain, and the motor neurons both in the hindbrain and the spinal cord. To study the role of Slit in early embryos, we overexpressed Slit2 in the whole embryos either by injection of its mRNA into one-cell stage embryos or by heat-shock treatment of the transgenic embryos which carries the slit2 gene under control of the heat-shock promoter. Overexpression of Slit2 in such ways impaired the convergent extension movement of the mesoderm and the rostral migration of the cells in the dorsal diencephalon and resulted in cyclopia. Our results shed light on a novel aspect of Slit function as a regulatory factor of mesodermal cell movement during gastrulation.
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Affiliation(s)
- S Y Yeo
- Laboratory for Development Gene Regulation, RIKEN Brain Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
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Loukas A, Hintz M, Linder D, Mullin NP, Parkinson J, Tetteh KK, Maizels RM. A family of secreted mucins from the parasitic nematode Toxocara canis bears diverse mucin domains but shares similar flanking six-cysteine repeat motifs. J Biol Chem 2000; 275:39600-7. [PMID: 10950959 DOI: 10.1074/jbc.m005632200] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Infective larvae of the parasitic nematode Toxocara canis secrete a family of mucin-like glycoproteins, which are implicated in parasite immune evasion. Analysis of T. canis expressed sequence tags identified a family of four mRNAs encoding distinct apomucins (Tc-muc-1-4), one of which had been previously identified in the TES-120 family of glycoproteins secreted by this parasite. The protein products of all four cDNAs contain signal peptides, a repetitive serine/threonine-rich tract, and varying numbers of 36-amino acid six-cysteine (SXC) domains. SXC domains are found in many nematode proteins and show similarity to cnidarian (sea anemone) toxins. Antibodies to the SXC domains of Tc-MUC-1 and Tc-MUC-3 recognize differently migrating members of TES-120. TES-120 proteins separated by chromatographic methods showed distinct amino acid composition, mass, and sequence information by both Edman degradation and matrix-assisted laser desorption ionization/time of flight mass spectrometry on peptide fragments. Tc-MUC-1, -2, and -3 were shown to be secreted mucins with real masses of 39.7, 47.8, and 45.0 kDa in contrast to their predicted peptide masses of 15.7, 16.2, and 26.0 kDa, respectively. The presence of SXC domains in all mucin products supports the suggestion that the SXC motif is required for mucin assembly or export. Homology modeling indicates that the six-cysteine domains of the T. canis mucins adopt a similar fold to the sea anemone potassium channel-blocking toxin BgK, forming three disulfide bonds within each subunit.
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MESH Headings
- Amino Acid Sequence
- Animals
- Blotting, Western
- Chromatography
- Chromatography, High Pressure Liquid
- Cysteine/chemistry
- DNA, Complementary/metabolism
- Electrophoresis, Polyacrylamide Gel
- Expressed Sequence Tags
- Female
- Gastric Mucins/chemistry
- Gastric Mucins/genetics
- Gene Library
- Models, Biological
- Models, Molecular
- Molecular Sequence Data
- Mucins/chemistry
- Mucins/genetics
- Multigene Family
- Protein Sorting Signals
- Protein Structure, Tertiary
- RNA, Messenger/metabolism
- Recombinant Proteins/metabolism
- Sequence Homology, Amino Acid
- Serine/chemistry
- Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
- Threonine/chemistry
- Toxocara canis/chemistry
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Affiliation(s)
- A Loukas
- Institute of Cell, Animal and Population Biology and Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, United Kingdom
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28
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Abstract
The existence of high-molecular-weight glycoproteins in saliva and salivary secretions has been recognized for nearly 30 years. These proteins, called mucins, are essential for oral health and perform many diverse functions in the oral cavity. Mucins have been intensively studied, and much has been learned about their biochemical properties and their interactions with oral micro-organisms and other salivary proteins. In the past several years, the major high-molecular-weight mucin in salivary secretions has been identified as MUC5B, one of a family of 11 human mucin gene products expressed in tissue-specific patterns in the gastrointestinal, respiratory, and reproductive tracts. MUC5B is one of four gel-forming mucins which exist as multimeric proteins with molecular weights greater than 20-40 million daltons. The heavily glycosylated mucin multimers form viscous layers which protect underlying epithelial surfaces from microbial, mechanical, and chemical assault. Another class of mucin molecules, the membrane-bound mucins, is structurally and functionally distinct from the gel-forming mucins. These proteins do not form multimers and can exist as both secreted and membrane-bound forms, with the latter anchored to epithelial cell membranes through a short membrane-spanning domain. In the present work, we show that two of the membrane-bound mucins, MUC1 and MUC4, are expressed in all major human salivary glands as well as in buccal epithelial cells. While the functions of these mucins in the oral environment are not understood, it is possible that they form a structural framework on the cell surface which not only is cytoprotective, but also may serve as a scaffold upon which MUC5B, and possibly other salivary proteins, assemble.
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Affiliation(s)
- G D Offner
- Department of Medicine, Boston University Medical Center, MA 02118, USA.
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29
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Abstract
Mucins, the major component of mucus, contain tandemly repeated sequences that differ from one mucin to another. Considerable advances have been made in recent years in our knowledge of mucin genes. The availability of the complete genomic and cDNA sequences of MUC5B, one of the four human mucin genes clustered on chromosome 11, provides an exemplary model for studying the molecular evolution of large mucins. The emerging picture is one of expansion of mucin genes by gene duplications, followed by internal repeat expansion that strictly preserves frameshift. Computational and phylogenetic analyses have permitted the proposal of an evolutionary history of the four human mucin genes located on chromosome 11 from an ancestor gene common to the human von Willebrand factor gene and the suggestion of a model for the evolution of the repeat coding portion of the MUC5B gene from a hypothetical ancestral minigene. The characterization of MUC5B, a member of the large secreted gel-forming mucin family, offers a new model for the comparative study of the structure-function relationship within this important family.
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30
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Marin F, Corstjens P, de Gaulejac B, de Vrind-De Jong E, Westbroek P. Mucins and molluscan calcification. Molecular characterization of mucoperlin, a novel mucin-like protein from the nacreous shell layer of the fan mussel Pinna nobilis (Bivalvia, pteriomorphia). J Biol Chem 2000; 275:20667-75. [PMID: 10770949 DOI: 10.1074/jbc.m003006200] [Citation(s) in RCA: 144] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
A cDNA expression library constructed from mantle tissue mRNA of the Mediterranean fan mussel Pinna nobilis was screened with antibodies raised against the acetic acid-soluble shell matrix of the same species. This resulted in the isolation of a 2138-base pair cDNA, containing 13 tandem repeats of 93 base pairs. The deduced protein has a molecular mass of 66.7 kDa and a isoelectric point of 4.8. This protein, which is enriched in serine and proline residues, was overexpressed, purified, and used for producing polyclonal antibodies. Immunological in situ and in vitro tests showed that the protein is localized in the nacreous aragonitic layer of P. nobilis, but not in the calcitic prisms. Because this protein of the nacre of P. nobilis exhibits some mucin-like characteristics, we propose the name mucoperlin. This is the first paper reporting the cloning of a molluscan mucin and the first molecular evidence for the involvement of a mucin in molluscan calcification. This finding corroborates our previous hypothesis that some of the proteinaceous constituents of the molluscan shell matrix would derive from mucins, common to many metazoan lineages of the late Precambrian (Marin, F., Smith, M., Isa, Y., Muyzer, G. and Westbroek, P. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 1554-1559). The adaptation of an ancestral mucin to a new function, the regulation of the mineralization process, may be one of the molecular events, among others, that would explain the simultaneous emergence of organized calcification in many metazoan lineages during the Cambrian explosion.
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Affiliation(s)
- F Marin
- Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.
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31
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Thornton DJ, Gray T, Nettesheim P, Howard M, Koo JS, Sheehan JK. Characterization of mucins from cultured normal human tracheobronchial epithelial cells. Am J Physiol Lung Cell Mol Physiol 2000; 278:L1118-28. [PMID: 10835316 DOI: 10.1152/ajplung.2000.278.6.l1118] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Early-passage normal human tracheobronchial epithelial (NHTBE) cells grown in air-liquid interface cultures in medium containing retinoids differentiate into a mucociliary epithelium over a 2- to 3-wk period and express increasing mRNA levels of the airway mucin genes MUC5AC and MUC5B as the cultures age; the levels of MUC2 mRNA were very low throughout the study. Using specific antibodies to MUC5AC and MUC5B mucins, we noted a gradual increase in these two mucins in the intracellular and apically secreted pools as a function of time. A low level of MUC2 mucin was detected, which did not change with time. The intracellular and apically secreted mucins isolated from day 14 and day 21 cultures by density gradient centrifugation were similar in density to those previously isolated from human respiratory mucus secretions. The sedimentation rate of the apically secreted mucins indicated that they were highly oligomerized, polydisperse macromolecules similar to those previously documented from in vivo secretions. In contrast, the cell-associated mucins from the cultured NHTBE cells were much smaller, possibly only monomers and dimers. Anion-exchange chromatography detected no differences in charge density between the reduced and carboxymethylated cell-associated and secreted forms of the MUC5AC and MUC5B mucins. The MUC5AC mucin was of similar charge density to its in vivo counterpart; however, MUC5B was more homogeneous than that found in vivo. Finally, evidence is presented for an intracellular NH(2)-terminal cleavage of the MUC5B mucins. These studies indicate that the mucins produced by cultured NHTBE cells are similar to those found in human airways, suggesting that this cell culture model is suited for studies of respiratory mucin biosynthesis, processing, and assembly.
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Affiliation(s)
- D J Thornton
- The Wellcome Trust Centre for Cell-Matrix Research, Division of Biochemistry, School of Biological Sciences, University of Manchester, United Kingdom.
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32
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Jiang W, Gupta D, Gallagher D, Davis S, Bhavanandan VP. The central domain of bovine submaxillary mucin consists of over 50 tandem repeats of 329 amino acids. Chromosomal localization of the BSM1 gene and relations to ovine and porcine counterparts. EUROPEAN JOURNAL OF BIOCHEMISTRY 2000; 267:2208-17. [PMID: 10759843 DOI: 10.1046/j.1432-1327.2000.01225.x] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
We previously elucidated five distinct protein domains (I-V) for bovine submaxillary mucin, which is encoded by two genes, BSM1 and BSM2. Using Southern blot analysis, genomic cloning and sequencing of the BSM1 gene, we now show that the central domain (V) consists of approximately 55 tandem repeats of 329 amino acids and that domains III-V are encoded by a 58.4-kb exon, the largest exon known for all genes to date. The BSM1 gene was mapped by fluorescence in situ hybridization to the proximal half of chromosome 5 at bands q2. 2-q2.3. The amino-acid sequence of six tandem repeats (two full and four partial) were found to have only 92-94% identities. We propose that the variability in the amino-acid sequences of the mucin tandem repeat is important for generating the combinatorial library of saccharides that are necessary for the protective function of mucins. The deduced peptide sequences of the central domain match those determined from the purified bovine submaxillary mucin and also show 68-94% identity to published peptide sequences of ovine submaxillary mucin. This indicates that the core protein of ovine submaxillary mucin is closely related to that of bovine submaxillary mucin and contains similar tandem repeats in the central domain. In contrast, the central domain of porcine submaxillary mucin is reported to consist of 81-amino-acid tandem repeats. However, both bovine submaxillary mucin and porcine submaxillary mucin contain similar N-terminal and C-terminal domains and the corresponding genes are in the conserved linkage regions of the respective genomes.
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Affiliation(s)
- W Jiang
- Department of Biochemistry and Molecular Biology, The Pennsylvania State University College of Medicine, Hershey, PA, USA
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33
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Affiliation(s)
- J Perez-Vilar
- Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, USA
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34
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Turner BS, Bhaskar KR, Hadzopoulou-Cladaras M, LaMont JT. Cysteine-rich regions of pig gastric mucin contain von willebrand factor and cystine knot domains at the carboxyl terminal(1). BIOCHIMICA ET BIOPHYSICA ACTA 1999; 1447:77-92. [PMID: 10500247 DOI: 10.1016/s0167-4781(99)00099-8] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
In order to sequence the cysteine-rich regions of pig gastric mucin (PGM), we used our previously identified pig gastric mucin clone PGM-2A to screen a pig stomach cDNA library and perform rapid amplification of cDNA ends to obtain two cysteine-rich clones, PGM-2X and PGM-Z13. PGM-2X has 1071 base pairs (bp) encoding 357 amino acids containing five serine-threonine-rich 16 amino acid tandem repeats, downstream from a cysteine-rich region similar to human and mouse MUC5AC. PGM-Z13 encodes the complete 3'-terminus of PGM and is composed of 3336 bp with a 2964 bp open reading frame encoding 988 amino acids with four serine-threonine-rich tandem repeats upstream from a cysteine-rich region similar to the carboxyl terminal regions of human and rat MUC5AC and human MUC5B. This region is homologous to von Willebrand factor C and D domains involved in acid induced polymerization, and to the carboxyl terminal cystine-knot domain of various mucins, TGF-beta, vWF and norrin, which is involved in dimerization. These newly sequenced cysteine-rich regions of pig gastric mucin may be critical for its gelation and for its observed increased viscosity induced by low pH.
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Affiliation(s)
- B S Turner
- Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, USA
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35
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Campbell B. Biochemical and Functional Aspects of Mucus and Mucin-Type Glycoproteins. DRUGS AND THE PHARMACEUTICAL SCIENCES 1999. [DOI: 10.1201/b14099-6] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
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36
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Nunes DP, Afdhal NH, Offner GD. A recombinant bovine gallbladder mucin polypeptide binds biliary lipids and accelerates cholesterol crystal appearance time. Gastroenterology 1999; 116:936-42. [PMID: 10092316 DOI: 10.1016/s0016-5085(99)70077-6] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
BACKGROUND & AIMS Mucin has a central role in the pathogenesis of cholesterol gallstones, in part because of its ability to bind biliary lipids and accelerate cholesterol crystal appearance time. Previous studies have localized these properties to nonglycosylated mucin domains, and we have recently shown that these domains contain a series of 127-amino acid, cysteine-rich repeats. The aim of this study was to express a recombinant mucin polypeptide containing these repeats and investigate its lipid-binding and pronucleating properties. METHODS A recombinant mucin polypeptide was expressed as a glutathione S-transferase fusion protein in Escherichia coli, purified by affinity chromatography, and compared with native bovine gallbladder mucin in lipid-binding and cholesterol crystal appearance time assays. RESULTS The recombinant mucin polypeptide bound a hydrophobic fluorescent probe and cholesterol in a concentration-dependent manner. It accelerated the appearance of cholesterol crystals from lithogenic model bile, an effect that was both time and concentration dependent. CONCLUSIONS The cysteine-rich repeats in the recombinant mucin polypeptide correspond to the protease-sensitive hydrophobic domains identified in earlier biochemical studies. Further delineation of the lipid-binding site(s) in these repeats will provide new insights into the mechanism of cholesterol crystal nucleation and stone growth.
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Affiliation(s)
- D P Nunes
- Section of Gastroenterology, Department of Medicine, Boston University School of Medicine and Boston Medical Center, Boston, Massachusetts 02118, USA
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37
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Perez-Vilar J, Hill RL. Identification of the half-cystine residues in porcine submaxillary mucin critical for multimerization through the D-domains. Roles of the CGLCG motif in the D1- and D3-domains. J Biol Chem 1998; 273:34527-34. [PMID: 9852122 DOI: 10.1074/jbc.273.51.34527] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Plasmids encoding the amino-terminal region of porcine submaxillary mucin were modified by site-specific mutagenesis to assess the roles of individual half-cystine residues in the assembly of disulfide-linked multimers of mucin. COS-7 cells with the plasmid containing C1199A expressed primarily monomers, suggesting that half-cystine 1199 in the D3-domain is involved in forming mucin multimers. This residue is in the sequence C1199SWRYEPCG, which is highly conserved in the D3-domain of other secreted mucins and human prepro-von Willebrand factor. In contrast, cells with the plasmid containing C1276A expressed trimers like those with unmutated plasmid, suggesting that half-cystine 1276 is not involved in formation of disulfide-bonded multimers. The roles of the half-cystines in the CGLCG motifs in the assembly of disulfide-bonded multimers of mucin were also assessed. Cells with plasmids in which both half-cystines in the motif in the D1- or D3-domain of mucin are replaced by alanine expressed proteins that were poorly secreted, suggesting that these mutations impair normal folding of the expressed proteins. A plasmid with a mutant D1-domain motif expressed monomers, whereas one with a mutant D3-domain motif expressed monomers and trimers. However, the trimers expressed by the latter plasmid were assembled in non-acidic compartments, as judged by expression studies in the presence of monensin, which inhibits trimer formation by unmutated plasmid, but not by the mutant plasmid. These results suggest that the CGLCG motif in the D1-domain is required for multimerization in the trans-Golgi complex. However, the CGLCG motif in the D3-domain appears to prevent formation of mucin multimers in non-acidic compartments of the cell. Plasmids encoding the D1- and D2-domains, the D1- and D3-domains, or only the D3-domain also expressed oligomers in the presence of monensin, suggesting that the three D-domains must be contiguous to avoid multimerization in non-acidic compartments. It is possible that these motifs in mucins are engaged in the thiol-disulfide interchange reactions during the assembly of disulfide-bonded multimers of mucin.
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Affiliation(s)
- J Perez-Vilar
- Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, USA
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38
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Itoh A, Miyabayashi T, Ohno M, Sakano S. Cloning and expressions of three mammalian homologues of Drosophila slit suggest possible roles for Slit in the formation and maintenance of the nervous system. BRAIN RESEARCH. MOLECULAR BRAIN RESEARCH 1998; 62:175-86. [PMID: 9813312 DOI: 10.1016/s0169-328x(98)00224-1] [Citation(s) in RCA: 143] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
In Drosophila embryogenesis, the slit gene has been shown to play a critical role in CNS midline formation. However, no slit homologues have been reported in vertebrates. Here, we have identified mammalian homologues of the slit gene (human Slit-1, Slit-2, Slit-3, and rat Slit-1). Each Slit gene encodes a putative secreted protein, which contains conserved protein-protein interaction domains including leucine-rich repeats (LRR) and epidermal growth factor (EGF)-like motifs, like that of the Drosophila protein. Northern blot analysis revealed that the human Slit-1, -2, and -3 mRNAs are exclusively expressed in the brain, spinal cord, and thyroid, respectively. In situ hybridization studies indicated that the rat Slit-1 mRNA is specifically expressed in the neurons of fetal and adult forebrains. Our data suggest that Slit genes form an evolutionary conserved group in vertebrates and invertebrates, and that the mammalian Slit proteins may participate in the formation and maintenance of the nervous and endocrine systems by protein-protein interactions.
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Affiliation(s)
- A Itoh
- Life Science Fundamental Research Laboratory, Asahi Chemical Industry Co., Ltd., 2-1 Samejima, Fuji City, Shizuoka 416-8501, Japan.
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39
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Jiang W, Woitach JT, Gupta D, Bhavanandan VP. Sequence of a second gene encoding bovine submaxillary mucin: implication for mucin heterogeneity and cloning. Biochem Biophys Res Commun 1998; 251:550-6. [PMID: 9792811 DOI: 10.1006/bbrc.1998.9515] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Secreted epithelial mucins are extremely large and heterogeneous glycoproteins. We report the 5 kilobase DNA sequence of a second gene, BSM2, which encodes bovine submaxillary mucin. The determined nucleotide and deduced amino acid sequences of BSM2 are 95.2% and 92. 2% identical, respectively, to those of the previously described BSM1 gene isolated from the same cow. Further, the five predicted protein domains of the two genes are 100%, 94%, 93%, 77%, and 88% identical. Based on the above results, we propose that expression of multiple homologous core proteins from a single animal is a factor in generating diversity of saccharides in mucins and in providing resistance of the molecules to proteolysis. In addition, this work raises several important issues in mucin cloning such as assembling sequences from seemingly overlapping clones and deducing consensus sequences for nearly identical tandem repeats.
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Affiliation(s)
- W Jiang
- Department of Biochemistry and Molecular Biology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania, 17033, USA.
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40
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Asker N, Axelsson MA, Olofsson SO, Hansson GC. Human MUC5AC mucin dimerizes in the rough endoplasmic reticulum, similarly to the MUC2 mucin. Biochem J 1998; 335 ( Pt 2):381-7. [PMID: 9761738 PMCID: PMC1219793 DOI: 10.1042/bj3350381] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
Biosynthetic studies on the human MUC5AC mucin were performed by immunoprecipitations with antisera recognizing only the non-O-glycosylated apomucin in the colon adenocarcinoma cell line LS 174T. Pulse-chase studies and subcellular fractionations showed that MUC5AC formed dimers in the rough endoplasmic reticulum within 15 min of the initiation of biosynthesis. No non-O-glycosylated species larger than dimers were identified. The dimerization was N-glycosylation-dependent, because tunicamycin treatment significantly lowered the rate of dimerization. When the biosynthesis of MUC5AC apomucin was compared with that of MUC2 apomucin, also produced in the LS 174T cell line, both apomucins were assembled in similar ways with respect to their rates of dimerization with and without inhibition of N-glycosylation. No heterodimerization was observed between the human MUC5AC and the MUC2 apomucins despite the extensive sequence similarities in the positions of the cysteine residues in the C-termini proposed to be involved in mucin dimerization.
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Affiliation(s)
- N Asker
- Department of Medical Biochemistry, Göteborg University, Medicinaregatan 9A, S-413 90 Gothenburg, Sweden
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41
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Abstract
Von Willebrand factor (VWF) is a blood glycoprotein that is required for normal hemostasis, and deficiency of VWF, or von Willebrand disease (VWD), is the most common inherited bleeding disorder. VWF mediates the adhesion of platelets to sites of vascular damage by binding to specific platelet membrane glycoproteins and to constituents of exposed connective tissue. These activities appear to be regulated by allosteric mechanisms and possibly by hydrodynamic shear forces. VWF also is a carrier protein for blood clotting factor VIII, and this interaction is required for normal factor VIII survival in the circulation. VWF is assembled from identical approximately 250 kDa subunits into disulfide-linked multimers that may be > 20,000 kDa. Mutations in VWD can disrupt this complex biosynthetic process at several steps to impair the assembly, intracellular targeting, or secretion of VWF multimers. Other VWD mutations impair the survival of VWF in plasma or the function of specific ligand binding sites. This growing body of information about VWF synthesis, structure, and function has allowed the reclassification of VWD based upon distinct pathophysiologic mechanisms that appear to correlate with clinical symptoms and the response to therapy.
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Affiliation(s)
- J E Sadler
- Howard Hughes Medical Institute, Department of Medicine, Washington University School of Medicine, Saint Louis, Missouri 63110, USA.
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42
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Asker N, Axelsson MA, Olofsson SO, Hansson GC. Dimerization of the human MUC2 mucin in the endoplasmic reticulum is followed by a N-glycosylation-dependent transfer of the mono- and dimers to the Golgi apparatus. J Biol Chem 1998; 273:18857-63. [PMID: 9668061 DOI: 10.1074/jbc.273.30.18857] [Citation(s) in RCA: 102] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Pulse-chase experiments in the colon cell line LS 174T combined with subcellular fractionation by sucrose density gradient centrifugation showed that the initial dimerization of the MUC2 apomucin started directly after translocation of the apomucin into the rough endoplasmic reticulum as detected by calnexin reactivity. As the mono- and dimers were chased, O-glycosylated MUC2 mono- and dimers were precipitated using an O-glycosylation-insensitive antiserum against the N-terminal domain of the MUC2 mucin. These O-glycosylated species were precipitated from the fractions that comigrated with the galactosyltransferase activity during the subcellular fractionation, indicating that not only MUC2 dimers but also a significant amount of monomers are transferred into the Golgi apparatus. Inhibition of N-glycosylation with tunicamycin treatment slowed down the rate of dimerization and introduced further oligomerization of the MUC2 apomucin in the endoplasmic reticulum. Results of two-dimensional gel electrophoresis demonstrated that these oligomers (putative tri- and tetramers) were stabilized by disulfide bonds. The non-N-glycosylated species of the MUC2 mucin were retained in the endoplasmic reticulum because no O-glycosylated species were precipitated after inhibition by tunicamycin. This suggests that N-glycans of MUC2 are necessary for the correct folding and dimerization of the MUC2 mucin.
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Affiliation(s)
- N Asker
- Department of Medical Biochemistry, University of Göteborg, Medicinaregatan 9A, S-413 90 Gothenburg, Sweden
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43
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Gipson IK, Inatomi T. Cellular origin of mucins of the ocular surface tear film. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 1998; 438:221-7. [PMID: 9634890 DOI: 10.1007/978-1-4615-5359-5_32] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
In summary, we have demonstrated that the ocular surface epithelia express at least three mucin genes. We suggest that the gel-forming mucin MUC5AC is a major mucin forming the mucus gel of the tear film. We further suggest that MUC1 facilitates the spread of the MUC5-containing mucus on the ocular surface and, along with the mucus gel, prevents cell and debris adhesion to the ocular surface. The function of MUC4 at the ocular surface remains to be elucidated.
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Affiliation(s)
- I K Gipson
- Schepens Eye Research Institute, Boston, Massachusetts, USA
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44
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Perez-Vilar J, Eckhardt AE, DeLuca A, Hill RL. Porcine submaxillary mucin forms disulfide-linked multimers through its amino-terminal D-domains. J Biol Chem 1998; 273:14442-9. [PMID: 9603957 DOI: 10.1074/jbc.273.23.14442] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
COS-7 cells expressing 1,360 residues from the amino terminus of porcine submaxillary mucin were used to determine whether this region, containing the D1, D2, and D3 domains, is involved in forming mucin multimers. Analysis of the proteins immunoprecipitated from the medium of transfected cells by reducing SDS-gel electrophoresis showed a single N-glycosylated protein with no indication of proteolytically processed forms. Without prior reduction, only two proteins, corresponding to monomeric and disulfide-linked trimeric species, were observed. The expressed protein devoid of N-linked oligosaccharides also formed trimers, but was secreted from cells in significantly less amounts than glycosylated trimers. Pulse-chase studies showed that the disulfide-linked trimers were assembled inside the cells no earlier than 30 min after protein synthesis commenced and after the intracellular precursors were N-glycosylated. Trimer formation was inhibited in cells treated with brefeldin A, monensin, chloroquine, or bafilomycin A1, although only brefeldin A prevented the secretion of the protein. These results suggest that trimerization takes place in compartments of the Golgi complex in which the vacuolar H+-ATPase maintains an acidic pH. Coexpression in the same cells of the amino-terminal region and the disulfide-rich carboxyl-terminal domain of the mucin showed that these structures were not disulfide-linked with one another. Cells expressing a DNA construct encoding a fusion protein between the amino- and carboxyl-terminal regions of the mucin secreted disulfide-linked dimeric and high molecular weight multimeric species of the recombinant mucin. The presence of monensin in the medium was without effect on dimerization, but inhibited the formation of disulfide-linked multimers. These studies suggest that disulfide-linked dimers of mucin are subsequently assembled into disulfide-linked multimers by the amino-terminal regions. They also suggest that the porcine mucin forms branched disulfide-linked multimers. This ability of the amino-terminal region of mucin to aid in the assembly of multimers is consistent with its amino acid identities to the amino-terminal region of human von Willebrand factor, which also serves to form disulfide-linked multimers of this protein.
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Affiliation(s)
- J Perez-Vilar
- Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, USA
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Jiang W, Woitach JT, Keil RL, Bhavanandan VP. Bovine submaxillary mucin contains multiple domains and tandemly repeated non-identical sequences. Biochem J 1998; 331 ( Pt 1):193-9. [PMID: 9512479 PMCID: PMC1219338 DOI: 10.1042/bj3310193] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
A number of cDNA fragments coding for bovine submaxillary mucin (BSM) were cloned, and the nucleotide sequence of the largest clone, BSM421, was determined. Two peptide sequences determined from the purified apoBSM were found near the N-terminus of the mucin-coding region of BSM421. This clone does not contain a start or stop codon, but its 3' end overlaps with the 5' end of a previously isolated clone, lambdaBSM10. The composite sequence of 1589 amino acid residues consists of five distinct protein domains, which are numbered from the C-terminus. The cysteine-rich domain I can be further divided into a von Willebrand factor type C repeat and a cystine knot. Domains III and V consist of similar repeated peptide sequences with an average of 47 residues. Domains II and IV do not contain such sequences but are similar to domains III and V in being rich in serine and threonine, many of which are predicted to be potential O-glycosylation sites. Domain III also contains two sequences that match the ATP/GTP-binding site motif A (P-loop). Only beta-strands and no alpha-helices are predicted for the partial deduced amino acid sequence. Northern analysis of submaxillary gland RNA with the BSM421 probe detected multiple messages of BSM with sizes from 1.1 to over 10 kb. The tandemly repeated, non-identical peptide sequences of approx. 47 residues in domains III and V of BSM differ from the tandemly repeated, identical 81-residue sequences of pig submaxillary mucin (PSM), although both BSM and PSM contain similar C-terminal domains. In contrast, two peptide sequences of ovine submaxillary mucin are highly similar (86% and 65% identical respectively) to the corresponding sequences in domain V of BSM.
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Affiliation(s)
- W Jiang
- Department of Biochemistry and Molecular Biology, The Pennsylvania State University College of Medicine, Hershey, PA 17033, USA
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Perez-Vilar J, Hill RL. The carboxyl-terminal 90 residues of porcine submaxillary mucin are sufficient for forming disulfide-bonded dimers. J Biol Chem 1998; 273:6982-8. [PMID: 9507005 DOI: 10.1074/jbc.273.12.6982] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
COS-7 cells transfected with expression vectors encoding 90 and 154 amino acid residues, respectively, from the carboxyl terminus of the disulfide-rich domain (240 residues) of porcine submaxillary mucin were shown to form disulfide-bonded dimers. Cells with expression vectors that encoded the disulfide-rich domain lacking the last 90 and 150 carboxyl-terminal residues, respectively, from the carboxyl terminus of the disulfide-rich domain were unable to secrete truncated domains. These results indicate that the information required to form disulfide-bonded dimers resides in only 90 residues, including 11 half-cystines. Site-specific mutagenesis was employed to change, one at a time, each codon for the 11 half-cystines to serine. Eight of the 11 mutants formed disulfide-bonded dimers indistinguishable from those produced by unmutated vector, although 6 of the 8 mutants also produced aggregates thought to be misfolded protein with scrambled disulfide bonds. Two additional mutant vectors encoding serine instead of half-cystine at residues 13244 and 13246 in submaxillary mucin expressed both monomers and dimers of the disulfide-rich domain but no aggregates. The final mutant vector, C13223S, expressed protein aggregates that were poorly secreted from transfected cells. A mutant vector with two codon changes, C13244A/C13246A, expressed both monomers and dimers, just like the single mutants at these half-cystines. These results suggest that three half-cystine residues (Cys13223, Cys13244, and Cys13246) may be involved in forming interchain disulfide bonds in mucin dimers. Two of these half-cystines, Cys13244 and Cys13246, are in the highly conserved sequence C13244LC13246C in the disulfide-rich domain of several other human mucins and in prepro-von Willebrand factor and norrin, a protein that in mutant forms gives rise to Norrie disease. Support for the involvement of these half-cystines in formation of disulfide-bonded dimers of these molecules is also provided by known mutations in prepro-von Willebrand factor and norrin.
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Affiliation(s)
- J Perez-Vilar
- Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, USA
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Shekels LL, Hunninghake DA, Tisdale AS, Gipson IK, Kieliszewski M, Kozak CA, Ho SB. Cloning and characterization of mouse intestinal MUC3 mucin: 3' sequence contains epidermal-growth-factor-like domains. Biochem J 1998; 330 ( Pt 3):1301-8. [PMID: 9494100 PMCID: PMC1219276 DOI: 10.1042/bj3301301] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
Mucin glycoproteins are a heterogeneous family of high-molecular-mass, heavily glycosylated proteins differentially expressed in epithelial tissue of the gastrointestinal, reproductive and respiratory tracts. We report here the cloning of a mouse caecal mucin (MCM). Amino acid analysis of purified MCM revealed a high content of serine (10.8%) and threonine (25.1%). Antibodies against deglycosylated MCM were prepared for immunohistochemical analysis and for screening a mouse caecal cDNA library. Immunohistochemical analysis showed strong staining of goblet cells and patchy staining of surface columnar cells in the duodenum, small intestine, caecum, colon and rectum. Screening of a mouse caecal cDNA library yielded clones containing tandem repeats of 18 bp with two predominant peptide sequences of TTTADV and TTTVVV. The tandem repeat domain is followed by 1137 bp of non-repetitive sequence and 521 bp of 3' untranslated sequence prior to the poly(A) tail. Two cysteine-rich regions lie within the 3' non-repetitive domain. The arrangement of the cysteines within these regions corresponds to epidermal growth factor-like domains. Following the second cysteine-rich region is a stretch of 19 hydrophobic amino acids which may act as a transmembrane domain or allow for interaction with hydrophobic molecules. Northern blot analysis indicates the mRNA is approximately 13.5 kb with greatest expression in the caecum and lesser amounts in the colon and small intestine. No MCM message is found in mouse stomach, trachea, lung, kidney, oesophagus or pancreas. In situ hybridization studies show that MCM message is expressed at the tips of villi in the intestine and in the upper crypts and surface cells of the caecum and colon. Chromosomal analysis assigns this gene to mouse chromosome 5 in a region of conserved linkage with human chromosome 7, the location of the human MUC3 gene. We conclude that we have identified a mouse caecal mucin which represents the mouse homologue of human MUC3. The mouse MUC3 cDNA sequence suggests that it is a novel non-polymerizing mucin which may participate in membrane or intermolecular interactions through its 3' non-repetitive region.
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Affiliation(s)
- L L Shekels
- Department of Medicine, University of Minnesota and VA Medical Center, Minneapolis, MN 55417, USA
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van Klinken BJ, Einerhand AW, Büller HA, Dekker J. The oligomerization of a family of four genetically clustered human gastrointestinal mucins. Glycobiology 1998; 8:67-75. [PMID: 9451015 DOI: 10.1093/glycob/8.1.67] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
Mucins are synthesized and secreted by many epithelia. They are complex glycoproteins that offer cytoprotection. In their functional configuration, mucins form oligomers by a biosynthetic process that is poorly understood. A family of four human gastrointestinal mucin genes (MUC2, MUC5AC, MUC5B, and MUC6) is clustered to chromosome 11p15.5. To study oligomerization of these related mucins, we performed metabolic labeling experiments with [35S]amino acids in LS174T cells, and isolated mucin precursors by specific immunoprecipitations that were analyzed on SDS-PAGE. Each of the precursors of MUC2, MUC5AC, MUC5B, and MUC6 formed a single species of disulfide-linked homo-oligomer within 1 h after pulse labeling. Based on apparent molecular masses, these oligomeric precursors were most likely dimers. Inhibition of vesicular RER-to-Golgi transport, with brefeldin A and CCCP, did not affect the dimerization of MUC2 precursors, localizing dimerization to the RER. O-Glycosylation of MUC2 followed dimerization. Inhibition of N-glycosylation by tunicamycin retarded, but did not inhibit, dimerization, indicating that N-glycans play a role in efficient dimerization of MUC2 precursors. Based on sequence homology, the ability of MUC2, MUC5AC, MUC5B and MUC6 to dimerize most likely resides in their C-terminal domains. Thus, the RER-localized dimerization of secretory mucins likely proceeds by similar mechanisms, which is an essential step in the formation of the human gastrointestinal mucus-gels.
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Affiliation(s)
- B J van Klinken
- Pediatric Gastroenterology and Nutrition, Academic Medical Center, Rm 68-260, University of Amsterdam, Amsterdam, The Netherlands
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Perez-Vilar J, Hill RL. Norrie disease protein (norrin) forms disulfide-linked oligomers associated with the extracellular matrix. J Biol Chem 1997; 272:33410-5. [PMID: 9407136 DOI: 10.1074/jbc.272.52.33410] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
COS-7 cells transfected with a DNA construct encoding the 133 amino acids in norrin plus six histidine residues at its carboxyl terminus were pulse-labeled with [35S]cysteine, and the labeled norrin was examined in cell lysates, the medium, and the extracellular matrix. SDS-gel electrophoresis under reducing conditions showed that the norrin expressed had an apparent Mr = 14,000 and was present only in cell lysates and the extracellular matrix. Under nonreducing conditions, most of the norrin in the extracellular matrix was oligomers that contained up to approximately 20 monomers. One of the major extracellular species of norrin under reducing conditions after cross-linking of norrin oligomers with bis(sulfosuccinimidyl)suberate had an apparent Mr = 28,000, consistent with covalent cross-linked dimers. Thus the covalently cross-linked dimers are key structural components of norrin oligomers. By site-directed mutagenesis, the codon for half-cystine 95 in norrin was changed to one encoding alanine. The norrin C95A found in the extracellular matrix of cells transfected with this mutant was the size of dimers, indicating that half-cystine 95 is involved in oligomer formation. The corresponding half-cystine residue in human prepro-von Willebrand factor is also involved in interchain disulfide bond formation, which is consistent with the sequence identity of the half-cystine residues in norrin and part of the half-cystine residues in a disulfide-rich domain of von Willebrand factor. Replacement of valine at residue 60 in norrin by glutamic acid, a mutation found in humans with a severe type of Norrie disease, results in a considerable reduction (50%) in the amount of norrin in the extracellular matrix of transfected COS-7 cells. Replacement of arginine at residue 121 by glutamine, which is associated with a less severe type of Norrie disease, results in a reduction in the amount of norrin R121Q in the extracellular matrix (26%). These studies suggest that norrin is a secreted protein that forms disulfide-bonded oligomers that are associated with the extracellular matrix upon secretion from cells. Moreover, the disulfide-rich motif of norrin and prepro-von Willebrand factor promotes interchain disulfide bond formation among polypeptides in which it is found.
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Affiliation(s)
- J Perez-Vilar
- Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, USA
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Eckhardt AE, Timpte CS, DeLuca AW, Hill RL. The complete cDNA sequence and structural polymorphism of the polypeptide chain of porcine submaxillary mucin. J Biol Chem 1997; 272:33204-10. [PMID: 9407109 DOI: 10.1074/jbc.272.52.33204] [Citation(s) in RCA: 45] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
The complete structure of the DNA encoding the polypeptide chain of porcine submaxillary mucin has been determined. The polypeptide is composed of distinct domains. A large central domain containing tandem repeats of 81 residues each is flanked by much shorter domains with sequences similar to the tandem repeats. Four disulfide-rich domains, three at the amino terminus and one at the carboxyl terminus, complete the chain. The disulfide-rich domains have significant sequence identity to those of other mucins and prepro-von Willebrand factor. The coding region of the mucin gene is highly polymorphic, and three alleles were identified in a single animal that encoded different numbers of the 81-residue tandem repeats. A single large exon devoid of introns encodes the tandem repeat domains. The largest allele with 135 tandem repeats encoded 13,288 amino acids to give a polypeptide with Mr = 1,184,106. The other two alleles contained 99 and 125 tandem repeats, respectively. Each allele also showed different restriction fragment length polymorphisms, which is consistent with the different patterns seen in individual animals. Fragment length polymorphism was also seen within two different families of animals, indicating that the polymorphism observed occurs in a single generation.
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Affiliation(s)
- A E Eckhardt
- Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710, USA
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