Purification of the Golgi adenosine 3'-phosphate 5'-phosphosulfate transporter, a homodimer within the membrane

Gateway to the Golgi: molecular mechanisms of nucleotide sugar transporters

The Golgi apparatus plays a central role in the secretory pathway as a hub for posttranslational modification, protein sorting and quality control. To date, there is little structural or biochemical information concerning the function of transporters that reside within this organelle. The SLC35 family of nucleotide sugar transporters link the synthesis of activated sugar molecules and sulfate in the cytoplasm, with the luminal transferases that catalyse their attachment to proteins and lipids during glycosylation and sulfation. A recent crystal structure of the GDP-mannose transporter has revealed key sequence motifs that direct ligand recognition and transport. Further biochemical studies unexpectedly found a requirement for short chain lipids in activating the transporter, suggesting a possible route for transport regulation within the Golgi.

My journey in the discovery of nucleotide sugar transporters of the Golgi apparatus

Defects in protein glycosylation can have a dramatic impact on eukaryotic cells and is associated with mental and developmental pathologies in humans. The studies outlined below illustrate how a basic biochemical problem in the mechanisms of protein glycosylation, specifically substrate transporters of nucleotide sugars, including ATP and 3'-phosphoadenyl-5'-phosphosulfate (PAPS), in the membrane of the Golgi apparatus and endoplasmic reticulum, expanded into diverse biological systems from mammals, including humans, to yeast, roundworms, and protozoa. Using these diverse model systems allowed my colleagues and me to answer fundamental biological questions that enabled us to formulate far-reaching hypotheses and expanded our knowledge of human diseases caused by malfunctions in the metabolic processes involved.

Overview of Nucleotide Sugar Transporter Gene Family Functions Across Multiple Species

Glycoproteins and glycolipids are crucial in a number of cellular processes, such as growth, development, and responses to external cues, among others. Polysaccharides, another class of sugar-containing molecules, also play important structural and signaling roles in the extracellular matrix. The additions of glycans to proteins and lipids, as well as polysaccharide synthesis, are processes that primarily occur in the Golgi apparatus, and the substrates used in this biosynthetic process are nucleotide sugars. These proteins, lipids, and polysaccharides are also modified by the addition of sulfate groups in the Golgi apparatus in a series of reactions where nucleotide sulfate is needed. The required nucleotide sugar substrates are mainly synthesized in the cytosol and transported into the Golgi apparatus by nucleotide sugar transporters (NSTs), which can additionally transport nucleotide sulfate. Due to the critical role of NSTs in eukaryotic organisms, any malfunction of these could change glycan and polysaccharide structures, thus affecting function and altering organism physiology. For example, mutations or deletion on NST genes lead to pathological conditions in humans or alter cell walls in plants. In recent years, many NSTs have been identified and functionally characterized, but several remain unanalyzed. This study examined existing information on functionally characterized NSTs and conducted a phylogenetic analysis of 257 NSTs predicted from nine animal and plant model species, as well as from protists and fungi. From this analysis, relationships between substrate specificity and the primary NST structure can be inferred, thereby advancing understandings of nucleotide sugar gene family functions across multiple species.

PAPST1 regulates sulfation of heparan sulfate proteoglycans in epithelial MDCK II cells

Proteoglycan (PG) sulfation depends on activated nucleotide sulfate, 3'-phosphoadenosine-5'-phosphosulfate (PAPS). Transporters in the Golgi membrane translocate PAPS from the cytoplasm into the organelle lumen where PG sulfation occurs. Silencing of PAPS transporter (PAPST) 1 in epithelial MDCK cells reduced PAPS uptake into Golgi vesicles. Surprisingly, at the same time sulfation of heparan sulfate (HS) was stimulated. The effect was pathway specific in polarized epithelial cells. Basolaterally secreted proteoglycans (PGs) displayed an altered HS sulfation pattern and increased growth factor binding capacity. In contrast, the sulfation pattern of apically secreted PGs was unchanged while the secretion was reduced. Regulation of PAPST1 allows epithelial cells to prioritize between PG sulfation in the apical and basolateral secretory routes at the level of the Golgi apparatus. This provides sulfation patterns that ensure PG functions at the extracellular level, such as growth factor binding.

Deficient import of acetyl-CoA into the ER lumen causes neurodegeneration and propensity to infections, inflammation, and cancer

The import of acetyl-CoA into the ER lumen by AT-1/SLC33A1 is essential for the N(ε)-lysine acetylation of ER-resident and ER-transiting proteins. A point-mutation (S113R) in AT-1 has been associated with a familial form of spastic paraplegia. Here, we report that AT-1S113R is unable to form homodimers in the ER membrane and is devoid of acetyl-CoA transport activity. The reduced influx of acetyl-CoA into the ER lumen results in reduced acetylation of ER proteins and an aberrant form of autophagy. Mice homozygous for the mutation display early developmental arrest. In contrast, heterozygous animals develop to full term, but display neurodegeneration and propensity to infections, inflammation, and cancer. The immune and cancer phenotypes are contingent on the presence of pathogens in the colony, whereas the nervous system phenotype is not. In conclusion, our results reveal a previously unknown aspect of acetyl-CoA metabolism that affects the immune and nervous systems and the risk for malignancies.

Roles of the nucleotide sugar transporters (SLC35 family) in health and disease

Nucleotide sugars and adenosine 3'-phospho 5'-phosphosulfate (PAPS) are transported from the cytosol to the endoplasmic reticulum (ER) and the Golgi apparatus where they serve as substrates for the glycosylation and sulfation of proteins, lipids and proteoglycans. The translocation is accomplished by the nucleotide sugar transporters (NSTs), a family of highly conserved hydrophobic proteins with multiple transmembrane domains that are part of the solute carrier family 35 (SLC35). NSTs are antiporters responsible not only for transporting nucleotide sugars and PAPS into the Golgi, but also for the transport of the reaction products back to the cytosol. The initial reaction products - the nucleoside diphosphates - must be first converted to nucleoside monophosphates by a group of enzymes called ectonucleoside triphosphate diphosphohydrolases (ENTPDs) before they can exit the Golgi. The transport role of NSTs is essential to glycosylation and development. Mutations in two NST genes, SLC35A1 and SLC35C1, have been related to congenital disorder of glycosylation II (CDG II).

Balancing metabolites in drought: the sulfur assimilation conundrum

A key plant response to drought is the accumulation of specific sets of metabolites that act as osmoprotectants, osmolytes, antioxidants, and/or stress signals. An emerging question is: how do plants regulate metabolism to balance the 'competing interests' between metabolites during stress? Recent research connects primary sulfur metabolism (e.g., sulfate transport in the vasculature, its assimilation in leaves, and the recycling of sulfur-containing compounds) with the drought stress response. In this review, we highlight key steps in sulfur metabolism that play significant roles in drought stress signaling and responses. We propose that a complex balancing act is required to coordinate primary and secondary sulfur metabolism during the drought stress response in plants.

Control of sulfur partitioning between primary and secondary metabolism in Arabidopsis

Sulfur is an essential nutrient for all organisms. Plants are able to take up inorganic sulfate and assimilate it into a range of bio-organic molecules either after reduction to sulfide or activation to 3'-phosphoadenosine 5'-phosphosulfate. While the regulation of the reductive part of sulfate assimilation and the synthesis of cysteine has been studied extensively in the past three decades, much less attention has been paid to the control of synthesis of sulfated compounds. Only recently the genes and enzymes activating sulfate and transferring it onto suitable acceptors have been investigated in detail with emphasis on understanding the diversity of the sulfotransferase gene family and the control of partitioning of sulfur between the two branches of sulfate assimilation. Here, the recent progress in our understanding of these processes will be summarized.

A nucleotide metabolite controls stress-responsive gene expression and plant development

Abiotic stress, such as drought and high salinity, activates a network of signaling cascades that lead to the expression of many stress-responsive genes in plants. The Arabidopsis FIERY1 (FRY1) protein is a negative regulator of stress and abscisic acid (ABA) signaling and exhibits both an inositol polyphosphatase and a 3′,5′-bisphosphate nucleotidase activity in vitro. The FRY1 nucleotidase degrades the sulfation byproduct 3′-phosphoadenosine-5′-phosphate (PAP), yet its in vivo functions and particularly its roles in stress gene regulation remain unclear. Here we developed a LC-MS/MS method to quantitatively measure PAP levels in plants and investigated the roles of this nucleotidase activity in stress response and plant development. It was found that PAP level was tightly controlled in plants and did not accumulate to any significant level either under normal conditions or under NaCl, LiCl, cold, or ABA treatments. In contrast, high levels of PAP were detected in multiple mutant alleles of FRY1 but not in mutants of other FRY1 family members, indicating that FRY1 is the major enzyme that hydrolyzes PAP in vivo. By genetically reducing PAP levels in fry1 mutants either through overexpression of a yeast PAP nucleotidase or by generating a triple mutant of fry1 apk1 apk2 that is defective in the biosynthesis of the PAP precursor 3′-phosphoadenosine-5′-phosphosulfate (PAPS), we demonstrated that the developmental defects and superinduction of stress-responsive genes in fry1 mutants correlate with PAP accumulation in planta. We also found that the hypersensitive stress gene regulation in fry1 requires ABH1 but not ABI1, two other negative regulators in ABA signaling pathways. Unlike in yeast, however, FRY1 overexpression in Arabidopsis could not enhance salt tolerance. Taken together, our results demonstrate that PAP is critical for stress gene regulation and plant development, yet the FRY1 nucleotidase that catabolizes PAP may not be an in vivo salt toxicity target in Arabidopsis.

AT-1 is the ER membrane acetyl-CoA transporter and is essential for cell viability

The transient or permanent modification of nascent proteins in the early secretory pathway is an essential cellular function that ensures correct folding and maturation of membrane and secreted proteins. We have recently described a new form of post-translational regulation of the membrane protein β-site APP cleaving enzyme 1 (BACE1) involving transient lysine acetylation in the lumen of the endoplasmic reticulum (ER). The essential components of this process are two ER-based acetyl-CoA:lysine acetyltransferases, ATase1 and ATase2, and a membrane transporter that translocates acetyl-CoA into the lumen of the ER. Here, we report the functional identification of acetyl-CoA transporter 1 (AT-1) as the ER membrane acetyl-CoA transporter. We show that AT-1 regulates the acetylation status of ER-transiting proteins, including the membrane proteins BACE1, low-density lipoprotein receptor and amyloid precursor protein (APP). Finally, we show that AT-1 is essential for cell viability as its downregulation results in widespread cell death and induction of features characteristic of autophagy.

Two Golgi-resident 3'-Phosphoadenosine 5'-phosphosulfate transporters play distinct roles in heparan sulfate modifications and embryonic and larval development in Caenorhabditis elegans

Synthesis of extracellular sulfated molecules requires active 3'-phosphoadenosine 5'-phosphosulfate (PAPS). For sulfation to occur, PAPS must pass through the Golgi membrane, which is facilitated by Golgi-resident PAPS transporters. Caenorhabditis elegans PAPS transporters are encoded by two genes, pst-1 and pst-2. Using the yeast heterologous expression system, we characterized PST-1 and PST-2 as PAPS transporters. We created deletion mutants to study the importance of PAPS transporter activity. The pst-1 deletion mutant exhibited defects in cuticle formation, post-embryonic seam cell development, vulval morphogenesis, cell migration, and embryogenesis. The pst-2 mutant exhibited a wild-type phenotype. The defects observed in the pst-1 mutant could be rescued by transgenic expression of pst-1 and hPAPST1 but not pst-2 or hPAPST2. Moreover, the phenotype of a pst-1;pst-2 double mutant were similar to those of the pst-1 single mutant, except that larval cuticle formation was more severely defected. Disaccharide analysis revealed that heparan sulfate from these mutants was undersulfated. Gene expression reporter analysis revealed that these PAPS transporters exhibited different tissue distributions and subcellular localizations. These data suggest that pst-1 and pst-2 play different physiological roles in heparan sulfate modification and development.

Adenosine-5'-phosphosulfate kinase is essential for Arabidopsis viability

In Arabidopsis thaliana, adenosine-5'-phosphosulfate kinase (APK) provides activated sulfate for sulfation of secondary metabolites, including the glucosinolates. We have successfully isolated three of the four possible triple homozygous mutant combinations of this family. The APK1 isoform alone was sufficient to maintain WT levels of growth and development. Analysis of apk1 apk2 apk3 and apk1 apk3 apk4 mutants suggests that APK3 and APK4 are functionally redundant, despite being located in cytosol and plastids, respectively. We were, however, unable to isolate apk1 apk3 apk4 mutants, most probably because the apk1 apk3 apk4 triple mutant combination is pollen lethal. Therefore, we conclude that APS kinase is essential for plant reproduction and viability.

Disruption of adenosine-5'-phosphosulfate kinase in Arabidopsis reduces levels of sulfated secondary metabolites

Plants can metabolize sulfate by two pathways, which branch at the level of adenosine 5'-phosphosulfate (APS). APS can be reduced to sulfide and incorporated into Cys in the primary sulfate assimilation pathway or phosphorylated by APS kinase to 3'-phosphoadenosine 5'-phosphosulfate, which is the activated sulfate form for sulfation reactions. To assess to what extent APS kinase regulates accumulation of sulfated compounds, we analyzed the corresponding gene family in Arabidopsis thaliana. Analysis of T-DNA insertion knockout lines for each of the four isoforms did not reveal any phenotypical alterations. However, when all six combinations of double mutants were compared, the apk1 apk2 plants were significantly smaller than wild-type plants. The levels of glucosinolates, a major class of sulfated secondary metabolites, and the sulfated 12-hydroxyjasmonate were reduced approximately fivefold in apk1 apk2 plants. Although auxin levels were increased in the apk1 apk2 mutants, as is the case for most plants with compromised glucosinolate synthesis, typical high auxin phenotypes were not observed. The reduction in glucosinolates resulted in increased transcript levels for genes involved in glucosinolate biosynthesis and accumulation of desulfated precursors. It also led to great alterations in sulfur metabolism: the levels of sulfate and thiols increased in the apk1 apk2 plants. The data indicate that the APK1 and APK2 isoforms of APS kinase play a major role in the synthesis of secondary sulfated metabolites and are required for normal growth rates.

A role for a lithium-inhibited Golgi nucleotidase in skeletal development and sulfation

Sulfation is an important biological process that modulates the function of numerous molecules. It is directly mediated by cytosolic and Golgi sulfotransferases, which use 3'-phosphoadenosine 5'-phosphosulfate to produce sulfated acceptors and 3'-phosphoadenosine 5'-phosphate (PAP). Here, we identify a Golgi-resident PAP 3'-phosphatase (gPAPP) and demonstrate that its activity is potently inhibited by lithium in vitro. The inactivation of gPAPP in mice led to neonatal lethality, lung abnormalities resembling atelectasis, and dwarfism characterized by aberrant cartilage morphology. The phenotypic similarities of gPAPP mutant mice to chondrodysplastic models harboring mutations within components of the sulfation pathway lead to the discovery of undersulfated chondroitin in the absence of functional enzyme. Additionally, we observed loss of gPAPP leads to perturbations in the levels of heparan sulfate species in lung tissue and whole embryos. Our data are consistent with a model that clearance of the nucleotide product of sulfotransferases within the Golgi plays an important role in glycosaminoglycan sulfation, provide a unique genetic basis for chondrodysplasia, and define a function for gPAPP in the formation of skeletal elements derived through endochondral ossification.

Overexpression of the 3'-phosphoadenosine 5'-phosphosulfate (PAPS) transporter 1 increases sulfation of chondroitin sulfate in the apical pathway of MDCK II cells

The canine 3'-phosphoadenosine 5'-phosphosulfate (PAPS) transporter 1 fused to GFP was stably expressed with a typical Golgi localization in MDCK II cells (MDCK II-PAPST1). The capacity for PAPS uptake into Golgi vesicles was enhanced to almost three times that of Golgi vesicles isolated from untransfected cells. We have previously shown that chondroitin sulfate proteoglycans (CSPGs) are several times more intensely sulfated in the basolateral than the apical secretory pathway in MDCK II cells (Tveit H, Dick G, Skibeli V, Prydz K. 2005. A proteoglycan undergoes different modifications en route to the apical and basolateral surfaces of Madin-Darby canine kidney cells. J Biol Chem. 280:29596-29603). Here we demonstrate that increased availability of PAPS in the Golgi lumen enhances the sulfation of CSPG in the apical pathway several times, while sulfation of CSPGs in the basolateral pathway shows minor changes. Sulfation of heparan sulfate proteoglycans is essentially unchanged. Our data indicate that CSPG sulfation in the apical pathway of MDCK II cells occurs at suboptimal conditions, either because the sulfotransferases involved have high K(m) values, or there is a lower PAPS concentration in the lumen of the apical secretory route than in the basolateral counterpart.

Nucleotide sugar transporters of the Golgi apparatus: from basic science to diseases

Approximately 80% of secreted and membrane proteins (40% of all proteins) of eukaryotes become covalently linked to sugars in the lumen of the Golgi apparatus, a cellular organelle that is part of the secretory system of all eukaryotes. The sugar donors are mostly nucleoside diphosphate sugars (nucleotide sugars) and must be translocated from the cytosol, their site of synthesis, across the Golgi apparatus membrane and into the lumen by specific transporters. These are hydrophobic, homodimeric proteins that span the membrane multiple times. Mutants of these proteins have developmental phenotypes including diseases in humans and cattle.

Characterization of a mutation and an alternative splicing of UDP-galactose transporter in MDCK-RCAr cell line

The UDP-galactose (UDP-Gal) transporter present in the Golgi apparatus is a member of a transporter family comprising hydrophobic proteins with multiple transmembrane domains. Co-immunoprecipitation experiments showed that the full-length UDP-Gal transporter protein forms oligomeric structures in the MDCK cell. A ricin-resistant mutant of the MDCK cell line (MDCK-RCA(r)) is deficient in galactose linked to macromolecules because of a lower UDP-Gal transport rate into the Golgi apparatus. We cloned this mutated protein and found that it contains a stop codon close to the 5' terminus of its open reading frame. We also detected a shorter splicing variant of the UDP-Gal transporter which contains a 183-nt in-frame deletion in both the wild-type and the mutant mRNA. We showed that the protein, when overexpressed, is localized in the Golgi apparatus and could partially correct the phenotype of the MDCK-RCA(r) and CHO-Lec8 mutant cell lines. The level of mRNA of the UDP-Gal transporter is much lower (25-30 copies per cell) than those of the CMP-sialic acid transporter (100 copies per cell), UDP-N-acetylglucosamine transporter (80 copies per cell), and GDP-fucose transporter (65 copies per cell). The transcript level of the shorter splicing variant of the UDP-Gal transporter is extremely rare in wild-type MDCK cells (a few copies per cell), but it is significantly increased in the mutant, RCA-resistant cells.

Reconstitution of GDP-mannose transport activity with purified Leishmania LPG2 protein in liposomes

Activated nucleotide sugars required for the synthesis of glycoconjugates within the secretory pathway of eukaryotes are provided by the action of nucleotide sugar transporters (NSTs). Typically, NSTs are studied in microsomal preparations from wild-type or mutant lines; however, in this setting it can be difficult to assess NST properties because of the presence of glycosyltransferases and other interfering activities. Here we have engineered Leishmania donovani to express high levels of an active LPG2 Golgi GDP-Man transporter bearing a C-terminal polyhistidine tag. The functional LPG2-HIS was solubilized, purified by metal affinity chromatography, and reconstituted into phosphatidylcholine-containing liposomes using polystyrene SM-2 beads. The proteoliposomes exhibited robust GDP-Man transport activity with an apparent K(m) of 6.6 mum. Transport activity was enhanced by preloading of GMP and showed specificity for multiple substrates (GDP-Ara and GDP-Fuc). In contrast to the activity in crude microsomes, transport was not dependent on the presence of divalent cations. Thus, reconstitution of transport activity using purified LPG2 protein in liposomes provides firm experimental evidence that a single polypeptide is solely required for NST activity and is able to mediate the uptake of multiple substrates. These studies are relevant to the study of NST structure and function in both protozoan parasites as well as their higher eukaryotic hosts.

Slalom encodes an adenosine 3'-phosphate 5'-phosphosulfate transporter essential for development in Drosophila

Sulfation of all macromolecules entering the secretory pathway in higher organisms occurs in the Golgi and requires the high-energy sulfate donor adenosine 3'-phosphate 5'-phosphosulfate. Here we report the first molecular identification of a gene that encodes a transmembrane protein required to transport adenosine 3'-phosphate 5'-phosphosulfate from the cytosol into the Golgi lumen. Mutations in this gene, which we call slalom, display defects in Wg and Hh signaling, which are likely due to the lack of sulfation of glycosaminoglycans by the sulfotransferase sulfateless. Analysis of mosaic mutant ovaries shows that sll function is also essential for dorsal-ventral axis determination, suggesting that sll transports the sulfate donor required for sulfotransferase activity of the dorsal-ventral determinant pipe.

Molecular cloning and identification of 3'-phosphoadenosine 5'-phosphosulfate transporter

Nucleotide sulfate, namely 3'-phosphoadenosine 5'-phosphosulfate (PAPS), is a universal sulfuryl donor for sulfation. Although a specific PAPS transporter is present in Golgi membrane, no study has reported the corresponding gene. We have identified a novel human gene encoding a PAPS transporter, which we have named PAPST1, and the Drosophila melanogaster ortholog, slalom (sll). The amino acid sequence of PAPST1 (432 amino acids) exhibited 48.1% identity with SLL (465 amino acids), and hydropathy analysis predicted the two to be type III transmembrane proteins. The transient expression of PAPST1 in SW480 cells showed a subcellular localization in Golgi membrane. The expression of PAPST1 and SLL in yeast Saccharomyces cerevisiae significantly increased the transport of PAPS into the Golgi membrane fraction. In human tissues, PAPST1 is highly expressed in the placenta and pancreas and present at lower levels in the colon and heart. An RNA interference fly of sll produced with a GAL4-UAS system revealed that the PAPS transporter is essential for viability. It is well known that mutations of some genes related to PAPS synthesis are responsible for human inherited disorders. Our findings provide insights into the significance of PAPS transport and post-translational sulfation.

Sulfation in the Golgi lumen of Madin-Darby canine kidney cells is inhibited by brefeldin A and depends on a factor present in the cytoplasm and on Golgi membranes

Madin-Darby canine kidney cells are more resistant than most other cell types to the classical effects of brefeldin A (BFA) treatment, the induction of retrograde transport of Golgi cisternae components to the endoplasmic reticulum. Here we show that sulfation of heparan sulfate proteoglycans (HSPGs), chondroitin sulfate proteoglycans (CSPGs), and proteins in the Golgi apparatus is dramatically reduced by low concentrations of BFA in which Golgi morphology is unaffected and secretion still takes place. BFA treatment seems to reduce sulfation by inhibition of the uptake of adenosine 3'-phosphate 5'-phosphosulfate (PAPS) into the Golgi lumen, and the inhibitory effect of BFA was similar for HSPGs, CSPGs, and proteins. This was different from the effect of chlorate, a well known inhibitor of PAPS synthesis in the cytoplasm. Low concentrations of chlorate (2-5 mm) inhibited sulfation of CSPGs and proteins only, whereas higher concentrations (15-30 mm) were required to inhibit sulfation of HSPGs. Golgi fractions pretreated with BFA had a reduced capacity for the synthesis of glycosaminoglycans (GAGs), but control level capacity could be restored by the addition of cytosol from various sources. This indicates that the PAPS pathway to the Golgi lumen depends on a BFA-sensitive factor that is present both on Golgi membranes and in the cytoplasm.

Lithocholic acid and sulphated lithocholic acid differ in the ability to promote matrix metalloproteinase secretion in the human colon cancer cell line CaCo-2

The human colon carcinoma cell line CaCo-2 has the ability to sulphate the secondary bile acid lithocholic acid (LA), whereas other primary or secondary bile acids were not sulphated [Halvorsen, Kase, Prydz, Gharagozlian, Andresen and Kolset (1999) Biochem. J. 343, 533--539]. To study the biological implications of this modification, CaCo-2 cells were incubated with either LA or sulphated lithocholic acid (3-sulpholithocholic acid, SLA), and in some experiments with taurine-conjugated lithocholic acid. Increased secretion of matrix metalloproteinases (MMPs) correlates with transformation of colon epithelial cells. When CaCo-2 cells were incubated with LA, the secretion of MMP-2 was found to increase approx. 60% when analysed by gelatin zymography, and 80% when analysed by Western blotting. SLA, in contrast, did not affect the level of MMP-2 secretion, and after zymography the level of enzyme activity was 78% of control values after 18 h incubation. The secretion of MMPs is linked to increased cellular invasion and, in tumours, to increased capacity for metastasis. The ability of CaCo-2 cells to invade in a chamber assay was stimulated after exposure to LA, whereas SLA-treated cells did not differ from control cells. LA therefore seems to induce a more invasive CaCo-2 cell phenotype, as judged by the two parameters tested, whereas the sulphated counterpart, SLA, did not have these effects. Sulphation of LA in the colon may be an important mechanism to decrease the potential LA has to promote a malignant epithelial phenotype.

Increasing the intra-Golgi pH of cultured LS174T goblet-differentiated cells mimics the decreased mucin sulfation and increased Thomsen-Friedenreich antigen (Gal beta1-3GalNac alpha-) expression seen in colon cancer

Mucins in ulcerative colitis and colon cancer share common properties of reduced sulfation and increased oncofetal carbohydrate antigen expression. It has previously been shown that there is no simple correlation between these changes and the activity of the relevant glycosyl-, sialyl-, and sulfo-transferases. We examined mucin sulfation and expression of oncofetal Thomsen-Friedenreich (TF) antigen (galactosyl beta1-3N-acetylgalactosamine alpha-) in the goblet cell-differentiated human colon cancer cell line LS174T following treatment with bafilomycin A(1, )which raises intra-Golgi pH, or monensin, which disrupts medial-trans Golgi transport. Cells were dual-labeled with sodium [(35)S]-sulfate and D-[6-(3)H(N)]-glucosamine hydrochloride, or labeled with L-[U-(14)C]-threonine alone. Mucin was purified using Sepharose CL-4B gel filtration. Mucin sulfo-Lewis(a) and TF antigen expression were assessed using the F2 anti-sulfo-Lewis(a) monoclonal antibody and peanut agglutinin binding respectively. Bafilomycin (0.01 microM; 48 h) reduced total mucin sulfation, expressed relative to incorporation of glucosamine, to 0.50 +/- 0.04 d.p.m. [(35)S]-sulfate per d.p.m. [(3)H]-glucosamine compared to control, 0.84 +/- 0.05 (p < 0.001, n = 16). This was accompanied by 50.3 +/- 8.0% increased expression of TF antigen (p < 0.01) and 50.1 +/- 5.5% decreased expression of sulfo-Lewis(a) (p < 0.01). The reduced sulfate:glucosamine ratio was largely due to increased incorporation of glucosamine into newly synthesized mucin rather than reduction in total sulfate incorporation. In contrast, monensin only reduced total mucin glycosylation at concentrations > 0.1 microM and had no significant effect on mucin sulfation or TF expression. Intra-Golgi alkalinization affects mucin glycosylation, resulting in decreased mucin sulfation and increased expression of TF antigen, changes that mimic those seen in cancerous and premalignant human colonic epithelium.

SQV-7, a protein involved in Caenorhabditis elegans epithelial invagination and early embryogenesis, transports UDP-glucuronic acid, UDP-N- acetylgalactosamine, and UDP-galactose

Caenorhabditis elegans sqv mutants are defective in vulval epithelial invagination and have a severe reduction in hermaphrodite fertility. The gene sqv-7 encodes a multitransmembrane hydrophobic protein resembling nucleotide sugar transporters of the Golgi membrane. A Golgi vesicle enriched fraction of Saccharomyces cerevisiae expressing SQV-7 transported UDP-glucuronic acid, UDP-N-acetylgalactosamine, and UDP-galactose (Gal) in a temperature-dependent and saturable manner. These nucleotide sugars are competitive, alternate, noncooperative substrates. The two mutant sqv-7 missense alleles resulted in a severe reduction of these three transport activities. SQV-7 did not transport CMP-sialic acid, GDP-fucose, UDP-N-acetylglucosamine, UDP-glucose, or GDP-mannose. SQV-7 is able to transport UDP-Gal in vivo, as shown by its ability to complement the phenotype of Madin-Darby canine kidney ricin resistant cells, a mammalian cell line deficient in UDP-Gal transport into the Golgi. These results demonstrate that unlike most nucleotide sugar transporters, SQV-7 can transport multiple distinct nucleotide sugars. We propose that SQV-7 translocates multiple nucleotide sugars into the Golgi lumen for the biosynthesis of glycoconjugates that play a pivotal role in development.

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.

A high-throughput assay for rat liver golgi and Saccharomyces cerevisiae-expressed murine CMP-N-acetylneuraminic acid transport proteins

Rat liver Golgi and Saccharomyces cerevisiae-expressed CMP-Neu5Ac transport protein were reconstituted in phosphatidylcholine liposomes and transport of CMP-Neu5Ac into these proteoliposomes was determined. The separation of transported substrate from free substrate was performed using Multiscreen minicolumns loaded with Sephadex G-50 resin (fine). The CMP-Neu5Ac transport characteristics of the rat liver Golgi and S. cerevisiae-expressed transporters, determined using this separation system, were very similar to those previously reported. Inhibition studies, utilizing the above procedure, revealed that the main structural features required for recognition of glycosyl nucleosides by the rat liver Golgi CMP-Neu5Ac transport protein were the nature of the nucleoside base and the anomeric configuration of the associated carbohydrate. In general, pyrimidine-based glycosyl nucleosides were found to inhibit transport to a far greater extent than purine-based glycosyl nucleosides, an observation that is in good agreement with previous reports. These results indicate that the reconstitution procedure, in conjunction with Multiscreen minicolumns, is an effective high-throughput method for the determination of CMP-Neu5Ac transport.

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.

Proteomics of rat liver Golgi complex: minor proteins are identified through sequential fractionation

The discovery of novel proteins resident to the Golgi complex will fuel our future studies of Golgi structure/function and provide justification for proteomic analysis of this organelle. Our approach to Golgi proteomics was to first isolate and characterize the intact organelle free of proteins in transit by use of tissue pretreated with cycloheximide. Then the stacked Golgi fraction was fractionated into biochemically defined subfractions: Triton X-114 insoluble, aqueous, and detergent phases. The aqueous and detergent phases were further fractionated by anion-exchange column chromatography. In addition, radiolabeled cytosol was incubated with stacked Golgi fractions containing proteins in transit, and the proteins bound to the Golgi stacks in an energy-dependent manner were characterized. All fractions were analyzed by two-dimensional (2-D) gel electrophoresis and identification numbers were given to 588 unique 2-D spots. Tandem mass spectrometry was used to analyze 93 of the most abundant 2-D spots taken from preparative Triton X-114 insoluble, aqueous and detergent phase 2-D gels. Fifty-one known and 22 unknown proteins were identified. This study represents the first installment in the mammalian Golgi proteome database. Our data suggest that cell fractionation followed by biochemical dissection of specific classes of molecules provides a significant advantage for the identification of low abundance proteins in organelles.

Lithocholic acid and sulphated lithocholic acid differ in the ability to promote matrix metalloproteinase secretion in the human colon cancer cell line CaCo-2

The human colon carcinoma cell line CaCo-2 has the ability to sulphate the secondary bile acid lithocholic acid (LA), whereas other primary or secondary bile acids were not sulphated [Halvorsen, Kase, Prydz, Gharagozlian, Andresen and Kolset (1999) Biochem. J. 343, 533--539]. To study the biological implications of this modification, CaCo-2 cells were incubated with either LA or sulphated lithocholic acid (3-sulpholithocholic acid, SLA), and in some experiments with taurine-conjugated lithocholic acid. Increased secretion of matrix metalloproteinases (MMPs) correlates with transformation of colon epithelial cells. When CaCo-2 cells were incubated with LA, the secretion of MMP-2 was found to increase approx. 60% when analysed by gelatin zymography, and 80% when analysed by Western blotting. SLA, in contrast, did not affect the level of MMP-2 secretion, and after zymography the level of enzyme activity was 78% of control values after 18 h incubation. The secretion of MMPs is linked to increased cellular invasion and, in tumours, to increased capacity for metastasis. The ability of CaCo-2 cells to invade in a chamber assay was stimulated after exposure to LA, whereas SLA-treated cells did not differ from control cells. LA therefore seems to induce a more invasive CaCo-2 cell phenotype, as judged by the two parameters tested, whereas the sulphated counterpart, SLA, did not have these effects. Sulphation of LA in the colon may be an important mechanism to decrease the potential LA has to promote a malignant epithelial phenotype.

Characterization of Yeast Yea4p, a uridine diphosphate-N-acetylglucosamine transporter localized in the endoplasmic reticulum and required for chitin synthesis

Chitin is an essential cell wall component, synthesis of which is regulated throughout the cell cycle in the yeast Saccharomyces cerevisiae. We cloned an S. cerevisiae gene, YEA4, whose product is homologous to the Kluyveromyces lactis uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) transporter. An epitope-tagged Yea4p localized mainly in the 10,000 x g pellet (P2), suggesting endoplasmic reticulum (ER) localization. Membrane vesicles from the P2 fraction showed an 8-fold higher UDP-GlcNAc transport activity in cells harboring a multicopy YEA4 plasmid than in cells harboring vector alone. The activity distribution is identical with the protein distribution in P2, whether the gene is overexpressed or not, suggesting its native localization in P2. Immunolocalization of epitope-tagged Yea4p further revealed ER localization. The increase in transport activity due to the YEA4 overexpression is specific for UDP-GlcNAc, but not for UDP-galactose and GDP-mannose. Deltayea4-disrupted cells showed a reduced rate of UDP-GlcNAc transport, contained less chitin, and were larger and rounder in shape than the wild type cells. Our results indicate that YEA4 encodes an ER-localized UDP-GlcNAc transporter that is required for cell wall chitin synthesis in S. cerevisiae.

Nuclear localization of PAPS synthetase 1: a sulfate activation pathway in the nucleus of eukaryotic cells

Sulfation is a major modification of many molecules in eukaryotes that is dependent on the enzymatic synthesis of an activated sulfate donor, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). While sulfate activation has long been assumed to occur in the cytosol, we show in this study that human PAPS synthetase 1 (PAPSS1), a bifunctional ATP sulfurylase/adenosine 5'-phosphosulfate (APS) kinase enzyme sufficient for PAPS synthesis, accumulates in the nucleus of mammalian cells. Nuclear targeting of the enzyme is mediated by its APS kinase domain and requires a catalytically dispensable 21 amino acid sequence at the amino terminus. Human PAPSS1 and Drosophila melanogaster PAPSS localize to the nucleus in yeast and relieve the methionine auxotrophy of ATP sulfurylase- or APS kinase-deficient strains, suggesting that PAPSS1 is fully functional in vivo when targeted to the nucleus. A second PAPS synthetase gene, designated PAPSS2, has recently been described, mutations of which are responsible for abnormal skeletal development in human spondyloepimetaphyseal dysplasia and murine brachymorphism. We found that PAPSS2, which localizes to the cytoplasm when ectopically expressed in mammalian cells, is relocated to the nucleus when coexpressed with PAPSS1. Taken together, these results indicate that a sulfation pathway might exist in the nucleus of eukaryotic cells. -Besset, S., Vincourt, J.-B., Amalric, F., Girard, J.-P. Nuclear localization of PAPS synthetase 1: a sulfate activation pathway in the nucleus of eukaryotic cells.

Synthesis and sorting of proteoglycans

Proteoglycans are widely expressed in animal cells. Interactions between negatively charged glycosaminoglycan chains and molecules such as growth factors are essential for differentiation of cells during development and maintenance of tissue organisation. We propose that glycosaminoglycan chains play a role in targeting of proteoglycans to their proper cellular or extracellular location. The variability seen in glycosaminoglycan chain structure from cell type to cell type, which is acquired by use of particular Ser-Gly sites in the protein core, might therefore be important for post-synthesis sorting. This links regulation of glycosaminoglycan synthesis to the post-Golgi fate of proteoglycans.

Reconstitution, identification, and purification of the rat liver golgi membrane GDP-fucose transporter

Glycosylation of glycoproteins, proteoglycans, and glycolipids occurring in the Golgi apparatus requires the translocation of nucleotide sugars from the cytosol into the lumen of the Golgi. Translocation is mediated by specific nucleotide sugar transporters, integral Golgi membrane proteins that regulate the above glycosylation reactions. A defect in GDP-fucose transport into the lumen of the Golgi apparatus has been recently identified in a patient affected by leukocyte adhesion deficiency type II syndrome (Lubke, T., Marquardt, T., von Figura, K., and Korner, C. (1999) J. Biol. Chem. 274, 25986-25989). We have now identified and purified the rat liver Golgi membrane GDP-fucose transporter, a protein with an apparent molecular mass of 39 kDa, by a combination of column chromatography, native functional size determination on a glycerol gradient, and photoaffinity labeling with 8-azidoguanosine-5'-[alpha-(32)P] triphosphate, an analog of GDP-fucose. The purified transporter appears to exist as a homodimer within the Golgi membrane. When reconstituted into phosphatidylcholine liposomes, it was active in GDP-fucose transport and was specifically photolabeled with 8-azidoguanosine-5'-[alpha-(32)P]triphosphate. Transport was also stimulated 2-3-fold after preloading proteoliposomes with GMP, the putative antiporter.

Identification, purification, and characterization of the rat liver golgi membrane ATP transporter

Phosphorylation of secretory and integral membrane proteins and of proteoglycans also occurs in the lumen of the Golgi apparatus. ATP, the phosphate donor in these reactions, must first cross the Golgi membrane before it can serve as substrate. The existence of a specific ATP transporter in the Golgi membrane has been previously demonstrated in vitro using intact Golgi membrane vesicles from rat liver and mammary gland. We have now identified and purified the rat liver Golgi membrane ATP transporter. The transporter was purified to apparent homogeneity by a combination of conventional ion exchange, dye color, and affinity chromatography. An approximately 70,000-fold purification (2% yield) was achieved starting from crude rat liver Golgi membranes. A protein with an apparent molecular mass of 60 kDa was identified as the putative transporter by a combination of column chromatography, photoaffinity labeling with an analog of ATP, and native functional size determination on a glycerol gradient. The purified transporter appears to exist as a homodimer within the Golgi membrane, and when reconstituted into phosphatidylcholine liposomes, was active in ATP but not nucleotide sugar or adenosine 3'-phosphate 5'-phosphosulfate transport. The transport activity was saturable with an apparent Km very similar to that of intact Golgi vesicles.

Membrane topology of the mammalian CMP-sialic acid transporter

Nucleotide sugar transporters form a family of distantly related membrane proteins of the Golgi apparatus and the endoplasmic reticulum. The first transporter sequences have been identified within the last 2 years. However, information about the secondary and tertiary structure for these molecules has been limited to theoretical considerations. In the present study, an epitope-insertion approach was used to investigate the membrane topology of the CMP-sialic acid transporter. Immunofluorescence studies were carried out to analyze the orientation of the introduced epitopes in semipermeabilized cells. Both an amino-terminally introduced FLAG sequence and a carboxyl-terminal hemagglutinin tag were found to be oriented toward the cytosol. Results obtained with CMP-sialic acid transporter variants that contained the hemagglutinin epitope in potential intermembrane loop structures were in good correlation with the presence of 10 transmembrane regions. This building concept seems to be preserved also in other mammalian and nonmammalian nucleotide sugar transporters. Moreover, the functional analysis of the generated mutants demonstrated that insertions in or very close to membrane-spanning regions inactivate the transport process, whereas those in hydrophilic loop structures have no detectable effect on the activity. This study points the way toward understanding structure-function relationships of nucleotide sugar transporters.

Identification and purification of the rat liver Golgi membrane UDP-N-acetylgalactosamine transporter

Glycosylation of glycoproteins, proteoglycans, and glycosphingolipids occurs mainly in the lumen of the endoplasmic reticulum and the Golgi apparatus. Nucleotide sugars, donors of all the sugars involved in Golgi glycosylation reactions, are synthesized in the cytoplasm and require specialized transporters to be translocated into the lumen of the Golgi apparatus. By controlling the supply of sugar nucleotides in the lumen of the Golgi apparatus, these transporters directly regulate the glycosylation of macromolecules transiting the Golgi. We have identified and purified the rat liver Golgi membrane UDP-N-acetylgalactosamine transporter. The transporter was purified to apparent homogeneity by a combination of conventional and dye color chromatography. An approximately 63,000-fold purification (6% yield) was achieved starting from crude rat liver Golgi membranes and resulting in a protein with an apparent molecular mass of 43 kDa. The transporter was active when reconstituted into phosphatidylcholine vesicles and could be specifically photolabeled with P3-(4-azidoanilido)-uridine-5'-[P1-32P]triphosphate, an analog of UDP-N-acetylgalactosamine. Native functional size determination on a glycerol gradient suggested that the transporter exists as a homodimer within the Golgi membrane.

Transporters of nucleotide sugars, ATP, and nucleotide sulfate in the endoplasmic reticulum and Golgi apparatus

The lumens of the endoplasmic reticulum and Golgi apparatus are the subcellular sites where glycosylation, sulfation, and phosphorylation of secretory and membrane-bound proteins, proteoglycans, and lipids occur. Nucleotide sugars, nucleotide sulfate, and ATP are substrates for these reactions. ATP is also used as an energy source in the lumen of the endoplasmic reticulum during protein folding and degradation. The above nucleotide derivatives and ATP must first be translocated across the membrane of the endoplasmic reticulum and/or Golgi apparatus before they can serve as substrates in the above lumenal reactions. Translocation of the above solutes is mediated for highly specific transporters, which are antiporters with the corresponding nucleoside monophosphates as shown by biochemical and genetic approaches. Mutants in mammals, yeast, and protozoa showed that a defect in a specific translocator activity results in selective impairments of the above posttranslational modifications, including loss of virulence of pathogenic protozoa. Several of these transporters have been purified and cloned. Experiments with yeast and mammalian cells demonstrate that these transporters play a regulatory role in the above reactions. Future studies will address the structure of the above proteins, how they are targeted to different organelles, their potential as drug targets, their role during development, and the possible occurrence of specific diseases.

Mutations in orthologous genes in human spondyloepimetaphyseal dysplasia and the brachymorphic mouse

The osteochondrodysplasias are a genetically heterogeneous group of disorders affecting skeletal development, linear growth and the maintenance of cartilage and bone. We have studied a large inbred Pakistani family with a distinct form of recessively inherited spondyloepimetaphyseal dysplasia (SEMD) and mapped a gene associated with this dwarfing condition to chromosome 10q23-24, a region syntenic with the locus for the brachymorphic mutation on mouse chromosome 19. We identified two orthologous genes, ATPSK2 and Atpsk2, encoding novel ATP sulfurylase/APS kinase orthologues in the respective regions of the human and mouse genomes. We characterized a nonsense mutation in ATPSK2 in the SEMD family and a missense mutation in the region of Atpsk2 encoding the APS kinase activity in the brachymorphic mouse. ATP sulfurylase/APS kinase catalyses the metabolic activation of inorganic sulfate to PAPS, the universal donor for post-translational protein sulfation in all cell types. The cartilage-specificity of the human and mouse phenotypes provides further evidence of the critical role of sulfate activation in the maturation of cartilage extracellular matrix molecules and the effect of defects in this process on the architecture of cartilage and skeletogenesis.

Nucleotide sugars, nucleotide sulfate, and ATP transporters of the endoplasmic reticulum and Golgi apparatus

The lumina of the endoplasmic reticulum and Golgi apparatus are the subcellular sites where glycosylation, sulfation, and phosphorylation of secretory and membrane-bound proteins, proteoglycans, and lipids occur. Nucleotide sugars, nucleotide sulfate, and ATP are substrates in the above reactions and must first be translocated from the cytosol into the lumen of these organelles. Translocation of these nucleotide derivatives is mediated by highly specific transporters, which are antiporters with the corresponding nucleoside monophosphate, as shown by genetic and biochemical approaches in mammals and yeast. Studies with mammalian, yeast, and protozoa mutants have shown that a defect in a specific translocator results in selective impairments of glycosylation of proteins, lipids and proteoglycans in vivo. Several of these transporters have been purified, cloned, and found to encode very hydrophobic proteins with multitransmembrane domains. Experiments with yeast and mammalian cells demonstrate that these transporters play a regulatory role in posttranslational modifications.

Functional expression of the murine Golgi CMP-sialic acid transporter in saccharomyces cerevisiae

We have functionally expressed the murine Golgi putative CMP-sialic acid transporter in Saccharomyces cerevisiae. Using a galactose-inducible expression system, S. cerevisiae vesicles were able to transport CMP-sialic acid. Transport was dependent on galactose induction and was temperature-dependent and saturable with an apparent Km of 2.9 microM. Transport was inhibited by CMP, and upon vesicle disruption with Triton X-100 parameters were very similar to the previously described CMP-sialic acid transport characteristics observed with mammalian Golgi vesicles. CMP-sialic acid transport induction was specific as no transport of UDP-galactose was observed even though the latter putative transporter has a high degree of amino acid sequence identity with the CMP-sialic acid transporter. Together, the above results demonstrate that the previously described cDNA encoding the putative CMP-sialic acid transporter encodes the transporter protein per se and suggests that this heterologous expression system may be used for further structural and functional studies of other Golgi membrane transporter proteins.

Expression cloning and characterization of a cDNA encoding a novel membrane protein required for the formation of O-acetylated ganglioside: a putative acetyl-CoA transporter

By expression cloning using COS-1 cells stably transfected with GD3-synthase (COS-1/GD3+) as a recipient cell line, we have isolated a cDNA, termed AT-1, encoding a novel protein required for the formation of O-acetylated (Ac) gangliosides. The cDNA encodes a protein with multitransmembrane spanning domains with a leucine zipper motif. It consists of 549 amino acids and has a molecular mass of 60.9 kDa. Although both O-Ac-GD3 and O-Ac-GT3 were barely detectable in recipient cells or cells transfected with the vector alone, their amount increased significantly in transfectants containing AT-1. When semi-intact cells prepared by treatment with streptolysin O were incubated with [Ac-14C]-Ac-CoA, increased incorporation of radioactivity was found in those cells transfected with AT-1 when compared with the mock transfectants. Northern blot analysis showed two major transcripts of 3.3 and 4.3 kb in all tissues examined. Immunohistochemical study with an antibody specific to the AT-1 protein suggested that it is most probably expressed in the endoplasmic reticulum membrane. Based on these results, the protein encoded by AT-1 is suggested to be an Ac-CoA transporter that is involved in the process of O-acetylation.

Expression cloning of the Golgi CMP-sialic acid transporter

Translocation of nucleotide sugars across the membrane of the Golgi apparatus is a prerequisite for the synthesis of complex carbohydrate structures. While specific transport systems for different nucleotide sugars have been identified biochemically in isolated microsomes and Golgi vesicles, none of these transport proteins has been characterized at the molecular level. Chinese hamster ovary (CHO) mutants of the complementation group Lec2 exhibit a strong reduction in sialylation of glycoproteins and glycolipids due to a defect in the CMP-sialic acid transport system. By complementation cloning in the mutant 6B2, belonging to the Lec2 complementation group, we were able to isolate a cDNA encoding the putative murine Golgi CMP-sialic acid transporter. The cloned cDNA encodes a highly hydrophobic, multiple membrane spanning protein of 36.4 kDa, with structural similarity to the recently cloned ammonium transporters. Transfection of a hemagglutinin-tagged fusion protein into the mutant 6B2 led to Golgi localization of the hemagglutinin epitope. Our results, together with the observation that the cloned gene shares structural similarities to other recently cloned transporter proteins, strongly suggest that the isolated cDNA encodes the CMP-sialic acid transporter.

Molecular cloning of the Golgi apparatus uridine diphosphate-N-acetylglucosamine transporter from Kluyveromyces lactis

The mannan chains of Kluyveromyces lactis mannoproteins are similar to those of Saccharomyces cerevisiae except that they lack mannose phosphate and have terminal alpha1-->2-linked N-acetylglucosamine. The biosynthesis of these chains probably occurs in the lumen of the Golgi apparatus, by analogy to S. cerevisiae. The sugar donors, GDP-mannose and UDP-GlcNAc, must first be transported from the cytosol, their site of synthesis, via specific Golgi membrane transporters into the lumen where they are substrates in the biosynthesis of these mannoproteins. A mutant of K. lactis, mnn2-2, that lacks terminal N-acetylglucosamine in its mannan chains in vivo, has recently been characterized and shown to have a specific defect in transport of UDP-GlcNAc into the lumen of Golgi vesicles in vitro. We have now cloned the gene encoding the K. lactis Golgi membrane UDP-GlcNAc transporter by complementation of the mnn2-2 mutation. The mnn2-2 mutant was transformed with a genomic library from wild-type K. lactis in a pKD1-derived vector; transformants were isolated and phenotypic correction was monitored following cell surface labeling with fluorescein isothiocyanate conjugated to Griffonia simplicifolia II lectin, which binds terminal N-acetylglucosamine, and a fluorescent activated cell sorter. A 2.4-kb DNA fragment was found to restore the wild-type lectin binding phenotype. Upon loss of the plasmid containing this fragment, reversion to the mutant phenotype occurred. The above fragment contained an open reading frame for a multitransmembrane spanning protein of 328 amino acids. The protein contains a leucine zipper motif and has high homology to predicted proteins from S. cerevisiae and C. elegans. In an assay in vitro, Golgi vesicles isolated from the transformant had regained their ability to transport UDP-GlcNAc. Taken together, the above results strongly suggest that the cloned gene encodes the Golgi UDP-GlcNAc transporter of K. lactis.

Purification and characterization of heparan sulfate 2-sulfotransferase from cultured Chinese hamster ovary cells

Heparan sulfate 2-sulfotransferase, which catalyzes the transfer of sulfate from adenosine 3'-phosphate 5'-phosphosulfate to position 2 of L-iduronic acid residue in heparan sulfate, was purified 51,700-fold to apparent homogeneity with a 6% yield from cultured Chinese hamster ovary cells. The isolation procedure included a combination of affinity chromatography on heparin-Sepharose CL-6B and 3',5'-ADP-agarose, which was repeated twice for each, and finally gel chromatography on Superose 12 . Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme showed two protein bands with molecular masses of 47 and 44 kDa. Both proteins appeared to be glycoproteins, because their molecular masses decreased after N-glycanase digestion. When completely desulfated and N-resulfated heparin and mouse Engelbreth-Holm-Swarm tumor heparan sulfate were used as acceptors, the purified enzyme transferred sulfate to position 2 of L-iduronic acid residue but did not transfer sulfate to the amino group of glucosamine residue or to position 6 of N-sulfoglucosamine residue. Heparan sulfates from pig aorta and bovine liver, however, were poor acceptors. The enzyme showed no activities toward chondroitin, chondroitin sulfate, dermatan sulfate, and keratan sulfate. The optimal pH for the enzyme activity was around 5.5. The enzyme activity was minimally affected by dithiothreitol and was stimulated strongly by protamine. The Km value for adenosine 3'-phosphate 5'-phosphosulfate was 0.20 microM.

Sac1p mediates the adenosine triphosphate transport into yeast endoplasmic reticulum that is required for protein translocation

Protein translocation into the yeast endoplasmic reticulum requires the transport of ATP into the lumen of this organelle. Microsomal ATP transport activity was reconstituted into proteoliposomes to characterize and identify the transporter protein. A polypeptide was purified whose partial amino acid sequence demonstrated its identity to the product of the SAC1 gene. Accordingly, microsomal membranes isolated from strains harboring a deletion in the SAC1 gene (sac1 delta) were found to be deficient in ATP-transporting activity as well as severely compromised in their ability to translocate nascent prepro-alpha-factor and preprocarboxypeptidase Y. Proteins isolated from the microsomal membranes of a sac1 delta strain were incapable of stimulating ATP transport when reconstituted into the in vitro assay system. When immunopurified to homogeneity and incorporated into artificial lipid vesicles, Sac1p was shown to reconstitute ATP transport activity. Consistent with the requirement for ATP in the lumen of the ER to achieve the correct folding of secretory proteins, the sac1 delta strain was shown to have a severe defect in transport of procarboxypeptidase Y out of the ER and into the Golgi complex in vivo. The collective data indicate an intimate role for Sac1p in the transport of ATP into the ER lumen.

Purification, photoaffinity labeling, and characterization of a single enzyme for 6-sulfation of both chondroitin sulfate and keratan sulfate

A soluble sulfotransferase that could 6-sulfate both chondroitin sulfate and corneal keratan sulfate was purified 27,500-fold using a sequence of affinity chromatographic steps with heparin-Sepharose, wheat germ agglutinin-agarose, and 3',5'-ADP-agarose. The essentially pure enzyme had a specific activity 40 times greater than the most purified chondroitin 6-sulfotransferase previously reported and exhibited a single sharp Coomassie Blue-stained and a heavy silver-stained protein band of 75 kDa on SDS-polyacrylamide gel electrophoresis. Chromatography of the purified enzyme on Sephacryl demonstrated a size of 150 kDa, which indicated that the native enzyme exists as a dimer. In addition to 6-sulfation of nonsulfated GalNAc, the purified serum enzyme had the ability to sulfate GalNAc 4-sulfate residues to give GalNAc 4,6-disulfate residues. The purified enzyme exhibited a Km of 40 microM for adenosine 3'-phosphate 5'-phosphosulfate when either chondroitin sulfate or corneal keratan sulfate were used as the acceptors. Use of both chondroitin sulfate and keratan sulfate in the same experiment demonstrated mutual competition, establishing that the sulfation of these substrates is by the same enzyme. Photoaffinity labeling of the purified enzyme with 2-azidoadenosine 3',5'-di[5'-32P]phosphate occurred only with the 75-kDa protein, confirming that this is the chondroitin 6-sulfotransferase/keratan sulfotransferase.

Purification and characterization of the GalNAc-4-sulfotransferase responsible for sulfation of GalNAc beta 1,4GlcNAc-bearing oligosaccharides

The pituitary glycoprotein hormone lutropin is characterized by its pulsatile appearance in the bloodstream which is important for the expression of its biological activity in the ovary. We have previously shown that lutropin bears unique Asn-linked oligosaccharides terminating with GalNAc-4-SO4 which allow the hormone to be rapidly cleared from the bloodstream via a specific receptor in the liver, thus contributing to its pulsatile appearance in the circulation. Furthermore, we have found that carbonic anhydrase VI, synthesized by the submaxillary gland and secreted into the saliva, also bears Asn-linked oligosaccharides terminating with GalNAc-4-SO4, suggesting that this unique sulfated structure mediates other biological functions in addition to rapid clearance from the circulation. We report here the purification of a GalNAc-4-sulfotransferase which transfers sulfate to terminal beta 1,4-linked GalNAc on Asn-linked oligosaccharides. We show that the purified submaxillary gland enzyme has kinetic parameters identical to the pituitary enzyme, indicating that the same sulfotransferase is responsible for the sulfation of lutropin oligosaccharides in pituitary and carbonic anhydrase VI oligosaccharides in submaxillary gland. This GalNAc-4-sulfotransferase has an apparent molecular mass of 128 kDa and can be specifically photoaffinity radiolabeled with 3',5'-ADP, a competitive inhibitor of sulfotransferase activity. The acceptor specificity of this GalNAc-4-sulfotransferase indicates that it is able to transfer sulfate to terminal GalNAc beta 1,4GlcNAc on both N- and O-glycosidically linked oligosaccharides, suggesting that this enzyme is also responsible for the sulfation of O-linked glycans on proopiomelanocortin.

Regulation of yeast Golgi glycosylation. Guanosine diphosphatase functions as a homodimer in the membrane

The Golgi lumenal GDPase plays an important role in the mannosylation of proteins and lipids of Saccharomyces cerevisiae by regulating the amount of GDP-mannose available in the Golgi lumen. The enzyme makes available GMP as an antiporter to be coupled with entry of GDP-mannose into the Golgi lumen from the cytosol. Using radiation inactivation and target analysis, we have now determined the functional molecular mass of the GDPase within the Golgi membrane and whether or not the enzyme has functional associations with other Golgi membrane proteins, including mannosyltransferases and the GDP-mannose transporter. The functional size of the GDPase was found to be approximately twice the estimated structural target size of the protein; this strongly suggests that the GDPase protein in situ functions as homodimer and does not require association with other membrane proteins for its function.