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Abstract
The brush border on the apical surface of enterocytes is a highly specialized structure well-adapted for efficient digestion and nutrient transport, whilst at the same time providing a protective barrier for the intestinal mucosa. The brush border is constituted of a densely ordered array of microvilli, protrusions of the plasma membrane, which are supported by actin-based microfilaments and interacting proteins and anchored in an apical network of actomyosin and intermediate filaments, the so-called terminal web. The highly dynamic, specialized apical domain is both an essential partner for the gut microbiota and an efficient signalling platform that enables adaptation to physiological stimuli from the external and internal milieu. Nevertheless, genetic alterations or various pathological stresses, such as infection, inflammation, and mechanical or nutritional alterations, can jeopardize this equilibrium and compromise intestinal functions. Long-time neglected, the intestinal brush-border shall be enlightening again as the central actor of the complex but essential intestinal homeostasis. Here, we review the processes and components involved in brush border organization and discuss pathological mechanisms that can induce brush border defects and their physiological consequences.
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Skau CT, Waterman CM. Specification of Architecture and Function of Actin Structures by Actin Nucleation Factors. Annu Rev Biophys 2016; 44:285-310. [PMID: 26098516 DOI: 10.1146/annurev-biophys-060414-034308] [Citation(s) in RCA: 75] [Impact Index Per Article: 9.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
The actin cytoskeleton is essential for diverse processes in mammalian cells; these processes range from establishing cell polarity to powering cell migration to driving cytokinesis to positioning intracellular organelles. How these many functions are carried out in a spatiotemporally regulated manner in a single cytoplasm has been the subject of much study in the cytoskeleton field. Recent work has identified a host of actin nucleation factors that can build architecturally diverse actin structures. The biochemical properties of these factors, coupled with their cellular location, likely define the functional properties of actin structures. In this article, we describe how recent advances in cell biology and biochemistry have begun to elucidate the role of individual actin nucleation factors in generating distinct cellular structures. We also consider how the localization and orientation of actin nucleation factors, in addition to their kinetic properties, are critical to their ability to build a functional actin cytoskeleton.
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Affiliation(s)
- Colleen T Skau
- Cell Biology and Physiology Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892; ,
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3
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Subunits of the Drosophila actin-capping protein heterodimer regulate each other at multiple levels. PLoS One 2014; 9:e96326. [PMID: 24788460 PMCID: PMC4008575 DOI: 10.1371/journal.pone.0096326] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2014] [Accepted: 04/07/2014] [Indexed: 01/23/2023] Open
Abstract
The actin-Capping Protein heterodimer, composed of the α and β subunits, is a master F-actin regulator. In addition to its role in many cellular processes, Capping Protein acts as a main tumor suppressor module in Drosophila and in humans, in part, by restricting the activity of Yorkie/YAP/TAZ oncogenes. We aimed in this report to understand how both subunits regulate each other in vivo. We show that the levels and capping activities of both subunits must be tightly regulated to control F-actin levels and consequently growth of the Drosophila wing. Overexpressing capping protein α and β decreases both F-actin levels and tissue growth, while expressing forms of Capping Protein that have dominant negative effects on F-actin promote tissue growth. Both subunits regulate each other's protein levels. In addition, overexpressing one of the subunit in tissues knocked-down for the other increases the mRNA and protein levels of the subunit knocked-down and compensates for its loss. We propose that the ability of the α and β subunits to control each other's levels assures that a pool of functional heterodimer is produced in sufficient quantities to restrict the development of tumor but not in excess to sustain normal tissue growth.
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Avenarius MR, Saylor KW, Lundeberg MR, Wilmarth PA, Shin JB, Spinelli KJ, Pagana JM, Andrade L, Kachar B, Choi D, David LL, Barr-Gillespie PG. Correlation of actin crosslinker and capper expression levels with stereocilia growth phases. Mol Cell Proteomics 2013; 13:606-20. [PMID: 24319057 DOI: 10.1074/mcp.m113.033704] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
During development of the chick cochlea, actin crosslinkers and barbed-end cappers presumably influence growth and remodeling of the actin paracrystal of hair cell stereocilia. We used mass spectrometry to identify and quantify major actin-associated proteins of the cochlear sensory epithelium from E14 to E21, when stereocilia widen and lengthen. Tight actin crosslinkers (i.e. fascins, plastins, and espin) are expressed dynamically during cochlear epithelium development between E7 and E21, with FSCN2 replacing FSCN1 and plastins remaining low in abundance. Capping protein, a barbed-end actin capper, is located at stereocilia tips; it is abundant during growth phase II, when stereocilia have ceased elongating and are increasing in diameter. Capping protein levels then decline during growth phase III, when stereocilia reinitiate barbed-end elongation. Although actin crosslinkers are readily detected by electron microscopy in developing chick cochlea stereocilia, quantitative mass spectrometry of stereocilia isolated from E21 chick cochlea indicated that tight crosslinkers are present there in stoichiometric ratios relative to actin that are much lower than their ratios for vestibular stereocilia. These results demonstrate the value of quantitation of global protein expression in chick cochlea during stereocilia development.
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Affiliation(s)
- Matthew R Avenarius
- Oregon Hearing Research Center and Vollum Institute, Oregon Health and Science University, Portland, Oregon 97239
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Eps8 controls dendritic spine density and synaptic plasticity through its actin-capping activity. EMBO J 2013; 32:1730-44. [PMID: 23685357 DOI: 10.1038/emboj.2013.107] [Citation(s) in RCA: 48] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2013] [Accepted: 04/15/2013] [Indexed: 12/13/2022] Open
Abstract
Actin-based remodelling underlies spine structural changes occurring during synaptic plasticity, the process that constantly reshapes the circuitry of the adult brain in response to external stimuli, leading to learning and memory formation. A positive correlation exists between spine shape and synaptic strength and, consistently, abnormalities in spine number and morphology have been described in a number of neurological disorders. In the present study, we demonstrate that the actin-regulating protein, Eps8, is recruited to the spine head during chemically induced long-term potentiation in culture and that inhibition of its actin-capping activity impairs spine enlargement and plasticity. Accordingly, mice lacking Eps8 display immature spines, which are unable to undergo potentiation, and are impaired in cognitive functions. Additionally, we found that reduction in the levels of Eps8 occurs in brains of patients affected by autism compared to controls. Our data reveal the key role of Eps8 actin-capping activity in spine morphogenesis and plasticity and indicate that reductions in actin-capping proteins may characterize forms of intellectual disabilities associated with spine defects.
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Drosophila actin-Capping Protein limits JNK activation by the Src proto-oncogene. Oncogene 2013; 33:2027-39. [PMID: 23644660 DOI: 10.1038/onc.2013.155] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2012] [Revised: 03/27/2013] [Accepted: 03/28/2013] [Indexed: 12/17/2022]
Abstract
The Src family kinases c-Src, and its downstream effectors, the Rho family of small GTPases RhoA and Jun N-terminal kinase (JNK) have a significant role in tumorigenesis. In this report, using the Drosophila wing disc epithelium as a model system, we demonstrate that the actin-Capping Protein (CP) αβ heterodimer, which regulates actin filament (F-actin) polymerization, limits Src-induced apoptosis or tissue overgrowth by restricting JNK activation. We show that overexpressing Src64B drives JNK-independent loss of epithelial integrity and JNK-dependent apoptosis via Btk29A, p120ctn and Rho1. However, when cells are kept alive with the Caspase inhibitor P35, JNK acts as a potent inducer of proliferation via activation of the Yorkie oncogene. Reducing CP levels direct apoptosis of overgrowing Src64B-overexpressing tissues. Conversely, overexpressing capping protein inhibits Src64B and Rho1, but not Rac1-induced JNK signaling. CP requires the actin-binding domain of the α-subunit to limit Src64B-induced apoptosis, arguing that the control of F-actin mediates this effect. In turn, JNK directs F-actin accumulation. Moreover, overexpressing capping protein also prevents apoptosis induced by ectopic JNK expression. Our data are consistent with a model in which the control of F-actin by CP limits Src-induced apoptosis or tissue overgrowth by acting downstream of Btk29A, p120ctn and Rho1, but upstream of JNK. In turn, JNK may counteract the effect of CP on F-actin, providing a positive feedback, which amplifies JNK activation. We propose that cytoskeletal changes triggered by misregulation of F-actin modulators may have a significant role in Src-mediated malignant phenotypes during the early stages of cellular transformation.
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Yang FH, Pyle WG. Reduced cardiac CapZ protein protects hearts against acute ischemia–reperfusion injury and enhances preconditioning. J Mol Cell Cardiol 2012; 52:761-72. [DOI: 10.1016/j.yjmcc.2011.11.013] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/24/2011] [Revised: 11/17/2011] [Accepted: 11/21/2011] [Indexed: 10/14/2022]
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8
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Lukman S, Robinson RC, Wales D, Verma CS. Conformational dynamics of capping protein and interaction partners: Simulation studies. Proteins 2012; 80:1066-77. [DOI: 10.1002/prot.24008] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2011] [Revised: 11/11/2011] [Accepted: 11/18/2011] [Indexed: 11/09/2022]
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Bernadskaya YY, Patel FB, Hsu HT, Soto MC. Arp2/3 promotes junction formation and maintenance in the Caenorhabditis elegans intestine by regulating membrane association of apical proteins. Mol Biol Cell 2011; 22:2886-99. [PMID: 21697505 PMCID: PMC3154884 DOI: 10.1091/mbc.e10-10-0862] [Citation(s) in RCA: 40] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
It has been proposed that Arp2/3, which promotes nucleation of branched actin, is needed for epithelial junction initiation but is less important as junctions mature. We focus here on how Arp2/3 contributes to the Caenorhabditis elegans intestinal epithelium and find important roles for Arp2/3 in the maturation and maintenance of junctions in embryos and adults. Electron microscope studies show that embryos depleted of Arp2/3 form apical actin-rich microvilli and electron-dense apical junctions. However, whereas apical/basal polarity initiates, apical maturation is defective, including decreased apical F-actin enrichment, aberrant lumen morphology, and reduced accumulation of some apical junctional proteins, including DLG-1. Depletion of Arp2/3 in adult animals leads to similar intestinal defects. The DLG-1/AJM-1 apical junction proteins, and the ezrin-radixin-moesin homologue ERM-1, a protein that connects F-actin to membranes, are required along with Arp2/3 for apical F-actin enrichment in embryos, whereas cadherin junction proteins are not. Arp2/3 affects the subcellular distribution of DLG-1 and ERM-1. Loss of Arp2/3 shifts both ERM-1 and DLG-1 from pellet fractions to supernatant fractions, suggesting a role for Arp2/3 in the distribution of membrane-associated proteins. Thus, Arp2/3 is required as junctions mature to maintain apical proteins associated with the correct membranes.
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Affiliation(s)
- Yelena Y Bernadskaya
- Department of Pathology and Laboratory Medicine, University of Medicine and Dentistry of New Jersey-Robert Wood Johnson Medical School, Piscataway, NJ 08854, USA
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Janody F, Treisman JE. Actin capping protein alpha maintains vestigial-expressing cells within the Drosophila wing disc epithelium. Development 2006; 133:3349-57. [PMID: 16887822 PMCID: PMC1544359 DOI: 10.1242/dev.02511] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Tissue patterning must be translated into morphogenesis through cell shape changes mediated by remodeling of the actin cytoskeleton. We have found that Capping protein alpha (Cpa) and Capping protein beta (Cpb), which prevent extension of the barbed ends of actin filaments, are specifically required in the wing blade primordium of the Drosophila wing disc. cpa or cpb mutant cells in this region, but not in the remainder of the wing disc, are extruded from the epithelium and undergo apoptosis. Excessive actin filament polymerization is not sufficient to explain this phenotype, as loss of Cofilin or Cyclase-associated protein does not cause cell extrusion or death. Misexpression of Vestigial, the transcription factor that specifies the wing blade, both increases cpa transcription and makes cells dependent on cpa for their maintenance in the epithelium. Our results suggest that Vestigial specifies the cytoskeletal changes that lead to morphogenesis of the adult wing.
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Affiliation(s)
- Florence Janody
- Developmental Biology Institute of Marseille Luminy, UMR6216 - Case 907, Parc Scientific de Luminy, 13288 Marseille Cedex 09, France.
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11
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Abstract
Temporal and spatial control of the actin cytoskeleton are crucial for a range of eukaryotic cellular processes. Capping protein (CP), a ubiquitous highly conserved heterodimer, tightly caps the barbed (fast-growing) end of the actin filament and is an important component in the assembly of various actin structures, including the dynamic branched filament network at the leading edge of motile cells. New research into the molecular mechanism of how CP interacts with the actin filament in vitro and the function of CP in vivo, including discoveries of novel interactions of CP with other proteins, has greatly enhanced our understanding of the role of CP in regulating the actin cytoskeleton.
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Affiliation(s)
- Martin A Wear
- Department of Cell Biology and Physiology, Washington University School of Medicine, St Louis, MI 63110, USA
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Mejillano MR, Kojima SI, Applewhite DA, Gertler FB, Svitkina TM, Borisy GG. Lamellipodial versus filopodial mode of the actin nanomachinery: pivotal role of the filament barbed end. Cell 2004; 118:363-73. [PMID: 15294161 DOI: 10.1016/j.cell.2004.07.019] [Citation(s) in RCA: 296] [Impact Index Per Article: 14.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2003] [Revised: 06/14/2004] [Accepted: 06/18/2004] [Indexed: 10/26/2022]
Abstract
Understanding how a particular cell type expresses the lamellipodial or filopodial form of the actin machinery is essential to understanding a cell's functional interactions. To determine how a cell "chooses" among these alternative modes of "molecular hardware," we tested the role of key proteins that affect actin filament barbed ends. Depletion of capping protein (CP) by short hairpin RNA (shRNA) caused loss of lamellipodia and explosive formation of filopodia. The knockdown phenotype was rescued by a CP mutant refractory to shRNA, but not by another barbed-end capper, gelsolin, demonstrating that the phenotype was specific for CP. In Ena/VASP deficient cells, CP depletion resulted in ruffling instead of filopodia. We propose a model for selection of lamellipodial versus filopodial organization in which CP is a negative regulator of filopodia formation and Ena/VASP has recruiting/activating functions downstream of actin filament elongation in addition to its previously suggested anticapping and antibranching activities.
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Affiliation(s)
- Marisan R Mejillano
- Department of Cell and Molecular Biology, Feinberg School of Medicine, Northwestern University, 303 E. Chicago Avenue, Chicago, IL 60611, USA
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Potter DA, Srirangam A, Fiacco KA, Brocks D, Hawes J, Herndon C, Maki M, Acheson D, Herman IM. Calpain regulates enterocyte brush border actin assembly and pathogenic Escherichia coli-mediated effacement. J Biol Chem 2003; 278:30403-12. [PMID: 12764139 PMCID: PMC2727654 DOI: 10.1074/jbc.m304616200] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
This study identifies calpain as being instrumental for brush border (BB) microvillus assembly during differentiation and effacement during bacterial pathogenesis. Calpain activity is decreased by 25-80% in Caco 2 lines stably overexpressing calpastatin, the physiological inhibitor of calpain, and the effect is proportional to the calpastatin/calpain ratio. These lines exhibit a 2.5-fold reduction in the rate of microvillus extension. Apical microvillus assembly is reduced by up to 50%, as measured by quantitative fluorometric microscopy (QFM) of ezrin, indicating that calpain recruits ezrin to BB microvilli. Calpain inhibitors ZLLYCHN2, MDL 28170, and PD 150606 block BB assembly and ezrin recruitment to the BB. The HIV protease inhibitor ritonavir, which inhibits calpain at clinically relevant concentrations, also blocks BB assembly, whereas cathepsin and proteasome inhibitors do not. Microvillus effacement is inhibited after exposure of calpastatin-overexpressing cells to enteropathogenic Escherichia coli. These results suggest that calpain regulates BB assembly as well as pathological effacement, and indicate that it is an important regulator involved in HIV protease inhibitor toxicity and host-microbial pathogen interactions.
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Affiliation(s)
- David A. Potter
- Division of Hematology/Oncology, Walther Oncology Center and Veterans Affairs Medical Center, Indiana University, Indianapolis, Indiana 46202
- Department of Biochemistry and Molecular Biology, Indiana University, Indianapolis, Indiana 46202
- To whom correspondence may be addressed: Dept. of Medicine, Indiana University School of Medicine, 1044 W. Walnut St., Indianapolis, IN 46202-5254. Tel.: 317-274-2221; Fax: 317-274-0396; E-mail:
| | - Anjaiah Srirangam
- Division of Hematology/Oncology, Walther Oncology Center and Veterans Affairs Medical Center, Indiana University, Indianapolis, Indiana 46202
| | - Kerry A. Fiacco
- Division of Hematology/Oncology, Walther Oncology Center and Veterans Affairs Medical Center, Indiana University, Indianapolis, Indiana 46202
| | - Daniel Brocks
- Division of Hematology/Oncology, Walther Oncology Center and Veterans Affairs Medical Center, Indiana University, Indianapolis, Indiana 46202
| | - John Hawes
- Department of Biochemistry and Molecular Biology, Indiana University, Indianapolis, Indiana 46202
| | - Carter Herndon
- Department of Biochemistry and Molecular Biology, Indiana University, Indianapolis, Indiana 46202
| | - Masatoshi Maki
- Department of Molecular Applied Biosciences, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Japan 464-01
| | - David Acheson
- Department of Public Health, University of Maryland, Baltimore, Maryland 21201
| | - Ira M. Herman
- Department of Physiology, Tufts University School of Medicine, Boston, Massachusetts 02111
- To whom correspondence may be addressed: Dept. of Physiology, Tufts University School of Medicine, 136 Harrison Ave., Boston, MA 02111. Tel.: 617-636-2991; Fax: 617-636-0445; E-mail:
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Yamashita A, Maeda K, Maéda Y. Crystal structure of CapZ: structural basis for actin filament barbed end capping. EMBO J 2003; 22:1529-38. [PMID: 12660160 PMCID: PMC152911 DOI: 10.1093/emboj/cdg167] [Citation(s) in RCA: 128] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
Capping protein, a heterodimeric protein composed of alpha and beta subunits, is a key cellular component regulating actin filament assembly and organization. It binds to the barbed ends of the filaments and works as a 'cap' by preventing the addition and loss of actin monomers at the end. Here we describe the crystal structure of the chicken sarcomeric capping protein CapZ at 2.1 A resolution. The structure shows a striking resemblance between the alpha and beta subunits, so that the entire molecule has a pseudo 2-fold rotational symmetry. CapZ has a pair of mobile extensions for actin binding, one of which also provides concomitant binding to another protein for the actin filament targeting. The mobile extensions probably form flexible links to the end of the actin filament with a pseudo 2(1) helical symmetry, enabling the docking of the two in a symmetry mismatch.
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Affiliation(s)
- Atsuko Yamashita
- Laboratory for Structural Biochemistry, RIKEN Harima Institute at SPring-8, 1-1-1 Kouto, Mikazuki, Sayo, Hyogo, 679-5148, Japan.
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Schott EJ, Vasta GR. The PmSOD1 gene of the protistan parasite Perkinsus marinus complements the sod2Delta mutant of Saccharomyces cerevisiae, and directs an iron superoxide dismutase to mitochondria. Mol Biochem Parasitol 2003; 126:81-92. [PMID: 12554087 DOI: 10.1016/s0166-6851(02)00271-2] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The facultative intracellular oyster parasite, Perkinsus marinus, taxonomically related to both dinoflagellates and apicomplexan parasites, possesses at least two distinct genes (PmSOD1 and PmSOD2) predicted to encode iron-containing superoxide dismutases (Fe-SOD). The present study demonstrates that PmSOD1 complements a Saccharomyces cerevisiae mutant lacking the mitochondrial manganese-containing SOD (Mn-SOD), whereas PmSOD2 complements an Escherichia coli mutant lacking genes for cytosolic SOD activities. Mitochondria isolated from complemented yeast contain an SOD activity susceptible to inhibition by hydrogen peroxide, but resistant to cyanide, both characteristics of Fe-SODs. In cultured P. marinus trophozoites, indirect immunofluorescence using anti-PmSOD1 antibodies shows colocalization of PmSOD1 product with the mitochondrial marker MitoTracker Red. Further analysis of the leader sequence of the predicted PmSOD1 product revealed similarities to a mitochondrial targeting domain, an unusual observation for Fe-SODs, which are typically localized in the cytoplasm. These results suggest that PmSOD1 encodes a mitochondrial Fe-SOD, which may contribute to P. marinus resistance to exogenous oxidative damage in host phagocytes. The present study constitutes the first report of an endogenous Fe-SOD that is directed to the mitochondria, and suggests that mitochondria targeting sequences have been conserved among diverse branches of the eukaryotes, including the early protista. It also illustrates the potential of complementation-based approaches for further gene discovery and characterization in P. marinus.
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Affiliation(s)
- Eric J Schott
- Center of Marine Biotechnology, University of Maryland Biotechnology Institute, 701 East Pratt Street, Baltimore 21202, USA
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Pyle WG, Hart MC, Cooper JA, Sumandea MP, de Tombe PP, Solaro RJ. Actin capping protein: an essential element in protein kinase signaling to the myofilaments. Circ Res 2002; 90:1299-306. [PMID: 12089068 DOI: 10.1161/01.res.0000024389.03152.22] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Actin capping protein (CapZ) binds the barbed ends of actin at sarcomeric Z-lines. In addition to anchoring actin, Z-discs bind protein kinase C (PKC). Although CapZ is crucial for myofibrillogenesis, its role in muscle function and intracellular signaling is unknown. We hypothesized that CapZ downregulation would impair myocardial function and disrupt PKC-myofilament signaling by impairing PKC-Z-disc interaction. To test these hypotheses, we examined transgenic (TG) mice in which cardiac CapZ protein is reduced. Fiber bundles were dissected from papillary muscles and detergent extracted. Some fiber bundles were treated with PKC activators phenylephrine (PHE) or endothelin (ET) before detergent extraction. We simultaneously measured Ca2+-dependent tension and actomyosin MgATPase activity. CapZ downregulation increased myofilament Ca2+ sensitivity without affecting maximum tension or actomyosin MgATPase activity. Maximum tension and actomyosin MgATPase activity were decreased after PHE or ET treatment of wild-type (WT) muscle. Fiber bundles from TG hearts did not respond to PHE or ET. Immunoblot analysis revealed an increase in myofilament-associated PKC-epsilon after PHE or ET exposure of WT preparations. In contrast, myofilament-associated PKC-epsilon was decreased after PHE or ET treatment in TG myocardium. Protein levels of myofilament-associated PKC-beta were decreased in TG ventricle. C-protein and troponin I phosphorylation was increased after PHE or ET treatment in WT and TG hearts. Basal phosphorylation levels of C-protein and troponin I were higher in TG myocardium. These results indicate that downregulation of CapZ, or other changes associated with CapZ downregulation, increases cardiac myofilament Ca2+ sensitivity, inhibits PKC-mediated control of myofilament activation, and decreases myofilament-associated PKC-beta.
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Affiliation(s)
- W Glen Pyle
- Department of Physiology and Biophysics, University of Illinois at Chicago, Chicago, Ill 60612, USA
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Cameron LA, Giardini PA, Soo FS, Theriot JA. Secrets of actin-based motility revealed by a bacterial pathogen. Nat Rev Mol Cell Biol 2000; 1:110-9. [PMID: 11253363 DOI: 10.1038/35040061] [Citation(s) in RCA: 148] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
Actin-based cell motility is a complex process involving a dynamic, self-organizing cellular system. Experimental problems initially limited our understanding of this type of motility, but the use of a model system derived from a bacterial pathogen has led to a breakthrough. Now, all the molecular components necessary for dynamic actin self-organization and motility have been identified, setting the stage for future mechanistic studies.
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Affiliation(s)
- L A Cameron
- Department of Biochemistry, Stanford University School of Medicine, 279 Campus Drive West, Stanford, California 94305-5307, USA
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18
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Abstract
Actin capping protein (CP) binds barbed ends of actin filaments to regulate actin assembly. CP is an alpha/beta heterodimer. Vertebrates have conserved isoforms of each subunit. Muscle cells contain two beta isoforms. beta1 is at the Z-line; beta2 is at the intercalated disc and cell periphery in general. To investigate the functions of the isoforms, we replaced one isoform with another using expression in hearts of transgenic mice. Mice expressing beta2 had a severe phenotype with juvenile lethality. Myofibril architecture was severely disrupted. The beta2 did not localize to the Z-line. Therefore, beta1 has a distinct function that includes interactions at the Z-line. Mice expressing beta1 showed altered morphology of the intercalated disc, without the lethality or myofibril disruption of the beta2-expressing mice. The in vivo function of CP is presumed to involve binding barbed ends of actin filaments. To test this hypothesis, we expressed a beta1 mutant that poorly binds actin. These mice showed both myofibril disruption and intercalated disc remodeling, as predicted. Therefore, CPbeta1 and CPbeta2 each have a distinct function that cannot be provided by the other isoform. CPbeta1 attaches actin filaments to the Z-line, and CPbeta2 organizes the actin at the intercalated discs.
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Affiliation(s)
- Marilyn C. Hart
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110
| | - John A. Cooper
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110
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Tardieux I, Baines I, Mossakowska M, Ward GE. Actin-binding proteins of invasive malaria parasites and the regulation of actin polymerization by a complex of 32/34-kDa proteins associated with heat shock protein 70kDa. Mol Biochem Parasitol 1998; 93:295-308. [PMID: 9662713 DOI: 10.1016/s0166-6851(98)00044-9] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
Movement of the malaria parasite into a host erythrocyte during invasion is thought to involve polymerization of parasite actin. We have used F-actin affinity chromatography to isolate actin-binding proteins from Plasmodium knowlesi merozoites, in an attempt to identify proteins responsible for regulating parasite actin polymerization during invasion. Five major proteins, of molecular masses 75, 70, 48, 40 and 34 kDa, were reproducibly eluted from the F-actin columns. The 70 kDa actin-binding protein was identified by tryptic peptide microsequencing as heat shock protein-70 kDa (HSC70); this identification was confirmed by Western blotting with anti-HSC70 antibody, and binding of the protein to ATP-agarose. A doublet of 32/34-kDa proteins coeluted with parasite HSC70 from the F-actin and ATP-agarose columns; a complex of these three proteins was also observed by gel filtration chromatography Highly enriched fractions containing the Plasmodium HSC70/32/34 complex inhibited the polymerization of rabbit skeletal muscle actin, in vitro. This capping activity was calcium-independent, and abrogated by phosphatidylinositol 4,5-bisphosphate. The average length of the actin filaments polymerized in presence of the HSC70/32/34-kDa complex was significantly shorter than in the absence of the complex, consistent with a capping activity. The capping or uncapping of actin filament ends by the HSC70/32/34-kDa complex during invasion could provide a mechanism for localized actin filament growth and movement of the parasite into the host cell.
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Affiliation(s)
- I Tardieux
- Laboratory of Parasitic Diseases, NIAID, NIH, Bethesda 20892, USA.
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Barkalow K, Witke W, Kwiatkowski DJ, Hartwig JH. Coordinated regulation of platelet actin filament barbed ends by gelsolin and capping protein. J Biophys Biochem Cytol 1996; 134:389-99. [PMID: 8707824 PMCID: PMC2120875 DOI: 10.1083/jcb.134.2.389] [Citation(s) in RCA: 121] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Exposure of cryptic actin filament fast growing ends (barbed ends) initiates actin polymerization in stimulated human and mouse platelets. Gelsolin amplifies platelet actin assembly by severing F-actin and increasing the number of barbed ends. Actin filaments in stimulated platelets from transgenic gelsolin-null mice elongate their actin without severing. F-actin barbed end capping activity persists in human platelet extracts, depleted of gelsolin, and the heterodimeric capping protein (CP) accounts for this residual activity. 35% of the approximately 5 microM CP is associated with the insoluble actin cytoskeleton of the resting platelet. Since resting platelets have an F-actin barbed end concentration of approximately 0.5 microM, sufficient CP is bound to cap these ends. CP is released from OG-permeabilized platelets by treatment with phosphatidylinositol 4,5-bisphosphate or through activation of the thrombin receptor. However, the fraction of CP bound to the actin cytoskeleton of thrombin-stimulated mouse and human platelets increases rapidly to approximately 60% within 30 s. In resting platelets from transgenic mice lacking gelsolin, which have 33% more F-actin than gelsolin-positive cells, there is a corresponding increase in the amount of CP associated with the resting cytoskeleton but no change with stimulation. These findings demonstrate an interaction between the two major F-actin barbed end capping proteins of the platelet: gelsolin-dependent severing produces barbed ends that are capped by CP. Phosphatidylinositol 4,5-bisphosphate release of gelsolin and CP from platelet cytoskeleton provides a mechanism for mediating barbed end exposure. After actin assembly, CP reassociates with the new actin cytoskeleton.
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Affiliation(s)
- K Barkalow
- Division of Experimental Medicine, Brigham and Women's Hospital, Boston, Massachusetts 02115, USA
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21
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Hopmann R, Cooper JA, Miller KG. Actin organization, bristle morphology, and viability are affected by actin capping protein mutations in Drosophila. J Biophys Biochem Cytol 1996; 133:1293-305. [PMID: 8682865 PMCID: PMC2120907 DOI: 10.1083/jcb.133.6.1293] [Citation(s) in RCA: 90] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Regulation of actin filament length and orientation is important in many actin-based cellular processes. This regulation is postulated to occur through the action of actin-binding proteins. Many actin-binding proteins that modify actin in vitro have been identified, but in many cases, it is not known if this activity is physiologically relevant. Capping protein (CP) is an actin-binding protein that has been demonstrated to control filament length in vitro by binding to the barbed ends and preventing the addition or loss of actin monomers. To examine the in vivo role of CP, we have performed a molecular and genetic characterization of the beta subunit of capping protein from Drosophila melanogaster. We have identified mutations in the Drosophila beta subunit-these are the first CP mutations in a multicellular organism, and unlike CP mutations in yeast, they are lethal, causing death during the early larval stage. Adult files that are heterozygous for a pair of weak alleles have a defect in bristle morphology that is correlated to disorganized actin bundles in developing bristles. Our data demonstrate that CP has an essential function during development, and further suggest that CP is required to regulate actin assembly during the development of specialized structures that depend on actin for their morphology.
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Affiliation(s)
- R Hopmann
- Department of Biology, Washington University, St. Louis, Missouri 63130, USA.
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22
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Nachmias VT, Golla R, Casella JF, Barron-Casella E. Cap Z, a calcium insensitive capping protein in resting and activated platelets. FEBS Lett 1996; 378:258-62. [PMID: 8557113 DOI: 10.1016/0014-5793(95)01474-8] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Capping of the barbed-ends of actin filaments is an important mechanism for control of the cytoskeleton. In platelets, a valuable model system, it has been thought that gelsolin was the major capping protein. We now report that platelets contain approximately 2 microM Cap Z, a calcium insensitive heterodimeric capping protein; two major and additional minor isoforms of both alpha and beta subunits are present. In lysates from resting platelets 75-80% of the Cap Z sediments with the high speed pellet, but if the platelets are activated with thrombin for 10 s, about 15% of the Cap Z leaves the pellet fraction and is found in the high speed supernatant where it is not bound to actin. This translocation of Cap Z to the supernatant is also observed when resting platelets are lysed into buffer containing 50-100 microM GTP gamma S and 10 mM EGTA. Our results suggest that release of Cap Z from some actin filaments could generate free filament barbed-ends. An increase in free barbed-ends has been reported in platelet lysates prepared shortly after thrombin activation.
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Affiliation(s)
- V T Nachmias
- Department of Cell and Developmental Biology, University of Pennsylvania School of Medicine, Philadelphia 19104-6058, USA
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23
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Sizonenko GI, Karpova TS, Gattermeir DJ, Cooper JA. Mutational analysis of capping protein function in Saccharomyces cerevisiae. Mol Biol Cell 1996; 7:1-15. [PMID: 8741835 PMCID: PMC278608 DOI: 10.1091/mbc.7.1.1] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
To investigate physiologic functions and structural correlates for actin capping protein (CP), we analyzed site-directed mutations in CAP1 and CAP2, which encode the alpha and beta subunits of CP in Saccharomyces cerevisiae. Mutations in four different regions caused a loss of CP function in vivo despite the presence of mutant protein in the cells. Mutations in three regions caused a complete loss of all aspects of function, including the actin distribution, viability with sac6, and localization of CP to actin cortical patches. Mutation of the fourth region led to partial loss of only one function-formation of actin cables. Some mutations retained function and exhibited the complete wild-type phenotype, and some mutations led to a complete loss of protein and therefore loss of function. The simplest hypothesis that can explain these results is that a single biochemical property is necessary for all in vivo functions. This biochemical property is most likely binding to actin filaments, because the nonfunctional mutant CPs no longer co-localize with actin filaments in vivo and because direct binding of CP to actin filaments has been well established by studies with purified proteins in vitro. More complex hypotheses, involving the existence of additional biochemical properties important for function, cannot be excluded by this analysis.
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Affiliation(s)
- G I Sizonenko
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA
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24
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The cytoskeleton of the intestinal epithelium. CYTOSKELETON IN SPECIALIZED TISSUES AND IN PATHOLOGICAL STATES 1996. [DOI: 10.1016/s1874-6020(96)80015-2] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
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25
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Karpova TS, Tatchell K, Cooper JA. Actin filaments in yeast are unstable in the absence of capping protein or fimbrin. J Cell Biol 1995; 131:1483-93. [PMID: 8522605 PMCID: PMC2120666 DOI: 10.1083/jcb.131.6.1483] [Citation(s) in RCA: 68] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
Many actin-binding proteins affect filament assembly in vitro and localize with actin in vivo, but how their molecular actions contribute to filament assembly in vivo is not understood well. We report here that capping protein (CP) and fimbrin are both important for actin filament assembly in vivo in Saccharomyces cerevisiae, based on finding decreased actin filament assembly in CP and fimbrin mutants. We have also identified mutations in actin that enhance the CP phenotype and find that those mutants also have decreased actin filament assembly in vivo. In vitro, actin purified from some of these mutants is defective in polymerization or binding fimbrin. These findings support the conclusion that CP acts to stabilize actin filaments in vivo. This conclusion is particularly remarkable because it is the opposite of the conclusion drawn from recent studies in Dictyostelium (Hug, C., P.Y. Jay, I. Reddy, J.G. McNally, P.C. Bridgman, E.L. Elson, and J.A. Cooper. 1995. Cell. 81:591-600). In addition, we find that the unpolymerized pool of actin in yeast is very small relative to that found in higher cells, which suggests that actin filament assembly is less dynamic in yeast than higher cells.
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Affiliation(s)
- T S Karpova
- Department of Cell Biology and Physiology, Washington University Medical School, St. Louis, Missouri, USA
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26
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Witke W, Sharpe AH, Hartwig JH, Azuma T, Stossel TP, Kwiatkowski DJ. Hemostatic, inflammatory, and fibroblast responses are blunted in mice lacking gelsolin. Cell 1995; 81:41-51. [PMID: 7720072 DOI: 10.1016/0092-8674(95)90369-0] [Citation(s) in RCA: 351] [Impact Index Per Article: 12.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Gelsolin, an 82 kDa actin-binding protein, has potent actin filament-severing activity in vitro. To investigate the in vivo function of gelsolin, transgenic gelsolin-null (Gsn-) mice were generated and found to have normal embryonic development and longevity. However, platelet shape changes are decreased in Gsn- mice, causing prolonged bleeding times. Neutrophil migration in vivo into peritoneal exudates and in vitro is delayed. Gsn- dermal fibroblasts have excessive actin stress fibers and migrate more slowly than wild-type fibroblasts, but have increased contractility in vitro. These observations establish the requirement of gelsolin for rapid motile responses in cell types involved in stress responses such as hemostasis, inflammation, and wound healing. Neither gelsolin nor other proteins with similar actin filament-severing activity are expressed in early embryonic cells, indicating that this mechanism of actin filament dynamics is not essential for motility during early embryogenesis.
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Affiliation(s)
- W Witke
- Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115, USA
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27
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Schafer DA, Hug C, Cooper JA. Inhibition of CapZ during myofibrillogenesis alters assembly of actin filaments. J Cell Biol 1995; 128:61-70. [PMID: 7822423 PMCID: PMC2120327 DOI: 10.1083/jcb.128.1.61] [Citation(s) in RCA: 109] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
The actin filaments of myofibrils are highly organized; they are of a uniform length and polarity and are situated in the sarcomere in an aligned array. We hypothesized that the barbed-end actin-binding protein, CapZ, directs the process of actin filament assembly during myofibrillogenesis. We tested this hypothesis by inhibiting the actin-binding activity of CapZ in developing myotubes in culture using two different methods. First, injection of a monoclonal antibody that prevents the interaction of CapZ and actin disrupts the non-striated bundles of actin filaments formed during the early stages of myofibril formation in skeletal myotubes in culture. The antibody, when injected at concentrations lower than that required for disrupting the actin filaments, binds at nascent Z-disks. Since the interaction of CapZ and the monoclonal antibody are mutually exclusive, this result indicates that CapZ binds nascent Z-disks independent of an interaction with actin filaments. In a second approach, expression in myotubes of a mutant form of CapZ that does not bind actin results in a delay in the appearance of actin in a striated pattern in myofibrils. The organization of alpha-actinin at Z-disks also is delayed, but the organization of titin and myosin in sarcomeres is not significantly altered. We conclude that the interaction of CapZ and actin is important for the organization of actin filaments of the sarcomere.
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Affiliation(s)
- D A Schafer
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110
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28
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Heintzelman MB, Hasson T, Mooseker MS. Multiple unconventional myosin domains of the intestinal brush border cytoskeleton. J Cell Sci 1994; 107 ( Pt 12):3535-43. [PMID: 7706404 DOI: 10.1242/jcs.107.12.3535] [Citation(s) in RCA: 76] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Representatives of class V and class VI unconventional myosins are identified as components of the intestinal brush border cytoskeleton. With brush border myosin-I and myosin-II, this brings to four the number of myosin classes associated with this one subcellular domain and represents the first characterization of four classes of myosins expressed in a single metazoan cell type. The distribution and cytoskeletal association of each myosin is distinct as assessed by both biochemical fractionation and immunofluorescence localization. Myosin-VI exists in both the microvillus and terminal web although the terminal web is the predominant site of concentration. Myosin-V is present in the terminal web and, most notably, at the distal ends of the microvilli, thus becoming the first actin-binding protein to be localized to this domain as assessed by both immunohistochemical and biochemical methods. In the undifferentiated enterocytes of the intestinal crypts, myosin-VI is expressed but not yet localized to the brush border, in contrast to myosin-V, which does demonstrate an apical distribution in these cells. An assessment of myosin abundance indicates that while myosin-II is the most abundant in the cell and in the brush border, brush border myosin-I is only slightly less abundant in contrast to myosins-V and -VI, both of which are two orders of magnitude less abundant than the others. Extraction studies indicate that of these four myosins, myosin-V is the most tightly associated with the brush border membrane, as detergent, in addition to ATP, is required for efficient solubilization.
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29
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Schafer DA, Korshunova YO, Schroer TA, Cooper JA. Differential localization and sequence analysis of capping protein beta-subunit isoforms of vertebrates. J Cell Biol 1994; 127:453-65. [PMID: 7929588 PMCID: PMC2120197 DOI: 10.1083/jcb.127.2.453] [Citation(s) in RCA: 118] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
Capping protein nucleates the assembly of actin filaments and stabilizes actin filaments by binding to their barbed ends. We describe here a novel isoform of the beta subunit of chicken capping protein, the beta 2 isoform, which arises by alternative splicing. The chicken beta 1 isoform and the beta 2 isoform are identical in their amino acid sequence except for a short region at the COOH terminus; this region of the beta subunit has been implicated in binding actin. Human and mouse cDNAs of the beta 1 and beta 2 isoforms also were isolated and among these vertebrates, the COOH-terminal region of each isoform is highly conserved. In contrast, comparison of the sequences of the vertebrate beta subunit COOH-termini to those of lower eukaryotes shows no similarities. The beta 2 isoform is the predominant isoform of nonmuscle tissues and the beta 1 isoform, which was first characterized in studies of capping protein from chicken muscle, is the predominant isoform of muscle tissues, as shown by immunoblots probed with isoform-specific antibodies and by RNAse protection analysis of mRNAs. The beta 2 isoform also is a component of dynactin complex from brain, which contains the actin-related protein Arp1. Both beta-subunit isoforms are expressed in cardiac muscle but they have non-overlapping subcellular distributions. The beta 1 isoform is at Z-discs of myofibrils, and the beta 2 isoform is enriched at intercalated discs; in cardiac myocytes grown in culture, the beta 2 isoform also is a component of cell-cell junctions and at sites where myofibrils contact the sarcolemma. The biochemical basis for the differential distribution of capping protein isoforms is likely due to interaction with specific proteins at Z-discs and cell-cell junctions, or to preferential association with different actin isoforms. Thus, vertebrates have developed isoforms of capping protein that associate with distinct actin-filament arrays.
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Affiliation(s)
- D A Schafer
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110
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30
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Schafer DA, Gill SR, Cooper JA, Heuser JE, Schroer TA. Ultrastructural analysis of the dynactin complex: an actin-related protein is a component of a filament that resembles F-actin. J Cell Biol 1994; 126:403-12. [PMID: 7518465 PMCID: PMC2200042 DOI: 10.1083/jcb.126.2.403] [Citation(s) in RCA: 237] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
The dynactin complex visualized by deepetch electron microscopy appears as a short filament 37-nm in length, which resembles F-actin, plus a thinner, laterally oriented filament that terminates in two globular heads. The locations of several of the constituent polypeptides were identified on this structure by applying antibodies to decorate the dynactin complex before processing for electron microscopy. Antibodies to the actin-related protein Arp1 (previously referred to as actin-RPV), bound at various sites along the filament, demonstrating that this protein assembles in a polymer similar to conventional actin. Antibodies to the barbed-end actin-binding protein, capping protein, bound to one end of the filament. Thus, an actin-binding protein that binds conventional actin may also bind to Arp1 to regulate its polymerization. Antibodies to the 62-kD component of the dynactin complex also bound to one end of the filament. An antibody that binds the COOH-terminal region of the 160/150-kD dynactin polypeptides bound to the globular domains at the end of the thin lateral filament, suggesting that the dynactin polypeptide comprises at least part of the sidearm structure.
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Affiliation(s)
- D A Schafer
- Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218
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31
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Casella J, Torres M. Interaction of Cap Z with actin. The NH2-terminal domains of the alpha 1 and beta subunits are not required for actin capping, and alpha 1 beta and alpha 2 beta heterodimers bind differentially to actin. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(17)37472-0] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
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32
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Abstract
The role of profilin in the regulation of actin assembly has been reexamined. The affinity of profilin for ATP-actin appears 10-fold higher than previously thought. In the presence of ATP, the participation of the profilin-actin complex to filament elongation at the barbed end is linked to a decrease in the steady-state concentration of globular actin. This surprising effect is made possible by the involvement of the irreversible ATP hydrolysis accompanying actin polymerization. As a consequence, in the presence of thymosin beta 4 (T beta 4), low amounts of profilin promote extensive actin assembly off of the pool of actin-T beta 4 complex. When barbed ends are capped, profilin simply sequesters globular actin. A model is proposed for the function of profilin in actin-based motility.
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Affiliation(s)
- D Pantaloni
- Laboratoire d'Enzymologie Centre National de la Recherche Scientifique, Gif-sur-Yvette, France
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33
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Karpova TS, Lepetit MM, Cooper JA. Mutations that enhance the cap2 null mutant phenotype in Saccharomyces cerevisiae affect the actin cytoskeleton, morphogenesis and pattern of growth. Genetics 1993; 135:693-709. [PMID: 8293974 PMCID: PMC1205713 DOI: 10.1093/genetics/135.3.693] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
Mutations conferring synthetic lethality in combination with null mutations in CAP2, the gene encoding the beta subunit of capping protein of Saccharomyces cerevisiae, were obtained in a colony color assay. Monogenic inheritance was found for four mutations, which were attributed to three genetic loci. One mutation, sac6-69, is in the gene encoding fimbrin, another actin-binding protein, which was expected because null mutations in SAC6 and CAP2 are known to be synthetic-lethal. The other two loci were designated slc for synthetic lethality with cap2. These loci include the mutations slc1-66, slc1-87 and slc2-107. The slc mutations are semi-dominant, as shown by incomplete complementation in slc/SLC cap2/cap2 heterozygotes. The slc mutations and sac6-69 interact with each other, as shown by enhanced phenotypes in diheterozygotes. Moreover, the haploid slc2-107 sac6-69 double mutant is inviable. In a CAP2 background, the slc mutations lead to temperature and osmotic sensitivity. They alter the distribution of the actin cytoskeleton, including deficits in the presence of actin cables and the polarization of cortical actin patches. The slc mutations also lead to a pseudomycelial growth pattern. Together these results suggest that slc1 and slc2 encode components of the actin cytoskeleton in yeast and that the actin cytoskeleton can regulate the patterns of growth.
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Affiliation(s)
- T S Karpova
- Department of Cell Biology and Physiology, Washington University Medical School, St. Louis, Missouri 63110-1093
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Waddle JA, Cooper JA, Waterston RH. The alpha and beta subunits of nematode actin capping protein function in yeast. Mol Biol Cell 1993; 4:907-17. [PMID: 8257793 PMCID: PMC275721 DOI: 10.1091/mbc.4.9.907] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
We cloned and analyzed two genes, cap-1 and cap-2, which encode the alpha and beta subunits of Caenorhabditis elegans capping protein (CP). The nematode CP subunits are 55% (cap-1) and 66% (cap-2) identical to the chicken CP subunits and 32% (cap-1) and 48% (cap-2) identical to the yeast CP subunits. Purified nematode CP made by expression of both subunits in yeast is functionally similar to chicken skeletal muscle CP in two different actin polymerization assays. The abnormal cell morphology and disorganized actin cytoskeleton of yeast CP null mutants are restored to wild-type by expression of the nematode CP subunits. Expression of the nematode CP alpha or beta subunit is sufficient to restore viability to yeast cap1 sac6 or cap2 sac6 double mutants, respectively. Therefore, despite evolution of the nematode actin cytoskeleton to a state far more complex than that of yeast, one important component can function in both organisms.
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Affiliation(s)
- J A Waddle
- Department of Genetics, Washington University School of Medicine, St. Louis, Missouri 63110
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35
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Abstract
Recent research on F-actin capping proteins has concentrated on three main areas. The discovery that controlled actin polymerization is the driving force for intracellular movement suggests an important role for capping proteins in regulating filament number and length. A capping protein from Dictyostelium (related to heat-shock protein HSP70) has been characterized that is activated by external stimuli. This provides a pivotal connection between extracellular signalling, cytoskeletal reorganization and locomotory behaviour. The roles of individual actin-binding sites in the gelsolin family of severing/capping proteins and binding sites for phosphatidylinositol 4,5-bisphosphate have been identified.
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Affiliation(s)
- A Weeds
- MRC Laboratory of Molecular Biology, Cambridge, UK
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36
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Schafer DA, Waddle JA, Cooper JA. Localization of CapZ during myofibrillogenesis in cultured chicken muscle. CELL MOTILITY AND THE CYTOSKELETON 1993; 25:317-35. [PMID: 8402953 DOI: 10.1002/cm.970250403] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
Actin filaments undergo dramatic changes in their organization during myofibrillogenesis. In mature skeletal muscle, both CapZ and the barbed end of the actin filaments are located at Z-discs. In vitro, CapZ binds the barbed end of actin filaments and prevents actin subunit addition and loss; CapZ also nucleates actin polymerization in vitro. Taken together, these properties suggest that CapZ may function to organize actin filaments during myofibrillogenesis. We report here that the amount of CapZ in myofibrils from adult chicken pectoral muscle is sufficient to "cap" each actin filament of the sacromere. Double immunofluorescence microscopy of skeletal muscle cells in culture was used to determine the spatial and temporal distributions of CapZ relative to actin, alpha-actinin, titin, and myosin during myofibrillogenesis. Of particular interest was the assembly of CapZ at nascent Z-discs in relation to the organization of actin filaments in nascent myofibrils. In myoblasts and young myotubes, CapZ was diffusely distributed in the cytoplasm. As myotubes matured, CapZ was initially observed in a uniform distribution along non-striated actin filaments called stress fiber-like structures (SFLS). CapZ was observed in a periodic pattern characteristic of mature Z-discs along the SFLS prior to the appearance of a striated staining pattern for actin. In older myotubes, when actin was observed in a pattern characteristic of I-bands, CapZ was distributed in a periodic pattern characteristic of mature Z-discs. The finding that CapZ was assembled at nascent Z-discs before actin was observed in a striated pattern is consistent with the hypothesis that CapZ directs the location and polarity of actin filaments during I-band formation in skeletal muscle cells. The assembly of CapZ at nascent Z-disc structures also was observed relative to the assembly of sarcomeric alpha-actinin, titin, and thick filaments. Titin and myosin were observed in structures having the organization of mature sarcomeres prior to the appearance of CapZ at nascent Z-discs. The distribution of CapZ and sarcomeric alpha-actinin in young myotubes was not coincident; in older myotubes, both CapZ and alpha-actinin were co-localized at Z-discs. In cardiac myocytes, CapZ was detected at Z-discs and was distributed in a punctate pattern throughout the cytoplasm. CapZ also was co-localized with A-CAM and vinculin at cell-cell junctions formed by the myocytes.
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Affiliation(s)
- D A Schafer
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri
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