The transmembrane domain of the molecular chaperone Cosmc directs its localization to the endoplasmic reticulum

SLC35A2 deficiency reduces protein levels of core 1 β-1,3-galactosyltransferase 1 (C1GalT1) and its chaperone Cosmc and affects their subcellular localization

Nucleotide sugar transporters (NSTs) are multitransmembrane proteins, localized in the Golgi apparatus and/or endoplasmic reticulum, which provide glycosylation enzymes with their substrates. It has been demonstrated that NSTs may form complexes with functionally related glycosyltransferases, especially in the N-glycosylation pathway. However, potential interactions of NSTs with enzymes mediating the biosynthesis of mucin-type O-glycans have not been addressed to date. Here we report that UDP-galactose transporter (UGT; SLC35A2) associates with core 1 β-1,3-galactosyltransferase 1 (C1GalT1; T-synthase). This provides the first example of an interaction between an enzyme that acts exclusively in the O-glycosylation pathway and an NST. We also found that SLC35A2 associated with the C1GalT1-specific chaperone Cosmc, and that the endogenous Cosmc was localized in both the endoplasmic reticulum and Golgi apparatus of wild-type HEK293T cells. Furthermore, in SLC35A2-deficient cells protein levels of C1GalT1 and Cosmc were decreased and their Golgi localization was less pronounced. Finally, we identified SLC35A2 as a novel molecular target for the antifungal agent itraconazole. Based on our findings we propose that NSTs may contribute to the stabilization of their interaction partners and help them to achieve target localization in the cell, most likely by facilitating their assembly into larger functional units.

Emerging Roles of the Unique Molecular Chaperone Cosmc in the Regulation of Health and Disease

The core-1 β1-3galactosyltransferase-specific chaperone 1 (Cosmc) is a unique molecular chaperone of core-1 β1-3galactosyltransferase(C1GALT1), which typically functions inside the endoplasmic reticulum (ER). Cosmc helps C1GALT1 to fold correctly and maintain activity. It also participates in the synthesis of the T antigen, O-glycan, together with C1GALT1. Cosmc is a multifaceted molecule with a wide range of roles and functions. It involves platelet production and the regulation of immune cell function. Besides that, the loss of function of Cosmc also facilitates the development of several diseases, such as inflammation diseases, immune-mediated diseases, and cancer. It suggests that Cosmc is a critical control point in diseases and that it should be regarded as a potential target for oncotherapy. It is essential to fully comprehend Cosmc's roles, as they may provide critical information about its involvement in disease development and pathogenesis. In this review, we summarize the recent progress in understanding the role of Cosmc in normal development and diseases.

Assistance for Folding of Disease-Causing Plasma Membrane Proteins

An extensive catalog of plasma membrane (PM) protein mutations related to phenotypic diseases is associated with incorrect protein folding and/or localization. These impairments, in addition to dysfunction, frequently promote protein aggregation, which can be detrimental to cells. Here, we review PM protein processing, from protein synthesis in the endoplasmic reticulum to delivery to the PM, stressing the main repercussions of processing failures and their physiological consequences in pathologies, and we summarize the recent proposed therapeutic strategies to rescue misassembled proteins through different types of chaperones and/or small molecule drugs that safeguard protein quality control and regulate proteostasis.

"Stuck on sugars - how carbohydrates regulate cell adhesion, recognition, and signaling"

We have explored the fundamental biological processes by which complex carbohydrates expressed on cellular glycoproteins and glycolipids and in secretions of cells promote cell adhesion and signaling. We have also explored processes by which animal pathogens, such as viruses, bacteria, and parasites adhere to glycans of animal cells and initiate disease. Glycans important in cell signaling and adhesion, such as key O-glycans, are essential for proper animal development and cellular differentiation, but they are also involved in many pathogenic processes, including inflammation, tumorigenesis and metastasis, and microbial and parasitic pathogenesis. The overall hypothesis guiding these studies is that glycoconjugates are recognized and bound by a growing class of proteins called glycan-binding proteins (GBPs or lectins) expressed by all types of cells. There is an incredible variety and diversity of GBPs in animal cells involved in binding N- and O-glycans, glycosphingolipids, and proteoglycan/glycosaminoglycans. We have specifically studied such molecular determinants recognized by selectins, galectins, and many other C-type lectins, involved in leukocyte recruitment to sites of inflammation in human tissues, lymphocyte trafficking, adhesion of human viruses to human cells, structure and immunogenicity of glycoproteins on the surfaces of human parasites. We have also explored the molecular basis of glycoconjugate biosynthesis by exploring the enzymes and molecular chaperones required for correct protein glycosylation. From these studies opportunities for translational biology have arisen, involving production of function-blocking antibodies, anti-glycan specific antibodies, and synthetic glycoconjugates, e.g. glycosulfopeptides, that specifically are recognized by GBPs. This invited short review is based in part on my presentation for the IGO Award 2019 given by the International Glycoconjugate Organization in Milan.

Loss of complex O-glycosylation impairs exocrine pancreatic function and induces MODY8-like diabetes in mice

Cosmc is ubiquitously expressed and acts as a specific molecular chaperone assisting the folding and stability of core 1 synthase. Thus, it plays a crucial role in the biosynthesis of O-linked glycosylation of proteins. Here, we show that ablation of Cosmc in the exocrine pancreas of mice causes expression of truncated O-glycans (Tn antigen), resulting in exocrine pancreatic insufficiency with decreased activities of digestive enzymes and diabetes. To understand the molecular causes of the pleiotropic phenotype, we used Vicia villosa agglutinin to enrich Tn antigen-modified proteins from Cosmc-KO pancreatic lysates and performed a proteomic analysis. Interestingly, a variety of proteins were identified, of which bile salt-activated lipase (also denoted carboxyl-ester lipase, Cel) was the most abundant. In humans, frameshift mutations in CEL cause maturity-onset diabetes of the young type 8 (MODY8), a monogenic syndrome of diabetes and pancreatic exocrine dysfunction. Here, we provide data suggesting that differentially O-glycosylated Cel could negatively affect beta cell function. Taken together, our findings demonstrate the importance of correct O-glycan formation for normal exocrine and endocrine pancreatic function, implying that aberrant O-glycans might be relevant for pathogenic mechanisms of the pancreas.

Signal-anchor sequences are an essential factor for the Golgi-plasma membrane localization of type II membrane proteins

Despite studies of the mechanism underlying the intracellular localization of membrane proteins, the specific mechanisms by which each membrane protein localizes to the endoplasmic reticulum, Golgi apparatus, and plasma membrane in the secretory pathway are unclear. In this study, a discriminant analysis of endoplasmic reticulum, Golgi apparatus and plasma membrane-localized type II membrane proteins was performed using a position-specific scoring matrix derived from the amino acid propensity of the sequences around signal-anchors. The possibility that the sequence around the signal-anchor is a factor for identifying each localization group was evaluated. The discrimination accuracy between the Golgi apparatus and plasma membrane-localized type II membrane proteins was as high as 90%, indicating that, in addition to other factors, the sequence around signal-anchor is an essential component of the selection mechanism for the Golgi and plasma membrane localization. These results may improve the use of membrane proteins for drug delivery and therapeutic applications.

The Cytokinin Oxidase/Dehydrogenase CKX1 Is a Membrane-Bound Protein Requiring Homooligomerization in the Endoplasmic Reticulum for Its Cellular Activity

Degradation of the plant hormone cytokinin is controlled by cytokinin oxidase/dehydrogenase (CKX) enzymes. The molecular and cellular behavior of these proteins is still largely unknown. In this study, we show that CKX1 is a type II single-pass membrane protein that localizes predominantly to the endoplasmic reticulum (ER) in Arabidopsis (Arabidopsis thaliana). This indicates that this CKX isoform is a bona fide ER protein directly controlling the cytokinin, which triggers the signaling from the ER. By using various approaches, we demonstrate that CKX1 forms homodimers and homooligomers in vivo. The amino-terminal part of CKX1 was necessary and sufficient for the protein oligomerization as well as for targeting and retention in the ER. Moreover, we show that protein-protein interaction is largely facilitated by transmembrane helices and depends on a functional GxxxG-like interaction motif. Importantly, mutations rendering CKX1 monomeric interfere with its steady-state localization in the ER and cause a loss of the CKX1 biological activity by increasing its ER-associated degradation. Therefore, our study provides evidence that oligomerization is a crucial parameter regulating CKX1 biological activity and the cytokinin concentration in the ER. The work also lends strong support for the cytokinin signaling from the ER and for the functional relevance of the cytokinin pool in this compartment.

Biochemical characterization of functional domains of the chaperone Cosmc

Cosmc is an endoplasmic reticulum chaperone necessary for normal protein O-GalNAc glycosylation through regulation of T-synthase, its single client. Loss-of-function of Cosmc results in expression of the Tn antigen, which is associated with multiple human diseases including cancer. Despite intense interest in dysregulated expression of the Tn antigen, little is known about the structure and function of Cosmc, including domain organization, secondary structure, oligomerization, and co-factors. Limited proteolysis experiments show that Cosmc contains a structured N-terminal domain (CosmcΔ256), and biochemical characterization of CosmcΔ256 reveals wild type chaperone activity. Interestingly, CosmcE152K, which shows loss of function in vivo, exhibits wild type-like activity in vitro. Cosmc and CosmcE152K heterogeneously oligomerize and form monomeric, dimeric, trimeric, and tetrameric species, while CosmcΔ256 is predominantly monomeric as characterized by chemical crosslinking and blue native page electrophoresis. Additionally, Cosmc selectively binds divalent cations in thermal shift assays and metal binding is abrogated by the CosmcΔ256 truncation, and perturbed by the E152K mutation. Therefore, the N-terminal domain of Cosmc mediates T-synthase binding and chaperone function, whereas the C-terminal domain is necessary for oligomerization and metal binding. Our results provide new structure-function insight to Cosmc, indicate that Cosmc behaves as a modular protein and suggests points of modulation or regulation of in vivo chaperone function.

Functional Consequences of Differential O-glycosylation of MUC1, MUC4, and MUC16 (Downstream Effects on Signaling)

Glycosylation is one of the most abundant post-translational modifications that occur within the cell. Under normal physiological conditions, O-linked glycosylation of extracellular proteins is critical for both structure and function. During the progression of cancer, however, the expression of aberrant and truncated glycans is commonly observed. Mucins are high molecular weight glycoproteins that contain numerous sites of O-glycosylation within their extracellular domains. Transmembrane mucins also play a functional role in monitoring the surrounding microenvironment and transducing these signals into the cell. In cancer, these mucins often take on an oncogenic role and promote a number of pro-tumorigenic effects, including pro-survival, migratory, and invasive behaviors. Within this review, we highlight both the processes involved in the expression of aberrant glycan structures on mucins, as well as the potential downstream impacts on cellular signaling.

The novel white spot syndrome virus-induced gene, PmERP15, encodes an ER stress-responsive protein in black tiger shrimp, Penaeus monodon

By microarray screening, we identified a white spot syndrome virus (WSSV)-strongly induced novel gene in gills of Penaeus monodon. The gene, PmERP15, encodes a putative transmembrane protein of 15 kDa, which only showed some degree of similarity (54-59%) to several unknown insect proteins, but had no hits to shrimp proteins. RT-PCR showed that PmERP15 was highly expressed in the hemocytes, heart and lymphoid organs, and that WSSV-induced strong expression of PmERP15 was evident in all tissues examined. Western blot analysis likewise showed that WSSV strongly up-regulated PmERP15 protein levels. In WSSV-infected hemocytes, immunofluorescence staining showed that PmERP15 protein was colocalized with an ER enzyme, protein disulfide isomerase, and in Sf9 insect cells, PmERP15-EGFP fusion protein colocalized with ER -Tracker™ Red dye as well. GRP78, an ER stress marker, was found to be up-regulated in WSSV-infected P. monodon, and both PmERP15 and GRP78 were up-regulated in shrimp injected with ER stress inducers tunicamycin and dithiothreitol. Silencing experiments showed that although PmERP15 dsRNA-injected shrimp succumbed to WSSV infection more rapidly, the WSSV copy number had no significant changes. These results suggest that PmERP15 is an ER stress-induced, ER resident protein, and its induction in WSSV-infected shrimp is caused by the ER stress triggered by WSSV infection. Furthermore, although PmERP15 has no role in WSSV multiplication, its presence is essential for the survival of WSSV-infected shrimp.

The Cosmc connection to the Tn antigen in cancer

The Tn antigen is a tumor-associated carbohydrate antigen that is not normally expressed in peripheral tissues or blood cells. Expression of this antigen, which is found in a majority of human carcinomas of all types, arises from a blockage in the normal O-glycosylation pathway in which glycans are extended from the common precursor GalNAcα1-O-Ser/Thr (Tn antigen). This precursor is generated in the Golgi apparatus on newly synthesized glycoproteins by a family of polypeptide α-N-acetylgalactosaminyltransferases (ppGalNAcTs) and then extended to the common core 1 O-glycan Galβ1-3GalNAcα1-O-Ser/Thr (T antigen) by a single enzyme termed the T-synthase (core 1 β3-galactosyltransferase or C1GalT). Formation of the active form of the T-synthase requires a unique molecular chaperone termed Cosmc, encoded by Cosmc on the X-chromosome (Xq24 in humans, Xc3 in mice). Cosmc resides in the endoplasmic reticulum (ER) and prevents misfolding, aggregation, and proteasome-dependent degradation of newly synthesized T-synthase. Loss of expression of active T-synthase or Cosmc can lead to expression of the Tn antigen, along with its sialylated version Sialyl Tn antigen as observed in several cancers. Both genetic and epigenetic pathways, in addition to potential metabolic regulation, can result in abnormal expression of the Tn antigen. Engineered expression of the Tn antigen by disruption of either C1GalT (T-syn) or Cosmc in mice is associated with a tremendous range of pathologies and engineered expression of the Tn antigen in mouse embryos leads to embryonic death. Studies indicate that many membrane glycoproteins expressing the Tn antigen and/or truncated O-glycans may be dysfunctional, due to degradation and/or misfolding. Thus, expression of normal O-glycans is associated with health and homeostasis whereas truncation of O-glycans, e.g. the Tn and/or Sialyl Tn antigens is associated with cancer and other pathologies.

Identification of a novel protein binding motif within the T-synthase for the molecular chaperone Cosmc

Prior studies suggested that the core 1 β3-galactosyltransferase (T-synthase) is a specific client of the endoplasmic reticulum chaperone Cosmc, whose function is required for T-synthase folding, activity, and consequent synthesis of normal O-glycans in all vertebrate cells. To explore whether the T-synthase encodes a specific recognition motif for Cosmc, we used deletion mutagenesis to identify a cryptic linear and relatively hydrophobic peptide in the N-terminal stem region of the T-synthase that is essential for binding to Cosmc (Cosmc binding region within T-synthase, or CBRT). Using this sequence information, we synthesized a peptide containing CBRT and found that it directly interacts with Cosmc and also inhibits Cosmc-assisted in vitro refolding of denatured T-synthase. Moreover, engineered T-synthase carrying mutations within CBRT exhibited diminished binding to Cosmc that resulted in the formation of inactive T-synthase. To confirm the general recognition of CBRT by Cosmc, we performed a domain swap experiment in which we inserted the stem region of the T-synthase into the human β4GalT1 and found that the CBRT element can confer Cosmc binding onto the β4GalT1 chimera. Thus, CBRT is a unique recognition motif for Cosmc to promote its regulation and formation of active T-synthase and represents the first sequence-specific chaperone recognition system in the ER/Golgi required for normal protein O-glycosylation.

Anchors aweigh: protein localization and transport mediated by transmembrane domains

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TMDs control the intracellular transport of many membrane proteins.

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The length and hydrophobicity of TMDs determine their sorting.

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Some membrane receptors for sorting TMDs have been identified.

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Lipid partitioning may also participate in the sorting of TMDs.

The transmembrane domains (TMDs) of integral membrane proteins have emerged as major determinants of intracellular localization and transport in the secretory and endocytic pathways. Unlike sorting signals in cytosolic domains, TMD sorting determinants are not conserved amino acid sequences but physical properties such as the length and hydrophilicity of the transmembrane span. The underlying sorting machinery is still poorly characterized, but several mechanisms have been proposed, including TMD recognition by transmembrane sorting receptors and partitioning into membrane lipid domains. Here we review the nature of TMD sorting determinants and how they may dictate transmembrane protein localization and transport.

The MARVEL transmembrane motif of occludin mediates oligomerization and targeting to the basolateral surface in epithelia

Occludin (Ocln), a MARVEL-motif-containing protein, is found in all tight junctions. MARVEL motifs are comprised of four transmembrane helices associated with the localization to or formation of diverse membrane subdomains by interacting with the proximal lipid environment. The functions of the Ocln MARVEL motif are unknown. Bioinformatics sequence- and structure-based analyses demonstrated that the MARVEL domain of Ocln family proteins has distinct evolutionarily conserved sequence features that are consistent with its basolateral membrane localization. Live-cell microscopy, fluorescence resonance energy transfer (FRET) and bimolecular fluorescence complementation (BiFC) were used to analyze the intracellular distribution and self-association of fluorescent-protein-tagged full-length human Ocln or the Ocln MARVEL motif excluding the cytosolic C- and N-termini (amino acids 60-269, FP-MARVEL-Ocln). FP-MARVEL-Ocln efficiently arrived at the plasma membrane (PM) and was sorted to the basolateral PM in filter-grown polarized MDCK cells. A series of conserved aromatic amino acids within the MARVEL domain were found to be associated with Ocln dimerization using BiFC. FP-MARVEL-Ocln inhibited membrane pore growth during Triton-X-100-induced solubilization and was shown to increase the membrane-ordered state using Laurdan, a lipid dye. These data demonstrate that the Ocln MARVEL domain mediates self-association and correct sorting to the basolateral membrane.

Tight complex formation between Cosmc chaperone and its specific client non-native T-synthase leads to enzyme activity and client-driven dissociation

The interaction of the endoplasmic reticulum molecular chaperone Cosmc with its specific client T-synthase (Core 1 β1-3-galactosyltransferase) is required for folding of the enzyme and eventual movement of the T-synthase to the Golgi, but the mechanism of interaction is unclear. Here we show that the lumenal domain of recombinant Cosmc directly interacts specifically in either free form or covalently bound to solid supports with denatured T-synthase but not with the active dimeric form of the enzyme. This leads to formation of a relatively stable complex of Cosmc and denatured T-synthase accompanied by formation of reactivated enzyme in an ATP-independent fashion that is not regulated by redox, calcium, pH, or intermolecular disulfide bond formation. The partly refolded and active T-synthase remains tightly bound noncovalently to Cosmc. Dissociation of T-synthase from the complex is promoted by further interactions of the complex with free forms of either native or non-native T-synthase. Taken together, these results demonstrate a novel mechanism in which Cosmc cycles to bind non-native T-synthase, leading to enzyme activity and release in a client-driven process.

Molecular cloning of a xylosyltransferase that transfers the second xylose to O-glucosylated epidermal growth factor repeats of notch

The extracellular domain of Notch contains epidermal growth factor (EGF) repeats that are extensively modified with different O-linked glycans. O-Fucosylation is essential for receptor function, and elongation with N-acetylglucosamine, catalyzed by members of the Fringe family, modulates Notch activity. Only recently, genes encoding enzymes involved in the O-glucosylation pathway have been cloned. In the Drosophila mutant rumi, characterized by a mutation in the protein O-glucosyltransferase, Notch signaling is impaired in a temperature-dependent manner, and a mouse knock-out leads to embryonic lethality. We have previously identified two human genes, GXYLT1 and GXYLT2, encoding glucoside xylosyltransferases responsible for the transfer of xylose to O-linked glucose. The identity of the enzyme further elongating the glycan to generate the final trisaccharide xylose-xylose-glucose, however, remained unknown. Here, we describe that the human gene C3ORF21 encodes a UDP-xylose:α-xyloside α1,3-xylosyltransferase, acting on xylose-α1,3-glucoseβ1-containing acceptor structures. We have, therefore, renamed it XXYLT1 (xyloside xylosyltransferase 1). XXYLT1 cannot act on a synthetic acceptor containing an α-linked xylose alone, but requires the presence of the underlying glucose. Activity on Notch EGF repeats was proven by in vitro xylosylation of a mouse Notch1 fragment recombinantly produced in Sf9 insect cells, a bacterially expressed EGF repeat from mouse Notch2 modified in vitro by Rumi and Gxylt2 and in vivo by co-expression of the enzyme with the Notch1 fragment. The enzyme was shown to be a typical type II membrane-bound glycosyltransferase localized in the endoplasmic reticulum.