Distinct protein domains of the yeast Golgi GDP-mannose transporter mediate oligomer assembly and export from the endoplasmic reticulum

The nucleotide-sugar transporter family: a phylogenetic approach

Nucleotide sugar transporters (NST) establish the functional link of membrane transport between the nucleotide sugars synthesized in the cytoplasm and nucleus, and the glycosylation processes that take place in the endoplasmic reticulum (ER) and Golgi apparatus. The aim of the present work was to perform a phylogenetic analysis of 87 bank annotated protein sequences comprising all the NST so far characterized and their homologues retrieved by BLAST searches, as well as the closely related triose-phosphate translocator (TPT) plant family. NST were classified in three comprehensive families by linking them to the available experimental data. This enabled us to point out both the possible ER subcellular targeting of these transporters mediated by the dy-lysine motif and the substrate recognition mechanisms specific to each family as well as an important acceptor site motif, establishing the role of evolution in the functional properties of each NST family.

Progression of the stacked Golgi compartments in the yeast Saccharomyces cerevisiae by overproduction of GDP-mannose transporter

The Golgi compartments of the yeast Saccharomyces cerevisiaeare dispersed within the cytoplasm, in contrast to the stacked cisternae in the mammalian cell, and consequently are observed as a punctate pattern by immunofluorescent staining of Golgi-marker proteins. The VIG4/VRG4 gene encodes the essential yeast GDP-mannose transporter, which is a polytopic membrane protein in the early and medial Golgi compartments. Upon overexpression of this gene by the aid of a strong promoter and multicopy vector, we found that stacked multivesicular structures, which resembled the cisternae of mammalian Golgi apparatus, had developed in S. cerevisiae. Immuno-electron microscopy showed that the GDP-mannose transporter was located on the stacked cisternae. Immuno-isolation and immunoblotting analyses of the vesicles showed that the overproduced GDP-mannose transporter also co-localized with the Golgi glycosyltransferases, but not with the ER- or late Golgi-marker proteins as in the control cell. We propose that the localization mechanism of the GDP-mannose transporter in the Golgi compartment would be efficient and hardly saturable, and therefore the overproduced protein induced a progression of Golgi-like compartments rather than being mislocalized in other compartments, such as the ER or a vacuole.

Identification of a Candida glabrata homologue of the S. cerevisiae VRG4 gene, encoding the Golgi GDP-mannose transporter

Mannoproteins on the cell wall of yeast and fungi help regulate cell shape, porosity, and cell-cell interactions, including those required for attachment to host cells by fungal pathogens. The mannose-containing oligosaccharides on proteins and lipids are extended in the Golgi by glycosyltransferases that use GDP-mannose as the sugar substrate. A membrane-bound transporter that, in Saccharomyces cerevisiae, is encoded by the VRG4 gene catalyses delivery of GDP-mannose into the lumen of the Golgi. We report here the cloning of the homologous VRG4 gene from the pathogenic yeast, Candida glabrata, by functional complementation of an S. cerevisiae vrg4 mutant. The sequence of the CgVrg4 protein displays significant homology to GDP-mannose transporters from other yeast, fungi, protozoa, and plants. CgVRG4 fully complements the glycosylation defect and other cell wall associated vrg4 mutant phenotypes. Like ScVRG4, CgVRG4 is essential for the viability of C. glabrata. These results suggest that, as in S. cerevisiae, CgVrg4p accounts for all of the GDP-mannose transport activity in the Golgi lumen.

Quaternary structure and function of transport proteins

Membranes are important sites for the regulation of metabolic functions because they contain transport molecules, which often catalyze the first step in a pathway, and signal-transduction components, which allow the cell to communicate with the environment. Given the catalytic importance of transport proteins and their role in membrane stability, it is possible that oligomerization is used to regulate their function. This review evaluates knowledge of the functions that are associated with the oligomeric organization of secondary transport proteins, which are a major class of solute-translocation systems in all living species.

Molecular and phenotypic analysis of CaVRG4, encoding an essential Golgi apparatus GDP-mannose transporter

Cell surface mannan is implicated in almost every aspect of pathogenicity of Candida albicans. In Saccharomyces cerevisiae, the Vrg4 protein acts as a master regulator of mannan synthesis through its role in substrate provision. The substrate for mannosylation of proteins and lipids in the Golgi apparatus is GDP-mannose, whose lumenal transport is catalyzed by Vrg4p. This nucleotide sugar is synthesized in the cytoplasm by pathways that are highly conserved in all eukaryotes, but its lumenal transport (and hence Golgi apparatus-specific mannosylation) is a fungus-specific process. To begin to study the role of Golgi mannosylation in C. albicans, we isolated the CaVRG4 gene and analyzed the effects of loss of its function. CaVRG4 encodes a functional homologue of the S. cerevisiae GDP-mannose transporter. CaVrg4p localized to punctate spots within the cytoplasm of C. albicans in a pattern reminiscent of localization of Vrg4p in the Golgi apparatus in S. cerevisiae. Like partial loss of ScVRG4 function, partial loss of CaVRG4 function resulted in mannosylation defects, which in turn led to a number of cell wall-associated phenotypes. While heterozygotes displayed no growth phenotypes, a hemizygous strain, containing a single copy of CaVRG4 under control of the methionine-repressible MET3 promoter, did not grow in the presence of methionine and cysteine, demonstrating that CaVRG4 is essential for viability. Mutant Candida vrg4 strains were defective in hyphal formation but exhibited a constitutive polarized mode of pseudohyphal growth. Because the VRG4 gene is essential for yeast viability but does not have a mammalian homologue, it is a particularly attractive target for development of antifungal therapies.

An acidic sequence of a putative yeast Golgi membrane protein binds COPII and facilitates ER export

We previously identified Sys1p as a high copy number suppressor of Ypt6 GTPase-deficient yeast mutants that are defective in endosome-to-Golgi transport. Here, we show that Sys1p is an integral membrane protein that resides on a post-endoplasmic reticulum (ER) organelle(s). Affinity studies with detergent- solubilized yeast proteins showed that the C-terminal 53 amino acid tail of Sys1p binds effectively to the cytoplasmic Sec23p-Sec24p COPII subcomplex. This binding required a di-acidic Asp-Leu-Glu (DXE) motif, previously shown to mediate efficient ER export of the vesicular stomatitis virus glycoprotein in mammalian cells. In Sys1p, a Glu-Leu-Glu (EXE) sequence could not substitute for the (DXE) motif. Mutations of the (DXE) sequence resulted in ER retention of approximately 30% of the protein at steady state, whereas addition of the Sys1p tail to an ER-resident membrane protein led to an intracellular redistribution of the chimeric protein. Our study demonstrates for the first time that, in yeast, a di-acidic sequence motif can act as a sorting signal for cargo selection during the formation of transport vesicles at the ER by direct binding to COPII component(s).

A Gip1p-Glc7p phosphatase complex regulates septin organization and spore wall formation

Sporulation of Saccharomyces cerevisiae is a developmental process in which a single cell is converted into four haploid spores. GIP1, encoding a developmentally regulated protein phosphatase 1 interacting protein, is required for spore formation. Here we show that GIP1 and the protein phosphatase 1 encoded by GLC7 play essential roles in spore development. The gip1Delta mutant undergoes meiosis and prospore membrane formation normally, but is specifically defective in spore wall synthesis. We demonstrate that in wild-type cells, distinct layers of the spore wall are deposited in a specific temporal order, and that gip1Delta cells display a discrete arrest at the onset of spore wall deposition. Localization studies revealed that Gip1p and Glc7p colocalize with the septins in structures underlying the growing prospore membranes. Interestingly, in the gip1Delta mutant, not only is Glc7p localization altered, but septins are also delocalized. Similar phenotypes were observed in a glc7-136 mutant, which expresses a Glc7p defective in interacting with Gip1p. These results indicate that a Gip1p-Glc7p phosphatase complex is required for proper septin organization and initiation of spore wall formation during sporulation.

Nucleotide sugar transporters: biological and functional aspects

The Golgi apparatus serves as the major site of glycosylation reactions. Nucleotide sugars which are substrates of the Golgi localized glycosyltransferases are synthesized in the cytoplasm (cell nucleus in case of CMP-sialic acid) and must be transported into the compartment lumen. This transport function is carried out by nucleotide sugar transporters. The first genes were cloned in the year 1996 and revealed a family of structurally conserved multi-transmembrane-spanning proteins. Due to the high structural and functional conservation, the identification of many putative nucleotide sugar transporter sequences has become possible in the existing gene data bases and accelerates the increase in knowledge on structure-function-relationships. Recent developments in the nucleotide sugar transporter field are discussed in this article.

A genomic view of yeast membrane transporters

The yeast membrane transporters play crucial roles in functions as diverse as nutrient uptake, drug resistance, salt tolerance, control of cell volume, efflux of undesirable metabolites and sensing of extracellular nutrients. A significant fraction of the many transporters inventoried after sequencing of the yeast genome has been characterised by classical experimental approaches. Post-genomic analysis has allowed a more extensive characterisation of transporter categories less tractable by genetics, for instance of transporters of intracellular membranes or transporters encoded by multigene families and displaying overlapping substrate specificities. A complete view of the role of membrane transporters in the metabolism of yeast may not be far off.

Point mutations identified in Lec8 Chinese hamster ovary glycosylation mutants that inactivate both the UDP-galactose and CMP-sialic acid transporters

Nucleotide-sugar transporters (NSTs) are critical components of glycosylation pathways in eukaryotes. The identification of structural elements that are involved in NST functions provides an important task. Chinese hamster ovary glycosylation mutants defective in nucleotide-sugar transport provide access to inactive transporters that can define such structure/function relationships. In this study, we have cloned the hamster UDP-galactose transporter (UGT) and identified defects in UGT gene transcripts from nine independent Chinese hamster ovary mutants that belong to the Lec8 complementation group. Reverse transcription polymerase chain reaction with primers that span the UGT open reading frame showed that three Lec8 mutants express a full-length open reading frame, while six Lec8 mutants predominantly express truncated UGT gene transcripts. Sequencing identified different single or triplet nucleotide changes in full-length UGT transcripts from three of the mutants. These mutations translate into three different amino acid changes at positions that are highly conserved in all the known mammalian NSTs. Transfection of a cDNA encoding either of the mutations Delta serine 213 or G281D failed to correct the UDP-galactose transport defect in Lec8 transfectants. Most importantly, introducing these same mutations into the homologous region of the murine CMP-sialic acid transporter caused inactivation of this transporter. Thus, identifying point mutations that inactivate UGT in Lec8 mutants resulted in the discovery of amino acids that are critical to the activity of both UGT and CST, the two most divergent mammalian NSTs.

Functional characterization of Gms1p/UDP-galactose transporter in Schizosaccharomyces pombe

Galactosylation of glycoproteins in the fission yeast Schizosaccharomyces pombe requires the transport of UDP-galactose as substrate for the galactosyltransferase into the lumen of the Golgi apparatus, which is achieved by the UDP-galactose transporter. We isolated a mutant (gms1) that is deficient in galactosylation of cell surface glycoproteins in Sz.pombe, and found that the gms1(+) gene encodes a UDP-galactose transporter. In the prediction of secondary structure of the Gms1 protein, an eight-membrane-spanning structure was obtained. Fluorescent microscopy revealed the functional Gms1-GFP fusion protein to be stably localized at the Golgi membrane. Sequencing analysis of the coding region of Gms1p derived from galactosylation-defective mutants identified a single amino acid mutation (A102T or A258E) located within the putative transmembrane region, helix 2 or helix 7, respectively. The mutagenized Gms1(A102T or A258E)p exhibited loss of UDP-galactose transport activity but no change in the localization to the Golgi membrane. The C-terminal truncated Gms1p mutants demonstrated that the C-terminal hydrophilic region was dispensable for targeting and function as UDP-galactose transporter at the Golgi membrane. We suggest that the putative eighth (the most C-terminus-proximal) transmembrane helix of Gms1p is critical to targeting from ER to the Golgi membrane.

The gene defective in leukocyte adhesion deficiency II encodes a putative GDP-fucose transporter

Leukocyte adhesion deficiency II (LAD II) is characterized by the lack of fucosylated glycoconjugates, including selectin ligands, causing immunodeficiency and severe mental and growth retardation. No deficiency in fucosyltransferase activities or in the activities of enzymes involved in GDP-fucose biosynthesis has been found. Instead, the transport of GDP-fucose into isolated Golgi vesicles of LAD II cells appeared to be reduced. To identify the gene mutated in LAD II, we cloned 12 cDNAs from Caenorhabditis elegans, encoding multi-spanning transmembrane proteins with homology to known nucleotide sugar transporters, and transfected them into fibroblasts from an LAD II patient. One of these clones re-established expression of fucosylated glycoconjugates with high efficiency and allowed us to identify a human homolog with 55% identity, which also directed re-expression of fucosylated glycoconjugates. Both proteins were localized to the Golgi. The corresponding endogenous protein in LAD II cells had an R147C amino acid change in the conserved fourth transmembrane region. Overexpression of this mutant protein in cells from a patient with LAD II did not rescue fucosylation, demonstrating that the point mutation affected the activity of the protein. Thus, we have identified the first putative GDP-fucose transporter, which has been highly conserved throughout evolution. A point mutation in its gene is responsible for the disease in this patient with LAD II.

Identification of a conserved motif in the yeast golgi GDP-mannose transporter required for binding to nucleotide sugar

Glycoproteins and lipids in the Golgi complex are modified by the addition of sugars. In the yeast Saccharomyces cerevisiae, these terminal Golgi carbohydrate modifications primarily involve mannose additions that utilize GDP-mannose as the substrate. The transport of GDP-mannose from its site of synthesis in the cytosol into the lumen of the Golgi is mediated by the VRG4 gene product, a nucleotide sugar transporter that is a member of a large family of related membrane proteins. Loss of VRG4 function leads to lethality, but several viable vrg4 mutants were isolated whose GDP-mannose transport activity was reduced but not obliterated. Mutations in these alleles mapped to a region of the Vrg4 protein that is highly conserved among other GDP-mannose transporters but not other types of nucleotide sugar transporters. Here, we present evidence that suggest an involvement of this region of the protein in binding GDP-mannose. Most of the mutations that were introduced within this conserved domain, spanning amino acids 280-291 of Vrg4p, lead to lethality, and none interfere with Vrg4 protein stability, localization, or dimer formation. The null phenotype of these mutant vrg4 alleles can be complemented by their overexpression. Vesicles prepared from vrg4 mutant strains were reduced in luminal GDP-mannose transport activity, but this effect could be suppressed by increasing the concentration of GDP-mannose in vitro. Thus, either an increased substrate concentration, in vitro, or an increased Vrg4 protein concentration, in vivo, can suppress these vrg4 mutant phenotypes. Vrg4 proteins with alterations in this region were reduced in binding to guanosine 5'-[gamma-(32)P]triphosphate gamma-azidoanilide, a photoaffinity substrate analogue whose binding to Vrg4-HAp was specifically inhibited by GDP-mannose. Taken together, these data are consistent with the model that amino acids in this region of the yeast GDP-mannose transporter mediate the recognition of or binding to nucleotide sugar prior to its transport into the Golgi.

Disulfide bonds of GM2 synthase homodimers. Antiparallel orientation of the catalytic domains

GM2 synthase is a homodimer in which the subunits are joined by lumenal domain disulfide bond(s). To define the disulfide bond pattern of this enzyme, we analyzed a soluble form by chemical fragmentation, enzymatic digestion, and mass spectrometry and a full-length form by site-directed mutagenesis. All Cys residues of the lumenal domain of GM2 synthase are disulfide bonded with Cys(429) and Cys(476) forming a disulfide-bonded pair while Cys(80) and Cys(82) are disulfide bonded in combination with Cys(412) and Cys(529). Partial reduction to produce monomers converted Cys(80) and Cys(82) to free thiols while the Cys(429) to Cys(476) disulfide remained intact. CNBr cleavage at amino acid 330 produced a monomer-sized band under nonreducing conditions which was converted upon reduction to a 40-kDa fragment and a 24-kDa myc-positive fragment. Double mutation of Cys(80) and Cys(82) to Ser produced monomers but not dimers. In summary these results demonstrate that Cys(429) and Cys(476) form an intrasubunit disulfide while the intersubunit disulfides formed by both Cys(80) and Cys(82) with Cys(412) and Cys(529) are responsible for formation of the homodimer. This disulfide bond arrangement results in an antiparallel orientation of the catalytic domains of the GM2 synthase homodimer.

Nucleotide sugar transporters of the Golgi apparatus

Glycosylation, sulfation and phosphorylation of proteins, proteoglycans and lipids occur in the lumen of the Golgi apparatus. The nucleotide substrates of these reactions must be first transported from the cytosol into the Golgi lumen by specific transporters. The topology and structure of these hydrophobic, multi-transmembrane-spanning proteins are beginning to be understood.