O-glycosylation of the coronavirus M protein. Differential localization of sialyltransferases in N- and O-linked glycosylation

Functional characterization of the YUR1, KTR1, and KTR2 genes as members of the yeast KRE2/MNT1 mannosyltransferase gene family

Eukaryotic glycan structures are progressively elaborated in the secretory pathway. Following the addition of a core N-linked carbohydrate in the endoplasmic reticulum, glycoproteins move to the Golgi complex where the elongation of O-linked sugar chains and processing of complex N-linked oligosaccharide structures take place. In order to better define how such post-translational modifications occur, we have been studying a yeast gene family in which at least one member, KRE2/MNT1, is involved in protein glycosylation. The family currently contains five other members: YUR1, KTR1, KTR2, KTR3 and KTR4 (Mallet, L., Bussereau, F., and Jacquet, M. (1994) Yeast 10, 819-831). All encode putative type II membrane proteins with a short cytoplasmic N terminus, a membrane-spanning region, and a highly conserved catalytic lumenal domain. Kre2p/Mnt1p is a alpha 1,2-mannosyltransferase involved in O- and N-linked glycosylation (Häusler, A., Ballou, L., Ballou, C.E., and Robbins, P.W. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 6846-6850); however, the role of the other proteins has not yet been established. We have carried out a functional analysis of Ktr1p, Ktr2p, and Yur1p. By in vitro assays, Ktr1p, Ktr2p, and Yur1p have been shown to be mannosyltransferase but, in vivo, do not appear to be involved in O-glycosylation. Examination of the electrophoretic mobility of the N-linked modified protein invertase in null mutant strains indicates that Ktr1p, Ktr2p, and Yur1p are involved in N-linked glycosylation, possibly as redundant enzymes. As found with Kre2p (Hill, K., Boone, C., Goebl, M., Puccia, R., Sdicu, A.-M., and Bussey, H. (1992) Genetics 130, 273-283), Ktr1p, Ktr2p, and Yur1p also seem to be implicated in the glycosylation of cell wall mannoproteins, since yeast cells containing different gene disruptions become K1 killer toxin-resistant. Immunofluorescence microscopy reveals that like Kre2p; Ktr1p, Ktr2p and Yur1p are localized in the Golgi complex.

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.

Oligomerization of a trans-Golgi/trans-Golgi network retained protein occurs in the Golgi complex and may be part of its retention

The mouse hepatitis virus M protein is a triple spanning membrane glycoprotein that, when expressed independently, localizes to trans-Golgi as well as to the trans-Golgi network (TGN). Passage of this protein from the endoplasmic reticulum through the intermediate compartment to the late Golgi and TGN can be conveniently followed by analyzing its O-linked sugars. Using pulse-chase analyses we studied the oligomerization of the M protein in sucrose gradients. The Golgi and TGN forms migrated as large heterogeneous complexes, whereas the endoplasmic reticulum and intermediate compartment forms of the protein appeared to migrate as monomer. Moreover, a mutant of the M protein lacking the 22 COOH-terminal amino acids, that is transported to the plasma membrane, gave rise to similar complexes, albeit smaller in size, that persisted at the plasma membrane. We propose that the trans-Golgi/TGN retention of the MHV-M protein is governed by two mechanisms: oligomerization possibly mediated by the transmembrane domains and binding of its cytoplasmic tail to cellular factors in trans Golgi/TGN.

Protein O-glycosylation in yeast. The PMT2 gene specifies a second protein O-mannosyltransferase that functions in addition to the PMT1-encoded activity

The PMT2 gene from Saccharomyces cerevisiae was identified as FUN25, a transcribed open reading frame on the left arm of chromosome I (Ouellette, B. F. F., Clark, M. W. C., Keng, T., Storms, R. G., Zhong, W., Zeng, B., Fortin, N., Delaney, S., Barton, A., Kaback, D.B., and Bussey, H. (1993) Genome 36, 32-42). The product encoded by the PMT2 gene shows significant similarity with the dolichyl phosphate-D-mannose:protein O-D-mannosyltransferase, Pmt1p (EC 2.4.1.109), which is required for initiating the assembly of O-linked oligosaccharides in S. cerevisiae (Strahl-Bolsinger, S., Immervoll, T., Deutzmann, R., and Tanner, W. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 8164-8168). The PMT2 gene encodes a new protein O-D-mannosyltransferase. Yeast cells carrying a PMT2 disruption show a diminished in vitro and in vivo O-mannosylation activity and resemble mutants with a nonfunctional PMT1 gene. Strains bearing a pmt1 pmt2 double disruption show a severe growth defect but retain residual O-mannosylation activity indicating the presence of at least one more protein-O-mannosyltransferase.

Coexpression and association of the spike protein and the membrane protein of mouse hepatitis virus

The M and S envelope glycoproteins of mouse hepatitis virus associate in the process of virus assembly. We have studied the intrinsic properties of M/S heterocomplexes by coexpressing M and S in the absence of other coronaviral proteins. The formation of M/S complexes under these conditions indicates that M and S can interact independently of other coronaviral factors. Pulse-chase analysis revealed that M and S associate in a pre-Golgi compartment. M/S complexes are efficiently transported beyond the coronavirus budding compartment to the Golgi complex. The failure to detect complexes at the surface of coexpressing cells demonstrated that they are retained intracellularly. Thus, coexpression of the envelope glycoproteins drastically affects the intracellular transport of the S protein: instead of being transported to the cell surface, S is retained intracellularly by its association with M.

Targeting of proteins to the Golgi apparatus

The Golgi apparatus maintains a highly organized structure in spite of the intense membrane traffic which flows into and out of this organelle. Resident Golgi proteins must have localization signals to ensure that they are targeted to the correct Golgi compartment and not swept further along the secretory pathway. There are a number of distinct groups of Golgi membrane proteins, including glycosyltransferases, recycling trans-Golgi network proteins, peripheral membrane proteins, receptors and viral glycoproteins. Recent studies indicate that there are a number of different Golgi localization signals and mechanisms for retaining proteins to the Golgi apparatus. This review focuses on the current knowledge in this field.

Complex formation between the spike protein and the membrane protein during mouse hepatitis virus assembly

Using different approaches we have demonstrated the formation of a complex between the S protein and the M protein in the process of mouse hepatitis virus (MHV) assembly. Preservation of the M/S heterocomplexes was critically dependent on the solubilization conditions. Pulse-chase labeling of MHV-infected cells followed by a co-immunoprecipitation assay revealed that newly synthesized S and M engage in complex formation with different kinetics, the S protein reacting much slower. Sedimentation experiments showed the M/S heteromultimer complexes to be very large. A model for the role of the complex formation in MHV assembly is presented.

Use of recombinant vaccinia virus vectors for cell biology

This chapter describes the use of several of the recombinant vaccinia expression systems, focuses on the systems that are most useful for cell biologists, and discusses their advantages and limitations. Vaccinia-mediated expression can be used for assessing cellular localization, posttranslational modifications, oligomerization, and transport and turnover rates. The system provides a rapid method for screening mutant proteins for expression and targeting. It is an excellent way of quickly deciding which mutant proteins might be worth further studying using stable expression systems. Expression of foreign genes using Vaccinia virus is based on recombinant viruses constructed by insertion of complementary DNA (cDNA) into the nonessential thymidine kinase (TK) gene. Both direct and indirect methods of expression are possible. The foreign gene can be inserted into the vaccinia genome by homologous recombination using a plasmid with flanking regions of vaccinia DNA. The recombinant virus is selected, expanded, and used to infect cells, which then express high levels of the foreign protein. Recombinant vaccinia viruses are generated by subcloning the foreign gene into a plasmid transfer vector so it is flanked by DNA from the vaccinia (TK) gene, which is nonessential for growth of the virus in tissue culture. This plasmid is then transfected into vaccinia-infected cells. Homologous recombination of the plasmid and the vaccinia genome generates a recombinant virus with an inactive TK gene.

Molecular biology of rubella virus

This chapter summarizes the present medical significance of rubella virus. Rubella virus infection is systemic in nature and the accompanying symptoms are generally benign, the most pronounced being a mild rash of short duration. The most common complication of rubella virus infection is transient joint involvement such as polyarthralgia and arthritis. The primary health impact of rubella virus is that it is a teratogenic agent. The vaccination strategy is aimed at elimination of rubella and includes both universal vaccination of infants at 15 months of age with the trivalent measles, mumps, rubella (MMR) vaccine and specific targeting with the rubella vaccine of seronegative women planning pregnancy and seronegative adults who could come in contact with women of childbearing age, although it is recommended that any individual over the age of 12 months without evidence of natural infection or vaccination be vaccinated. Medically, the current challenge posed by rubella virus is to achieve complete vaccination coverage to prevent resurgences.

Folding of the mouse hepatitis virus spike protein and its association with the membrane protein

Coronaviruses are assembled by budding into pre-Golgi membranes. Using different approaches we have demonstrated that the spike (S) protein and the membrane (M) protein of mouse hepatitis virus (MHV) associate to form large complexes. Newly synthesized M was found in these complexes almost immediately after its synthesis, whereas the S protein started to appear in heterocomplexes after 10-20 min. This is consistent with the slow rate of folding of S and with the observation that folding of S preceeds its association with M. While the folding of S involves the formation of multiple disulfide bonds, folding of M is disulfide-independent. This contrast was reflected by the differential sensitivity of the two proteins to reduction with dithiothreitol (DTT). Addition of DTT to the culture medium of MHV-infected cells drastically impaired the folding of S, but not of M. Consequently, the S protein was unable to interact with M. Under these conditions, S stayed in the ER while M was transported efficiently beyond the site of budding to the Golgi complex. We conclude that the association of S with M is an essential step in the formation of the viral envelope and in the accumulation of both proteins at the site of virus assembly.

Disulfide bonds in folding and transport of mouse hepatitis coronavirus glycoproteins

We have analyzed the effects of reducing conditions on the folding of the spike (S) protein and on the intracellular transport of the membrane (M) protein of the mouse hepatitis coronavirus. These proteins differ in their potential to form disulfide bonds in the lumen of the endoplasmic reticulum (ER). Intrachain disulfide bonds are formed in the S protein but not in M, which was demonstrated in a pulse-chase experiment by analyzing the viral proteins under nonreducing conditions. To reduce disulfide bonds in vivo, we added dithiothreitol (DTT) to the culture medium of mouse hepatitis coronavirus-infected cells following a procedure recently described by Braakman et al. (I. Braakman, J. Helenius, and A. Helenius, EMBO J. 11:1717-1722, 1992). Short exposure to DTT resulted in the complete reduction of newly synthesized S protein and affected its conformation as judged by the change in mobility in nonreducing gels and by the loss of recognition by a conformation-specific monoclonal antibody. Using this antibody in an immunofluorescence assay, we monitored the reducing effect of DTT in situ. DTT was found to initially affect only the S protein present in the ER; also, after longer treatment, the remaining signal also gradually disappeared. In contrast, folding and transport of the M protein were not inhibited by DTT. Under reducing conditions, M was transported efficiently to the trans side of the Golgi complex, indicating that cellular processes such as ER-to-Golgi transport, O-glycosylation, and Golgi retention were unaffected. In the presence of DTT, the M protein even moved at an increased rate to the Golgi complex, which is probably because of its failure to interact with unfolded S protein. The effects of in vivo reduction were reversible. When DTT was removed from pulse-labeled cells, the S protein folded posttranslationally and aberrantly; during its oxidation, most of S now transiently aggregated into large disulfide-linked complexes from which subsequently folded S molecules dissociated.

Targeting and retention of Golgi membrane proteins

Recent cloning of genes encoding membrane proteins of the Golgi complex has allowed investigation of protein targeting to this organelle. Targeting signals have been identified in three glycosyltransferases, a viral envelope protein and several proteins of the trans-Golgi network. Interestingly, the targeting signals for membrane proteins of the Golgi stacks seem to be contained in transmembrane domains. Information in the cytoplasmic tails is required for the targeting of trans-Golgi network proteins. Mechanisms involving both retention and retrieval have been invoked.

Targeting of viral glycoproteins to the Golgi complex

Certain enveloped viruses are known to assemble on membranes of the Golgi complex. Intracellular budding is facilitated by targeting of the viral glycoproteins to this organelle. It is likely that these viral glycoproteins are retained in the Golgi by the same means as are endogenous Golgi proteins. Consequently, the study of Golgi-specific viral proteins has provided important clues to the nature of Golgi retention signals.

Membrane assembly of the triple-spanning coronavirus M protein. Individual transmembrane domains show preferred orientation

The M protein of mouse hepatitis virus strain A59 is a triple-spanning membrane protein which assembles with an uncleaved internal signal sequence, adopting an NexoCcyt orientation. To study the insertion mechanism of this protein, domains potentially involved in topogenesis were deleted and the effects analyzed in topogenesis were deleted and the effects analyzed in several ways. Mutant proteins were synthesized in a cell-free translation system in the presence of microsomal membranes, and their integration and topology were determined by alkaline extraction and by protease-protection experiments. By expression in COS-1 and Madin-Darby canine kidney-II cells, the topology of the mutant proteins was also analyzed in vivo. Glycosylation was used as a biochemical marker to assess the disposition of the NH2 terminus. An indirect immunofluorescence assay on semi-intact Madin-Darby canine kidney-II cells using domain-specific antibodies served to identify the cytoplasmically exposed domains. The results show that each membrane-spanning domain acts independently as an insertion and anchor signal and adopts an intrinsic preferred orientation in the lipid bilayer which corresponds to the disposition of the transmembrane domain in the wild-type assembled protein. These observations provide further insight into the mechanism of membrane integration of multispanning proteins. A model for the insertion of the coronavirus M protein is proposed.

Membrane assembly of the triple-spanning coronavirus M protein. Individual transmembrane domains show preferred orientation

The M protein of mouse hepatitis virus strain A59 is a triple-spanning membrane protein which assembles with an uncleaved internal signal sequence, adopting an NexoCcyt orientation. To study the insertion mechanism of this protein, domains potentially involved in topogenesis were deleted and the effects analyzed in topogenesis were deleted and the effects analyzed in several ways. Mutant proteins were synthesized in a cell-free translation system in the presence of microsomal membranes, and their integration and topology were determined by alkaline extraction and by protease-protection experiments. By expression in COS-1 and Madin-Darby canine kidney-II cells, the topology of the mutant proteins was also analyzed in vivo. Glycosylation was used as a biochemical marker to assess the disposition of the NH2 terminus. An indirect immunofluorescence assay on semi-intact Madin-Darby canine kidney-II cells using domain-specific antibodies served to identify the cytoplasmically exposed domains. The results show that each membrane-spanning domain acts independently as an insertion and anchor signal and adopts an intrinsic preferred orientation in the lipid bilayer which corresponds to the disposition of the transmembrane domain in the wild-type assembled protein. These observations provide further insight into the mechanism of membrane integration of multispanning proteins. A model for the insertion of the coronavirus M protein is proposed.