Subcellular localization of the UDP-N-acetyl-D-galactosamine: polypeptide N-acetylgalactosaminyltransferase-mediated O-glycosylation reaction in the submaxillary gland

Pathways of O-glycan biosynthesis in cancer cells

Glycoproteins with O-glycosidically linked carbohydrate chains of complex structures and functions are found in secretions and on the cell surfaces of cancer cells. The structures of O-glycans are often unusual or abnormal in cancer, and greatly contribute to the phenotype and biology of cancer cells. Some of the mechanisms of changes in O-glycosylation pathways have been determined in cancer model systems. However, O-glycan biosynthesis is a complex process that is still poorly understood. The glycosyltransferases and sulfotransferases that synthesize O-glycans appear to exist as families of related enzymes of which individual members are expressed in a tissue- and growth-specific fashion. Studies of their regulation in cancer may reveal the connection between cancerous transformation and glycosylation which may help to understand and control the abnormal biology of tumor cells. Cancer diagnosis may be based on the appearance of certain glycosylated epitopes, and therapeutic avenues have been designed to attack cancer cells via their glycans.

Scanning the available Dictyostelium discoideum proteome for O-linked GlcNAc glycosylation sites using neural networks

Dictyostelium discoideum has been suggested as a eukaryotic model organism for glycobiology studies. Presently, the characteristics of acceptor sites for the N-acetylglucosaminyl-transferases in Dictyostelium discoideum, which link GlcNAc in an alpha linkage to hydroxyl residues, are largely unknown. This motivates the development of a species specific method for prediction of O-linked GlcNAc glycosylation sites in secreted and membrane proteins of D. discoideum. The method presented here employs a jury of artificial neural networks. These networks were trained to recognize the sequence context and protein surface accessibility in 39 experimentally determined O-alpha-GlcNAc sites found in D. discoideum glycoproteins expressed in vivo. Cross-validation of the data revealed a correlation in which 97% of the glycosylated and nonglycosylated sites were correctly identified. Based on the currently limited data set, an abundant periodicity of two (positions-3, -1, +1, +3, etc.) in Proline residues alternating with hydroxyl amino acids was observed upstream and downstream of the acceptor site. This was a consequence of the spacing of the glycosylated residues themselves which were peculiarly found to be situated only at even positions with respect to each other, indicating that these may be located within beta-strands. The method has been used for a rapid and ranked scan of the fraction of the Dictyostelium proteome available in public databases, remarkably 25-30% of which were predicted glycosylated. The scan revealed acceptor sites in several proteins known experimentally to be O-glycosylated at unmapped sites. The available proteome was classified into functional and cellular compartments to study any preferential patterns of glycosylation. A sequence based prediction server for GlcNAc O-glycosylations in D. discoideum proteins has been made available through the WWW at http://www.cbs.dtu.dk/services/DictyOGlyc/ and via E-mail to DictyOGlyc@cbs.dtu.dk.

Dynamic epigenetic regulation of initial O-glycosylation by UDP-N-Acetylgalactosamine:Peptide N-acetylgalactosaminyltransferases. site-specific glycosylation of MUC1 repeat peptide influences the substrate qualities at adjacent or distant Ser/Thr positions

In search of possible epigenetic regulatory mechanisms ruling the initiation of O-glycosylation by polypeptide:N-acetylgalactosaminyltransferases, we studied the influences of mono- and disaccharide substituents of glycopeptide substrates on the site-specific in vitro addition of N-acetylgalactosamine (GalNAc) residues by recombinant GalNAc-Ts (rGalNAc-T1, -T2, and -T3). The substrates were 20-mers (HGV20) or 21-mers (AHG21) of the MUC1 tandem repeat peptide carrying GalNAcalpha or Galbeta1-3GalNAcalpha at different positions. The enzymatic products were analyzed by MALDI mass spectrometry and Edman degradation for the number and sites of incorporated GalNAc. Disaccharide placed on the first position of the diad Ser-16-Thr-17 prevents glycosylation of the second, whereas disaccharide on the second position of Ser-16-Thr-17 and Thr-5-Ser-6 does not prevent GalNAc addition to the first. Multiple disaccharide substituents suppress any further glycosylation at the remaining sites. Glycosylation of Ser-16 is negatively affected by glycosylation at position -6 (Thr-10) or -10 (Ser-6) and is inhibited by disaccharide at position -11 (Thr-5), suggesting the occurrence of glycosylation-induced effects on distant acceptor sites. Kinetic studies revealed the accelerated addition of GalNAc to Ser-16 adjacent to GalNAc-substituted Thr-17, demonstrating positive regulatory effects induced by glycosylation on the monosaccharide level. These antagonistic effects of mono- and disaccharides could underlie a postulated regulatory mechanism.

A different intracellular distribution of a single reporter protein is determined at steady state by KKXX or KDEL retrieval signals

To establish the specific contribution to protein topology of KKXX and KDEL retrieval motifs, we have determined by immunogold electron microscopy and cell fractionation the intracellular distribution at steady state of the transmembrane and anchorless versions of human CD8 protein, tagged with KKXX (CD8-E19) and KDEL (CD8-K), respectively, and stably expressed in epithelial rat cells (Martire, G., Mottola, G., Pascale, M. C., Malagolini, N., Turrini, I., Serafini-Cessi, F., Jackson, M. R., and Bonatti, S. (1996) J. Biol. Chem. 271, 3541-3547). The CD8-E19 protein is represented by a single form, initially O-glycosylated: only about half of it is located in the endoplasmic reticulum, whereas more than 30% of the total is present in the intermediate compartment and cis-Golgi complex. In the latter compartments, CD8-E19 colocalizes with beta-coat protein (COP) (COPI component) and shows the higher density of labeling. Conversely, about 90% of the total CD8-KDEL protein is localized in clusters on the endoplasmic reticulum, where significant co-localization with Sec-23p (COPII component) is observed, and unglycosylated and initially O-glycosylated forms apparently constitute a single pool. Altogether, these results suggest that KKXX and KDEL retrieval motifs have different topological effects on theirs own at steady state: the first results in a specific enrichment in the intermediate compartment and cis-Golgi complex, and the latter dictates residency in the endoplasmic reticulum.

The acceptor specificity of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases

The in vitro and in vivo specificity of the family of peptide:N-acetylgalactosaminyltransferases (GalNAcT) is analyzed on the basis of the reactivity and/or inhibitory activity of peptides and protein segments. The transferases appear to be multi-substrate enzymes with extended active sites containing a least nine subsites that interact cooperatively with a linear segment of at least nine amino acid residues on the acceptor polypeptide. Functional acceptor sites are located on the surface of the protein and extended conformations (beta-strand conformation) are preferred. The acceptor specificity of GalNAc-T can be predicted from the primary structure of the acceptor peptide with an accuracy of 70 to 80%. The same GalNAc-T enzymes catalyze the glycosylation of both serine and threonine residues. The higher in vitro catalytic efficiency toward threonine versus serine is the result of enhanced binding as well as increased reaction velocity, both effects being the result of steric interactions between the active site of the enzyme and the methyl group of threonine. Results from substrate binding studies suggest that GalNAc-T catalyzed transfer proceeds via an ordered sequential mechanism.

Expression of polypeptide GalNAc-transferases in stratified epithelia and squamous cell carcinomas: immunohistological evaluation using monoclonal antibodies to three members of the GalNAc-transferase family

Mucin-type O-glycosylation is initiated by a large family of UDP-GalNAc: polypeptide N -acetyl-galactosaminyltransferases (GalNAc-transferases). Individual GalNAc-transferases appear to have different functions and Northern analysis indicates that they are differently expressed in different organs. This suggests that O-glycosylation may vary with the repertoire of GalNAc-transferases expressed in a given cell. In order to study the repertoire of GalNAc-transferases in situ in tissues and changes in tumors, we have generated a panel of monoclonal antibodies (MAbs) with well defined specificity for human GalNAc-T1, -T2, and -T3. Application of this panel of novel antibodies revealed that GalNAc- transferases are differentially expressed in different cell lines, in spermatozoa, and in oral mucosa and carcinomas. For example, GalNAc-T1 and -T2 but not -T3 were highly expressed in WI38 cells, and GalNAc-T3 but not GalNAc-T1 or -T2 was expressed in spermatozoa. The expression patterns in normal oral mucosa were found to vary with cell differentiation, and for GalNAc-T2 and -T3 this was reflected in oral squamous cell carcinomas. The expression pattern of GalNAc-T1 was on the other hand changed in tumors to either total loss or expression in cytological poorly differentiated tumor cells, where the normal undifferentiated cells lacked expression. These results demonstrate that the repertoire of GalNAc-transferases is different in different cell types and vary with cellular differentiation, and malignant transformation. The implication of this is not yet fully understood, but it suggests that specific changes in sites of O-glycosylation of proteins may occur as a result of changes in the repertoire of GalNAc-transferases.

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

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.

Viral glycoproteins accumulate in newly formed annulate lamellae following infection of lymphoid cells by human herpesvirus 6

Ultrastructural analysis of HSB-2 T-lymphoid cells and human cord blood mononuclear cells infected with human herpesvirus 6 revealed the presence, in the cell cytoplasm, of annulate lamellae (AL), which were absent in uninfected cells. Time course analysis of the appearance of AL following viral infection showed that no AL were visible within the first 72 h postinfection and that their formation correlated with the expression of the late viral glycoprotein gp116. The requirement of active viral replication for AL neoformation was further confirmed by experiments using inactivated virus or performed in presence of the viral DNA polymerase inhibitor phosphonoacetic acid. Both conventional electron microscopic examination and immunogold fracture labeling with anti-endoplasmic reticulum antibodies indicated a close relationship of AL with the endoplasmic reticulum and nuclear membranes. However, when the freeze-fractured cells were immunogold labeled with an anti-gp116 monoclonal antibody, AL membranes were densely labeled, whereas nuclear membranes and endoplasmic reticulum cisternae appeared virtually unlabeled, showing that viral envelope glycoproteins selectively accumulate in AL. In addition, gold labeling with Helix pomatia lectin and wheat germ agglutinin indicated that AL cisternae, similar to cis-Golgi membranes, contain intermediate, but not terminal, forms of glycoconjugates. Taken together, these results suggest that in this cell-virus system, AL function as a viral glycoprotein storage compartment and as a putative site of O-glycosylation.

The recycling pathway of protein ERGIC-53 and dynamics of the ER-Golgi intermediate compartment

To establish recycling routes in the early secretory pathway we have studied the recycling of the ER-Golgi intermediate compartment (ERGIC) marker ERGIC-53 in HepG2 cells. Immunofluorescence microscopy showed progressive concentration of ERGIC-53 in the Golgi area at 15 degreesC. Upon rewarming to 37 degreesC ERGIC-53 redistributed into the cell periphery often via tubular processes that largely excluded anterograde transported albumin. Immunogold labeling of cells cultured at 37 degreesC revealed ERGIC-53 predominantly in characteristic beta-COP-positive tubulo-vesicular clusters both near the Golgi apparatus and in the cell periphery. Concentration of ERGIC-53 at 15 degreesC resulted from both accumulation of ERGIC-53 in the ERGIC and movement of ERGIC membranes closer to the Golgi apparatus. Upon rewarming to 37 degreesC the labeling of ERGIC-53 in the ERGIC rapidly returned to normal levels whereas ERGIC-53's labeling in the cis-Golgi was unchanged. Temperature manipulations had no effect on the average number of ERGIC-53 clusters. Density gradient centrifugation indicated that the surplus ERGIC-53 accumulating in the ERGIC at 15 degreesC was rapidly transported to the ER upon rewarming. These results suggest that the ERGIC is a dynamic membrane system composed of a constant average number of clusters and that the major recycling pathway of ERGIC-53 bypasses the Golgi apparatus.

Cloning of a human UDP-N-acetyl-alpha-D-Galactosamine:polypeptide N-acetylgalactosaminyltransferase that complements other GalNAc-transferases in complete O-glycosylation of the MUC1 tandem repeat

A fourth human UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase, designated GalNAc-T4, was cloned and expressed. The genomic organization of GalNAc-T4 is distinct from GalNAc-T1, -T2, and -T3, which contain multiple coding exons, in that the coding region is contained in a single exon. GalNAc-T4 was placed at human chromosome 12q21.3-q22 by in situ hybridization and linkage analysis. GalNAc-T4 expressed in Sf9 cells or in a stably transfected Chinese hamster ovary cell line exhibited a unique acceptor substrate specificity. GalNAc-T4 transferred GalNAc to two sites in the MUC1 tandem repeat sequence (Ser in GVTSA and Thr in PDTR) using a 24-mer glycopeptide with GalNAc residues attached at sites utilized by GalNAc-T1, -T2, and -T3 (TAPPAHGVTSAPDTRPAPGSTAPPA, GalNAc attachment sites underlined). Furthermore, GalNAc-T4 showed the best kinetic properties with an O-glycosylation site in the P-selectin glycoprotein ligand-1 molecule. Northern analysis of human organs revealed a wide expression pattern. Immunohistology with a monoclonal antibody showed the expected Golgi-like localization in salivary glands. A single base polymorphism, G1516A (Val to Ile), was identified (allele frequency 34%). The function of GalNAc-T4 complements other GalNAc-transferases in O-glycosylation of MUC1 showing that glycosylation of MUC1 is a highly ordered process and changes in the repertoire or topology of GalNAc-transferases will result in altered pattern of O-glycan attachments.

Structural requirements for O-glycosylation of the mouse hepatitis virus membrane protein

The mouse hepatitis virus (MHV) membrane (M) protein contains only O-linked oligosaccharides. We have used this protein as a model to study the structural requirements for O-glycosylation. We show that MHV M is modified by the addition of a single oligosaccharide side chain at the cluster of 4 hydroxylamino acids present at its extreme amino terminus and identified Thr at position 5 as the functional acceptor site. The hydroxylamino acid cluster, which is quite conserved among O-glycosylated coronavirus M proteins, is not in itself sufficient for O-glycosylation. Downstream amino acids are required to introduce a functional O-glycosylation site into a foreign protein. In a mutagenic analysis O-glycosylation was found to be sensitive to some particular changes but no unique sequence motif for O-glycosylation could be identified. Expression of mutant M proteins in cells revealed that substitution of any 1 residue was tolerated, conceivably due to the occurrence of multiple UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc transferases). Indeed, MHV M served as a substrate for GalNac-T1, -T2, and -T3, as was demonstrated using an in situ glycosylation assay based on the co-expression of endoplasmic reticulum-retained forms of the GalNAc transferases with endoplasmic reticulum-resident MHV M mutants. The GalNAc transferases were found to have largely overlapping, but distinct substrate specificities. The requirement for a threonine as acceptor rather than a serine residue and the requirement for a proline residue three positions downstream of the acceptor site were found to be distinctive features.

Multiple functions of Pmt1p-mediated protein O-mannosylation in the fungal pathogen Candida albicans

Protein mannosylation by Pmt proteins initiates O-glycosylation in fungi. We have identified the PMT1 gene and analyzed the function of Pmt1p in the fungal human pathogen Candida albicans. Mutants defective in PMT1 alleles lacked Pmt in vitro enzymatic activity, showed reduced growth rates, and tended to form cellular aggregates. In addition, multiple specific deficiencies not known in Saccharomyces cerevisiae (including defective hyphal morphogenesis; supersensitivity to the antifungal agents hygromycin B, G418, clotrimazole, and calcofluor white; and reduced adherence to Caco-2 epithelial cells) were observed in pmt1 mutants. PMT1 deficiency also led to faster electrophoretic mobility of the Als1p cell wall protein and to elevated extracellular activities of chitinase. Homozygous pmt1 mutants were avirulent in a mouse model of systemic infection, while heterozygous PMT1/pmt1 strains showed reduced virulence. The results indicate that protein O-mannosylation by Pmt proteins occurs in different fungal species, where PMT1 deficiency can lead to defects in multiple cellular functions.

Concepts and principles of O-linked glycosylation

The biosynthesis, structures, and functions of O-glycosylation, as a complex posttranslational event, is reviewed and compared for the various types of O-glycans. Mucin-type O-glycosylation is initiated by tissue-specific addition of a GalNAc-residue to a serine or a threonine of the fully folded protein. This event is dependent on the primary, secondary, and tertiary structure of the glycoprotein. Further elongation and termination by specific transferases is highly regulated. We also describe some of the physical and biological properties that O-glycosylation confers on the protein to which the sugars are attached. These include providing the basis for rigid conformations and for protein stability. Clustering of O-glycans in Ser/Thr(/Pro)-rich domains allows glycan determinants such as sialyl Lewis X to be presented as multivalent ligands, essential for functional recognition. An additional level of regulation, imposed by exon shuffling and alternative splicing of mRNA, results in the expression of proteins that differ only by the presence or absence of Ser/Thr(/Pro)-rich domains. These domains may serve as protease-resistant spacers in cell surface glycoproteins. Further biological roles for O-glycosylation discussed include the role of isolated mucin-type O-glycans in recognition events (e.g., during fertilization and in the immune response) and in the modulation of the activity of enzymes and signaling molecules. In some cases, the O-linked oligosaccharides are necessary for glycoprotein expression and processing. In contrast to the more common mucin-type O-glycosylation, some specific types of O-glycosylation, such as the O-linked attachment of fucose and glucose, are sequon dependent. The reversible attachment of O-linked GlcNAc to cytoplasmic and nuclear proteins is thought to play a regulatory role in protein function. The recent development of novel technologies for glycan analysis promises to yield new insights in the factors that determine site occupancy, structure-function relationship, and the contribution of O-linked sugars to physiological and pathological processes. These include diseases where one or more of the O-glycan processing enzymes are aberrantly regulated or deficient, such as HEMPAS and cancer.

Molecular analysis of CaMnt1p, a mannosyl transferase important for adhesion and virulence of Candida albicans

There is an immediate need for identification of new antifungal targets in opportunistic pathogenic fungi like Candida albicans. In the past, efforts have focused on synthesis of chitin and glucan, which confer mechanical strength and rigidity upon the cell wall. This paper describes the molecular analysis of CaMNT1, a gene involved in synthesis of mannoproteins, the third major class of macromolecule found in the cell wall. CaMNT1 encodes an alpha-1, 2-mannosyl transferase, which adds the second mannose residue in a tri-mannose oligosaccharide structure which represents O-linked mannan in C. albicans. The deduced amino acid sequence suggests that CaMnt1p is a type II membrane protein residing in a medial Golgi compartment. The absence of CaMnt1p reduced the ability of C. albicans cells to adhere to each other, to human buccal epithelial cells, and to rat vaginal epithelial cells. Both heterozygous and homozygous Camnt1 null mutants of C. albicans showed strong attenuation of virulence in guinea pig and mouse models of systemic candidosis, which, in guinea pigs, could be attributed to a decreased ability to reach and/or adhere internal organs. Therefore, correct CaMnt1p-mediated O-linked mannosylation of proteins is critical for adhesion and virulence of C. albicans.

Porcine submaxillary mucin forms disulfide-linked multimers through its amino-terminal D-domains

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.

NetOglyc: prediction of mucin type O-glycosylation sites based on sequence context and surface accessibility

The specificities of the UDP-GalNAc:polypeptide Nacetylgalactosaminyltransferases which link the carbohydrate GalNAc to the side-chain of certain serine and threonine residues in mucin type glycoproteins, are presently unknown. The specificity seems to be modulated by sequence context, secondary structure and surface accessibility. The sequence context of glycosylated threonines was found to differ from that of serine, and the sites were found to cluster. Non-clustered sites had a sequence context different from that of clustered sites. Charged residues were disfavoured at position -1 and +3. A jury of artificial neural networks was trained to recognize the sequence context and surface accessibility of 299 known and verified mucin type O-glycosylation sites extracted from O-GLYCBASE. The cross-validated NetOglyc network system correctly found 83% of the glycosylated and 90% of the non-glycosylated serine and threonine residues in independent test sets, thus proving more accurate than matrix statistics and vector projection methods. Predictions of O-glycosylation sites in the envelope glycoprotein gp120 from the primate lentiviruses HIV-1, HIV-2 and SIV are presented. The most conserved O-glycosylation signals in these evolutionary-related glycoproteins were found in their first hypervariable loop, V1. However, the strain variation for HIV-1 gp120 was significant. A computer server, available through WWW or E-mail, has been developed for prediction of mucin type O-glycosylation sites in proteins based on the amino acid sequence. The server addresses are http://www.cbs.dtu.dk/services/NetOGlyc/ and netOglyc@cbs.dtu.dk.

Localization of three human polypeptide GalNAc-transferases in HeLa cells suggests initiation of O-linked glycosylation throughout the Golgi apparatus

O-glycosylation of proteins is initiated by a family of UDP-N-acetylgalactosamine:polypeptide N-acetylgalactos-aminyltransferases (GalNAc-T). In this study, we have localized endogenous and epitope-tagged human GalNAc-T1, -T2 and -T3 to the Golgi apparatus in HeLa cells by subcellular fractionation, immunofluorescence and immunoelectron microscopy. We show that all three GalNAc-transferases are concentrated about tenfold in Golgi stacks over Golgi associated tubular-vesicular membrane structures. Surprisingly, we find that GalNAc-T1, -T2 and -T3 are present throughout the Golgi stack suggesting that initiation of O-glycosylation may not be restricted to the cis Golgi, but occur at multiple sites within the Golgi apparatus. GalNAc-T1 distributes evenly across the Golgi stack whereas GalNAc-T2 and -T3 reside preferentially on the trans side and in the medial part of the Golgi stack, respectively. Moreover, we have investigated the possibility of O-glycan initiation in pre-Golgi compartments such as the ER. We could not detect endogenous polypeptide GalNAc-transferase activity in the ER of HeLa cells, neither by subcellular fractionation nor by situ glycosylation of an ER-retained form of CD8 (CD8/E19). However, upon relocation of chimeric GalNAc-T1 or -T2 to the ER, CD8/E19 is glycosylated with different efficiencies indicating that all components required for initiation of O-glycosylation are present in the ER except for polypeptide GalNAc-transferases.

Substrate specificities of three members of the human UDP-N-acetyl-alpha-D-galactosamine:Polypeptide N-acetylgalactosaminyltransferase family, GalNAc-T1, -T2, and -T3

Mucin-type O-glycosylation is initiated by UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc-transferases). The role each GalNAc-transferase plays in O-glycosylation is unclear. In this report we characterized the specificity and kinetic properties of three purified recombinant GalNAc-transferases. GalNAc-T1, -T2, and -T3 were expressed as soluble proteins in insect cells and purified to near homogeneity. The enzymes have distinct but partly overlapping specificities with short peptide acceptor substrates. Peptides specifically utilized by GalNAc-T2 or -T3, or preferentially by GalNAc-T1 were identified. GalNAc-T1 and -T3 showed strict donor substrate specificities for UDP-GalNAc, whereas GalNAc-T2 also utilized UDP-Gal with one peptide acceptor substrate. Glycosylation of peptides based on MUC1 tandem repeat showed that three of five potential sites in the tandem repeat were glycosylated by all three enzymes when one or five repeat peptides were analyzed. However, analysis of enzyme kinetics by capillary electrophoresis and mass spectrometry demonstrated that the three enzymes react at different rates with individual sites in the MUC1 repeat. The results demonstrate that individual GalNAc-transferases have distinct activities and the initiation of O-glycosylation in a cell is regulated by a repertoire of GalNAc-transferases.

Altered Golgi localization of core 2 beta-1,6-N-acetylglucosaminyltransferase leads to decreased synthesis of branched O-glycans

Mucin type O-glycans with core 2 branches are distinct from nonbranched O-glycans, and the amount of core 2 branched O-glycans changes dramatically during T cell differentiation. This oligosaccharide is synthesized only when core 2 beta-1, 6-N-acetylglucosaminyltransferase (C2GnT) is present, and the expression of this glycosyltransferase is highly regulated. To understand how O-glycan synthesis is regulated by the orderly appearance of glycosyltransferases that form core 2 branched O-glycans, the subcellular localization of C2GnT was determined by using antibodies generated that are specific to C2GnT. The studies using confocal light microscopy demonstrated that C2GnT was localized mainly in cis to medial-cisternae of the Golgi. We then converted C2GnT to a trans-Golgi enzyme by replacing its Golgi retention signal with that of alpha-2,6-sialyltransferase, which resides in trans-Golgi. Chinese hamster ovary cells expressing wild type C2GnT and the chimeric C2GnT were then subjected to oligosaccharide analysis. The results obtained clearly indicate that the conversion of C2GnT into a trans-Golgi enzyme resulted in a substantial decrease of core 2 branched oligosaccharides. These results, taken together, strongly suggest that the predominance of core 2 branched oligosaccharides in those cells expressing C2GnT is due to the fact that C2GnT is located earlier in the Golgi than alpha-2,3-sialyltransferase that competes with C2GnT for the common substrate. Furthermore, alteration of Golgi localization renders the chimeric C2GnT much less efficient in synthesizing core 2 branched oligosaccharides, indicating the critical role of orderly subcellular localization of glycosyltransferases.

Discovery of the shortest sequence motif for high level mucin-type O-glycosylation

The consensus primary amino acid sequence for mucin-type O-glycosylation sites has not been identified. To determine the shortest motif sequence required for high level mucin-type O-glycosylation, we prepared more than 100 synthetic peptides and assayed in vitro O-GalNAc transfer to serine or threonine in these peptides using a bovine colostrum UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyl transferase (O-GalNAcT). We chose the sequence PDAASAAP from human erythropoietin (hEPO) for further systematic substitutions because it accepted GalNAc and was a fairly simple sequence consisting only of four kinds of amino acids. Several substitutions showed that threonine is approximately 40-fold better than serine as the glycosylated amino acid and a proline at position +3 on the C-terminal side is very important. To define the effect of proline residues around the glycosylation site, we analyzed a series of peptides containing one to three proline residues in a parent peptide AAATAAA. The results clearly indicated that prolines at positions +1 and +3 had a positive effect. The O-GalNAc transfer level of AAATPAP was increased approximately 90-fold from AAATAAA. The deletion of amino acids from the N-terminal side of the glycosylation site suggested that five amino acids from position -1 to +3 were especially important for glycosylation. Moreover, the influence of all 20 amino acids at positions -1, +2, and +4 was analyzed. Uncharged amino acids were preferred at position -1, and small or positively charged amino acids were preferred at position +2. No preference was observed at position +4. We propose a mucin-type O-glycosylation motif, XTPXP, which may be suitable as a signal for protein O-glycosylation. The features observed in this study also appear to be very useful for prediction of mucin-type O-glycosylation sites in glycoproteins.

A transfected sialyltransferase that is elevated in breast cancer and localizes to the medial/trans-Golgi apparatus inhibits the development of core-2-based O-glycans

The alpha2,3 sialyltransferase, alpha2,3 SAT (O), catalyzes the transfer of sialic acid to Galbeta1,3 N-acetyl-D-galactosamine (GalNAc) (core-1) in mucin type O-glycosylation, and thus terminates chain extension. A Core-2 branch can also be formed from core-1 by the core-2 beta1,6 N-acetyl-d-glucosamine transferase (beta1,6 GlcNAc T) that leads to chain extension. Increased levels of the alpha2,3 SAT (O) and decreased levels of the core-2 beta1,6 GlcNAc T are seen in breast cancer cells and correlate with differences in the structure of the O-glycans synthesized (Brockhausen et al., 1995; Lloyd et al., 1996). Since in mucin type O-glycosylation sugars are added individually and sequentially in the Golgi apparatus, the position of the transferases, as well as their activity, can determine the final structure of the O-glycans synthesized. A cDNA coding for the human alpha2,3 SAT (O) tagged with an immunoreactive epitope from the myc gene has been used to map the position of the glycosyltransferase in nontumorigenic (MTSV1-7) and malignant (T47D) breast epithelial cell lines. Transfectants were analyzed for expression of the enzyme at the level of message and protein, as well as for enzymic activity. In T47D cells, which do not express core-2 beta1,6 GlcNAc T, the increased activity of the sialyltransferase correlated with increased sialylation of core-1 O-glycans on the epithelial mucin MUC1. Furthermore, in MTSV1-7 cells, which do express core-2 beta1,6 GlcNAc T, an increase in sialylated core-1 structures is accompanied by a reduction in the ratio of GlcNAc: GalNAc in the O-glycans attached to MUC1, implying a decrease in branching. Using quantitative immunoelectron microscopy, the sialyltransferase was mapped to the medial- and trans-Golgi cisternae, with some being present in the TGN. The data represent the first fine mapping of a sialyltransferase specifically active in O-glycosylation and demonstrate that the structure of O-glycans synthesized by a cell can be manipulated by transfecting with recombinant glycosyltransferases.

Aberrant O-glycosylation in the collagenous domain of pro alpha2(I) procollagen subunits synthesized by chemically transformed hamster fibroblasts

Chemically transformed Syrian hamster embryo fibroblasts (NQT-SHE) do not synthesize the pro alpha1(I) subunit of type I collagen, but they secrete two forms of the pro alpha2(I) subunit (N33 and N50) with abnormal post-translational modifications localized in the alpha2CB3,5 cyanogen bromide peptide of the collagenous domain (B. Peterkofsky and W. Prather (1992) J. Biol. Chem. 267 5388-5395). Isoelectric focusing and treatment of the modified chains with glycosidases and biotinylated Jacalin lectin identified the modifications as Gal beta1,3-GalNAc-O-Ser/Thr with or without a terminal sialic acid in an alpha2,6 linkage. Unhydroxylated N33 alpha-chains also reacted with Jacalin, confirming that the abnormal modification was O-glycosylation and not hyperhydroxylation of proline or lysine. Cells were treated with benzyl GalNAc, a competitive inhibitor of galactosyl transferase that prevents addition of Gal to GalNAc-O-Ser/Thr and thus blocks elongation of O-glycosyl chains. Treated cells secreted pro alpha2(I) chains containing GalNAc-O-Ser/Thr but no galactose or sialic acid, which suggested that Gal addition takes place before sialylation. Treatment of NQT-SHE cells with monensin and brefeldin A inhibited secretion and led to intracellular accumulation of pro alpha2(I) chains that contained only GalNAc. Therefore, it appears that GalNAc addition to pro alpha2(I) chains in NQT-SHE cells occurs in the cis-Golgi, while sialic acid and galactose are added in the trans-Golgi network. The pro alpha2(I) chains produced by NQT-SHE cells most likely are modified because they are in the denatured state, and thus potential O-glycosylation sites become available that would not be exposed in normal triple helical procollagen.

GTP-bound forms of rab6 induce the redistribution of Golgi proteins into the endoplasmic reticulum

rab6 is a ubiquitous ras-like GTPase involved in intra-Golgi transport. We have studied at both morphological and biochemical levels the behavior of Golgi resident proteins in HeLa cells overexpressing wild-type rab6 and GTP- and GDP-bound mutants of rab6 (rab6 Q72L and rab6 T27N, respectively). We show that wild-type rab6 and rab6 Q72L overexpression induces the redistribution of the trans-Golgi protein beta-1,4-galactosyltransferase into the endoplasmic reticulum (ER) and allows the addition of sialylated O-glycans on an ER-retained protein, the major histocompatibility complex class II-associated invariant chain. Remarkably, rab6 Q72L effects, which require the integrity of microtubules, were almost indistinguishable from those induced by brefeldin A, a fungic metabolite that causes a mixing of Golgi and ER membranes. In contrast, overexpression of rab6 T27N does not cause the redistribution of Golgi proteins, but inhibits basal O-glycosylation of the major histocompatibility complex class II-associated invariant chain.

Synthesis and intracellular trafficking of Muc-1 and mucins by polarized mouse uterine epithelial cells

Mucins function as a protective layer rendering the apical surface of epithelial cells nonadhesive to a variety of microorganisms and macromolecules. Muc-1 is a transmembrane mucin expressed at the apical cell surface of mouse uterine epithelial cells (UEC) that disappears as UEC become receptive for embryo implantation (Surveyor, G. A., Gendler, S. J., Pemberton, L., Das, S. K., Chakraborty, I., Julian, J., Pimental, R. A., Wegner, C. W., Dey, S. K., and Carson, D. D. (1995) Endocrinology 136, 3639-3647). In the present study, the kinetics of Muc-1 assembly, cell surface expression, release, and degradation were examined in polarized mouse UEC in vitro. Mucins were identified as the predominant glycoconjugates synthesized, apically expressed, and vectorially released in both wild-type and Muc-1 null mice. When mucins were released, greater than 95% were directed to the apical compartment. Approximately half of the cell-associated mucins lost during a 24-h period were found in the apical compartment. Vectorial biotinylation detected apically disposed, cell-surface mucin and indicated that at least 34% of these mucins are released apically within 24 h. This suggests that release of mucin ectodomains is part of the mechanism of mucin removal from the apical cell surface of UEC. The half-lives of total cell-associated mucins and Muc-1 were 19.5 +/- 1 and 16.5 +/- 0.8 h, respectively. Muc-1 represented approximately 10% of the [3H]glucosamine-labeled, cell-associated mucins. Studies of the kinetics of intracellular transport of Muc-1 indicated transit times of 21 +/- 15 min from the rough endoplasmic reticulum to Golgi apparatus and 111 +/- 28 min from the Golgi apparatus to the cell surface. Collectively, these studies provide the first comprehensive description of Muc-1 and mucin maturation, metabolism, and release by polarized cells, as well as defining a major metabolic fate for mucins expressed by UEC. Normal metabolic processing appears to be sufficient to account for the removal of Muc-1 protein during the transition of UEC to a receptive state.

The PMT gene family: protein O-glycosylation in Saccharomyces cerevisiae is vital

The transfer of mannose to seryl and threonyl residues of secretory proteins is catalyzed by a family of protein mannosyltransferases coded for by seven genes (PMT1-7). Mannose dolichylphosphate is the sugar donor of the reaction, which is localized at the endoplasmic reticulum. By gene disruption and crosses all single, double and triple mutants of genes PMT1-4 were constructed. Two of the double and three of the triple mutants were not able to grow under normal conditions; three of these mutants could grow, however, when osmotically stabilized. The various mutants were extensively characterized concerning growth, morphology and their sensitivity to killer toxin K1, caffeine and calcofluor white. O-Mannosylation of gp115/Gas1p was affected only in pmt4 mutants, whereas glycosylation of chitinase was mainly affected in pmt1 and pmt2 mutants. The results show that protein O-glycosylation is essential for cell wall rigidity and cell integrity and that this protein modification, therefore, is vital for Saccharomyces cerevisiae.

Protein glycosylation in the endoplasmic reticulum and the Golgi apparatus and cell type-specificity of cell surface glycoconjugate expression: analysis by the protein A-gold and lectin-gold techniques

High resolution immunolabeling applying the protein A-gold technique and carbohydrate cytochemistry using lectin-gold labeling on Lowicryl K4M and thawed-frozen thin sections are most useful approaches for the detection of protein antigens and lectin binding sites in intracellular organelles and the plasma membrane. They provided the basis for modern electron microscopic studies on protein glycosylation reactions and the identification of their subcellular localization as reviewed here. These studies have demonstrated organelle subcompartments and the cell type-specific compartmentation of endoplasmic reticulum and Golgi apparatus-associated glycosylation reactions. The other subject reviewed in this paper is cell surface glycoconjugates, as they are expressed in relation to specific cell types present in various organs and during cellular differentiation processes.

Different fate of a single reporter protein containing KDEL or KKXX targeting signals stably expressed in mammalian cells

In mammalian cells, resident luminal and type I transmembrane proteins of the endoplasmic reticulum usually contain KDEL and KKXX at the carboxyl terminus. These sequences induce retrieval from compartments located downstream in the secretory pathway. It has been suggested that the retrieval may occur from multiple sites, ranging from the intermediate compartment to the trans-Golgi network. To compare the retrieval of luminal and type I membrane proteins, we have used different forms of a single reporter, the human CD8 glycoprotein, stably expressed in FRT cells. Metabolic labeling and oligosaccharide analysis show that the mechanism based on the KDEL signal is leaky. With time, the KDEL-containing CD8 form reaches the trans/trans-Golgi network compartments, where the protein is terminally glycosylated. At this stage, the retrieval mechanism stops being effective and the protein is consequently secreted. Conversely, the mechanism based on the KKXX signal guarantees that most of the KKXX-containing CD8 form resides in the endoplasmic reticulum, little in the Golgi complex and undetectable levels at the plasma membrane. The O-glycosylation of this protein comprises for the vast majority the sole addition of peptide-bound GalNAc that occurs in an early Golgi compartment.

T-cell-specific deletion of a polypeptide N-acetylgalactosaminyl-transferase gene by site-directed recombination

UDP-N-acetylgalactosamine (GalNAc): polypeptide N-acetylgalactosaminyltransferase (polypeptide GalNAc-T) catalyzes transfer of the monosaccharide GalNAc to serine and threonine residues, thereby initiating O-linked oligosaccharide biosynthesis. Previous studies have suggested the possibility of multiple polypeptide GalNAc-Ts, although attachment of saccharide units to polypeptide or lipid in generating oligosaccharide structures in vertebrates has been dependent upon the activity of single gene products. To address this issue and to determine the relevance of Oglycosylation variation in T-cell ontogeny, we have directed Cre/loxP mutagenic recombination to the polypeptide GalNAc-T locus in gene-targeted mice. Resulting deletion in the catalytic region of polypeptide GalNAc-T occurred to completion on both alleles in thymocytes and was found in peripheral T cells, but not among other cell types. Thymocyte O-linked oligosaccharide formation persisted in the absence of a functional targeted polypeptide GalNAc-T allele as determined by O-glycan-specific lectin binding. T-cell development and colonization of secondary lymphoid organs were also normal. These results indicate a complexity in vertebrate O-glycan biosynthesis that involves multiple polypeptide GalNAc-Ts. We infer the potential for protein-specific O-glycan formation governed by distinct polypeptide GalNAc-Ts.

Cloning and sequence homology of a rat UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase

A UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (polypeptide GalNAc transferase) cDNA was amplified from rat sublingual, submandibular and parotid glands, brain, skeletal muscle, and liver, using the polymerase chain reaction (PCR) and sequences derived from bovine polypeptide GalNAc transferase-Type 1 (polypeptide GalNAc transferase-T1). The transcripts encoding the rat sublingual gland and bovine enzymes were 91% identical in nucleotide sequence, except in their 5' and 3' untranslated regions. The enzymes encoded by the rat and bovine cDNAs were 559 amino acids in length and were virtually identical (98% amino acid sequence identity and 99.5% homologous overall). Northern blot analysis indicates that the polypeptide GalNAc transferase-T1 transcripts are expressed in many tissues but at widely differing levels. Although the amino acid sequence of polypeptide GalNAc transferase-T1 is conserved among mammals, the pattern of tissue expression varies between rats and humans. For example, the steady-state level of polypeptide GalNAc transferase-T1 transcript is quite low in lung relative to other rat tissues, whereas high expression of this transcript is detected in human lung. Therefore, we surmise that isoforms of polypeptide GalNAc transferase must exist and that isoforms are expressed in a tissue-dependent fashion. Searches of the GenBank database have revealed homologous sequences for several isoforms derived from several human tissues. In addition, hypothetical proteins from C. elegans also display strong homology; evidence suggests six ancestral isoforms of polypeptide GalNAc transferases may exist in C. elegans.

Evidence for recycling of the resident medial/trans Golgi enzyme, N-acetylglucosaminyltransferase I, in ldlD cells

ldlD cells, which lack the UDP-Gal/UDP-GalNAc 4-epimerase, were stably transfected with a Myc-tagged version of N-acetylglucosaminyltransferase I (Myc-Glc-NAc-T I). In the absence of GalNAc and Gal, newly synthesized GlcNAc-T I did not acquire O-linked oligosaccharides but was catalytically active and was transported to the Golgi region as defined using both immunofluorescence and immunoelectron microscopy. After addition of cycloheximide to prevent further synthesis, GalNAc and Gal were added, and the unglycosylated GlcNAc-T I was found to acquire mature, O-linked oligosaccharides with a half-time of about 150 min. The addition of these sugars was sensitive to N-ethylmaleimide and okadaic acid, both inhibitors of vesicle-mediated traffic. Together, these results suggest that Myc-Glc-NAc-T I undergoes retrograde transport to the early part of the Golgi apparatus where the first O-linked sugar, GalNAc, is added followed by anterograde transport back to the Golgi stack, where addition of Gal and sialic acid occurs.

UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase. Identification and separation of two distinct transferase activities

Using a defined acceptor substrate peptide as an affinity chromatography ligand we have developed a purification scheme for a unique human polypeptide, UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (GalNAc-transferase) (White, T., Bennett, E.P., Takio, K., Sørensen, T., Bonding, N., and Clausen, H. (1995) J. Biol. Chem. 270, 24156-24165). Here we report detailed studies of the acceptor substrate specificity of GalNAc-transferase purified by this scheme as well as the Gal-NAc-transferase activity, which, upon repeated affinity chromatography, evaded purification by this affinity ligand. Using a panel of acceptor peptides, a qualitative difference in specificity between these separated transferase activities in four rat organs and two human organs also revealed qualitative differences in specificity. The results support the existence of multiple Gal-NAc-transferase activities and suggest that these are differentially expressed in different organs. As the number of GalNAc-transferases existing is unknown, as is the specificity of the until now cloned and expressed GalNAc-transferases (T1 and T2), it is as yet impossible to relate the results obtained to specific enzyme proteins. The identification of acceptor peptides that can be used to discriminate GalNAc-transferase activities is an important step toward understanding the molecular basis of GalNAc O-linked glycosylation in cells and organs and in pathological conditions.

Targeting of protein ERGIC-53 to the ER/ERGIC/cis-Golgi recycling pathway

ERGIC-53 is a lectin-type membrane protein that continuously recycles between the ER, ER-Golgi intermediate compartment (ERGIC) and the cis-Golgi. To identify the targeting signals that mediate this recycling, N-glycosylated and myc-tagged variants of ERGIC-53 were constructed. By monitoring endoglycosidase H resistance, we measured the loss from the ER-ERGIC-cis-Golgi cycle of ERGIC-53. A domain exchange approach with the plasma membrane reporter protein CD4 showed that the transmembrane and the lumenal domains are not sufficient, while the cytoplasmic domain of ERGIC-53 is required and sufficient for pre-medial-Golgi localization. However, the ERGIC-53 cytoplasmic domain on CD4 lead to increased ER-staining by immunofluorescence microscopy indicating that this domain alone cannot provide for unbiased recycling through the ER-ERGIC-cis-Golgi compartments. Complete progress through the ER-ERGIC-cis-Golgi recycling pathway requires the cytoplasmic domain acting together with the lumenal domain of ERGIC-53. Dissection of the cytoplasmic domain revealed a COOH-terminal di-lysine ER-retrieval signal, KKFF, and an RSQQE targeting determinant adjacent to the transmembrane domain. Surprisingly, the two COOH-terminal phenylalanines influence the targeting. They reduce the ER-retrieval capacity of the di-lysine signal and modulate the RSQQE determinant.

Biosynthesis of GlyCAM-1, a mucin-like ligand for L-selectin

L-selectin, a member of the selectin family of leukocyte-endothelial adhesion proteins, mediates the initial attachment of lymphocytes to lymph node high endothelial venules during lymphocyte recirculation. One of the endothelial-associated ligands for L-selectin is GlyCAM-1, a mucin-like glycoprotein, which presents novel sulfated, sialylated and fucosylated O-glycans. In order to understand the generation of these glycans, we have examined the biosynthesis of GlyCAM-1 in lymph node organ culture. Using peptide-specific antibodies, lectins, and recombinant L-selectin, we detected the following species of GlyCAM-1: unglycosylated (< 28 kDa); modified with GalNAc only (28-33 kDa); modified with sialic acid, fucose, and sulfate but lacking L-selectin reactivity (40-50 kDa); and mature (L-selectin-reactive) ligand (50-60 kDa). Pulse-chase labeling at 15 degrees C suggested that GalNAc is added in a pre-Golgi compartment. Treatment with brefeldin A almost completely blocked sulfation, indicating that this modification occurs in the trans-Golgi network. Two distinct sialylation events occurred in the presence of brefeldin A, while fucosylation was partially blocked. We conclude that sialylation precedes both fucosylation and sulfation during biosynthesis. This ordering will help to identify the critical acceptor structures recognized by lymph node glycosyltransferases and sulfotransferases.

A C-terminally-anchored Golgi protein is inserted into the endoplasmic reticulum and then transported to the Golgi apparatus

Unlike conventional membrane proteins of the secretory pathway, proteins anchored to the cytoplasmic surface of membranes by hydrophobic sequences near their C termini follow a posttranslational, signal recognition particle-independent insertion pathway. Many such C-terminally-anchored proteins have restricted intracellular locations, but it is not known whether these proteins are targeted directly to the membranes in which they will ultimately reside. Here we have analyzed the intracellular sorting of the Golgi protein giantin, which consists of a rod-shaped 376-kDa cytoplasmic domain followed by a hydrophobic C-terminal anchor sequence. Unexpectedly, we find that giantin behaves like a conventional secretory protein in that it inserts into the endoplasmic reticulum (ER) and then is transported to the Golgi. A deletion mutant lacking a portion of the cytoplasmic domain adjacent to the membrane anchor still inserts into the ER but fails to reach the Golgi, even though this mutant has a stable folded structure. These findings suggest that the localization of a C-terminally-anchored Golgi protein involves at least three steps: insertion into the ER membrane, controlled incorporation into transport vesicles, and retention within the Golgi.