The signal anchor and stem regions of the beta-galactoside alpha 2,6-sialyltransferase may each act to localize the enzyme to the Golgi apparatus

Sialic acid in the regulation of blood cell production, differentiation and turnover

Sialic acid is a unique sugar moiety that resides in the distal and most accessible position of the glycans on mammalian cell surface and extracellular glycoproteins and glycolipids. The potential for sialic acid to obscure underlying structures has long been postulated, but the means by which such structural changes directly affect biological processes continues to be elucidated. Here, we appraise the growing body of literature detailing the importance of sialic acid for the generation, differentiation, function and death of haematopoietic cells. We conclude that sialylation is a critical post-translational modification utilized in haematopoiesis to meet the dynamic needs of the organism by enforcing rapid changes in availability of lineage-specific cell types. Though long thought to be generated only cell-autonomously within the intracellular ER-Golgi secretory apparatus, emerging data also demonstrate previously unexpected diversity in the mechanisms of sialylation. Emphasis is afforded to the mechanism of extrinsic sialylation, whereby extracellular enzymes remodel cell surface and extracellular glycans, supported by charged sugar donor molecules from activated platelets.

The stem region of group A transferase is crucial for its specificity, and its alteration promotes heterologous Forssman synthase activity

Some stem region mutants of human blood group A transferase (hAT) possess Forssman synthase (FS) activity, but very little is known about the mechanisms responsible for this enzymatic crosstalk. We performed confocal microscopy and image analysis to determine whether different intra-Golgi localization was accountable for this acquired activity. We also performed structural modeling and mutational and normal mode analyses. We introduced new mutations in the stem region and tested its FS and AT activities. No differences in subcellular localization were found between hAT and FS-positive mutants. AlphaFold models of hAT and mFS (mouse Forssman synthase) showed that the hAT stem region has a tether-like stem region, while in mFS, it encircles its catalytic domain. In silico analysis of FS-positive mutants indicated that stem region mutations induced structural changes, decreasing interatomic interactions and mobility of hAT that correlated with FS activity. Several additional mutations introduced in that region also bestowed FS activity without altering the AT activity: hAT 37-55 aa substitution by mFS 34-52, 37-55 aa deletion, and missense mutations: S46P, Q278Y, and Q286M. Stem region structure, mobility, and interactions are crucial for hAT specificity. Moreover, stem region mutations can lead to heterologous Forssman activity without changes in the catalytic machinery.

Endocytosed HIV-1 Envelope Glycoprotein Traffics to Rab14+ Late Endosomes and Lysosomes to Regulate Surface Levels in T-Cell Lines

Production of infectious HIV-1 particles requires incorporation of the viral envelope glycoprotein (Env) at the plasma membrane (PM) of infected CD4+ T cells. Env trafficking to the PM exposes viral epitopes that can be exploited by the host immune system; however, HIV-1 can evade this response by endocytosis of excess Env from the PM. The fate of Env after internalization remains unclear, with evidence suggesting several different vesicular trafficking steps may be involved, including recycling pathways. To date, there have been very few studies documenting the trafficking pathways of native Env in infected T cells. Furthermore, it remains unclear whether there are T-cell-specific endosomal pathways regulating the fate of endocytic Env. Here, we use a pulse-labeling approach with a monovalent anti-Env Fab probe to characterize the trafficking of internalized Env within infected CD4+ T-cell lines, together with CRISPR/Cas9-mediated endogenous protein tagging, to assess the role of host cell Rab GTPases in Env trafficking. We show that endocytosed Env traffics to Rab14+ compartments that possess hallmarks of late endosomes and lysosomes. We also demonstrate that Env can recycle back to the PM, although we find that recycling does not occur at high rates when compared to the model recycling protein transferrin. These results help to resolve open questions about the fate and relevance of endocytosed Env in HIV-infected cells and suggest a novel role for Rab14 in a cell-type-specific late-endosomal/lysosomal trafficking pathway in T cells. IMPORTANCE HIV-1 envelope glycoprotein (Env) evades immune neutralization through many mechanisms. One immune evasion strategy may result from the internalization of excess surface-exposed Env to prevent antibody-dependent cellular cytotoxicity or neutralization. Characterization of the fate of endocytosed Env is critical to understand which vesicular pathways could be targeted to promote display of Env epitopes to the immune system. In this study, we characterize the endocytic fate of native Env, expressed from infected human T-cell lines. We demonstrate that Env is rapidly trafficked to a late-endosome/lysosome-like compartment and can be recycled to the cell surface for incorporation into virus assembly sites. This study implicates a novel intracellular compartment, marked by host-cell Rab14 GTPases, for the sequestration of Env. Therapeutic approaches aimed at mobilizing this intracellular pool of Env could lead to stronger immune control of HIV-1 infection via antibody-dependent cell-mediated cytotoxicity.

Getting Sugar Coating Right! The Role of the Golgi Trafficking Machinery in Glycosylation

The Golgi is the central organelle of the secretory pathway and it houses the majority of the glycosylation machinery, which includes glycosylation enzymes and sugar transporters. Correct compartmentalization of the glycosylation machinery is achieved by retrograde vesicular trafficking as the secretory cargo moves forward by cisternal maturation. The vesicular trafficking machinery which includes vesicular coats, small GTPases, tethers and SNAREs, play a major role in coordinating the Golgi trafficking thereby achieving Golgi homeostasis. Glycosylation is a template-independent process, so its fidelity heavily relies on appropriate localization of the glycosylation machinery and Golgi homeostasis. Mutations in the glycosylation enzymes, sugar transporters, Golgi ion channels and several vesicle tethering factors cause congenital disorders of glycosylation (CDG) which encompass a group of multisystem disorders with varying severities. Here, we focus on the Golgi vesicle tethering and fusion machinery, namely, multisubunit tethering complexes and SNAREs and their role in Golgi trafficking and glycosylation. This review is a comprehensive summary of all the identified CDG causing mutations of the Golgi trafficking machinery in humans.

The dimeric structure of wild-type human glycosyltransferase B4GalT1

Most glycosyltransferases, including B4GalT1 (EC 2.4.1.38), are known to assemble into enzyme homomers and functionally relevant heteromers in vivo. However, it remains unclear why and how these enzymes interact at the molecular/atomic level. Here, we solved the crystal structure of the wild-type human B4GalT1 homodimer. We also show that B4GalT1 exists in a dynamic equilibrium between monomer and dimer, since a purified monomer reappears as a mixture of both and as we obtained crystal forms of the monomer and dimer assemblies in the same crystallization conditions. These two crystal forms revealed the unliganded B4GalT1 in both the open and the closed conformation of the Trp loop and the lid regions, responsible for donor and acceptor substrate binding, respectively. The present structures also show the lid region in full in an open conformation, as well as a new conformation for the GlcNAc acceptor loop (residues 272–288). The physiological relevance of the homodimer in the crystal was validated by targeted mutagenesis studies coupled with FRET assays. These showed that changing key catalytic amino acids impaired homomer formation in vivo. The wild-type human B4GalT1 structure also explains why the variant proteins used for crystallization in earlier studies failed to reveal the homodimers described in this study.

The extended cytoplasmic tail of the human B4GALNT2 is critical for its Golgi targeting and post-Golgi sorting

The Sd^a /Cad antigen reported on glycoconjugates of human tissues has an increasingly recognized wide impact on the physio-pathology of different biological systems. The last step of its biosynthesis relies on the enzymatic activity of the β1,4-N-acetylgalactosaminyltransferase-II (B4GALNT2), which shows the highest expression level in healthy colon. Previous studies reported the occurrence in human colonic cells of two B4GALNT2 protein isoforms that differ in the length of their cytoplasmic tail, the long isoform showing an extended 66-amino acid tail. We examined here, the subcellular distribution of the two B4GALNT2 protein isoforms in stably transfected colonic LS174T cells and in transiently transfected HeLa cells using fluorescence microscopy. While a similar subcellular distribution at the trans-Golgi cisternae level was observed for the two isoforms, our study pointed to an atypical subcellular localization of the long B4GALNT2 isoform into dynamic vesicles. We demonstrated a critical role of its extended cytoplasmic tail for its Golgi targeting and post-Golgi sorting and highlighted the existence of a newly described post-Golgi sorting signal as well as a previously undescribed fate of a Golgi glycosyltransferase.

DATABASE

The proteins β1,4GalNAcT II, β1,4-GalT1, FucT I, FucT VI and ST3Gal IV are noted B4GALNT2, B4GALT1, FUT1, FUT6 and ST3GAL4, whereas the corresponding human genes are noted B4GALNT2, B4GALT1, FUT1, FUT6 and ST3GAL4 according to the HUGO nomenclature.

Engineering β1,4-galactosyltransferase I to reduce secretion and enhance N-glycan elongation in insect cells

β1,4-galactosyltransferase I (B4GALT1) is a Golgi-resident enzyme that elongates glycoprotein glycans, but a subpopulation of this enzyme is secreted following proteolytic cleavage in its stem domain. We hypothesized that engineering B4GALT1 to block cleavage and secretion would enhance its retention and, therefore, its function. To test this hypothesis, we replaced the cytoplasmic/transmembrane/stem (CTS) domains of B4GALT1 with those from human α1,3-fucosyltransferase 7 (FUT7), which is not cleaved and secreted. Expression of FUT7-CTS-B4GALT1 in insect cells produced lower levels of secreted and higher levels of intracellular B4GALT1 activity than the native enzyme. We also noted that the B4GALT1 used in our study had a leucine at position 282, whereas all other animal B4GALT1 sequences have an aromatic amino acid at this position. Thus, we examined the combined impact of changing the CTS domains and the amino acid at position 282 on intracellular B4GALT1 activity levels and N-glycan processing in insect cells. The results demonstrated a correlation between the levels of intracellular B4GALT1 activity and terminally galactosylated N-glycans, N-glycan branching, the appearance of hybrid structures, and reduced core fucosylation. Thus, engineering B4GALT1 to reduce its cleavage and secretion is an approach that can be used to enhance N-glycan elongation in insect cells.

Golgi phosphoprotein 3 triggers signal-mediated incorporation of glycosyltransferases into coatomer-coated (COPI) vesicles

Newly synthesized membrane and secreted proteins undergo a series of posttranslational modifications in the Golgi apparatus, including attachment of carbohydrate moieties. The final structure of so-formed glycans is determined by the order of execution of the different glycosylation steps, which seems intimately related to the spatial distribution of glycosyltransferases and glycosyl hydrolases within the Golgi apparatus. How cells achieve an accurate localization of these enzymes is not completely understood but might involve dynamic processes such as coatomer-coated (COPI) vesicle-mediated trafficking. In yeast, this transport is likely to be regulated by vacuolar protein sorting 74 (Vps74p), a peripheral Golgi protein able to interact with COPI coat as well as with a binding motif present in the cytosolic tails of some mannosyltransferases. Recently, Golgi phosphoprotein 3 (GOLPH3), the mammalian homolog of Vps74, has been shown to control the Golgi localization of core 2 N-acetylglucosamine-transferase 1. Here, we highlight a role of GOLPH3 in the spatial localization of α-2,6-sialyltransferase 1. We show, for the first time, that GOLPH3 supports incorporation of both core 2 N-acetylglucosamine-transferase 1 and α-2,6-sialyltransferase 1 into COPI vesicles. Depletion of GOLPH3 altered the subcellular localization of these enzymes. In contrast, galactosyltransferase, an enzyme that does not interact with GOLPH3, was neither incorporated into COPI vesicles nor was dependent on GOLPH3 for proper localization.

The origin and function of platelet glycosyltransferases

Platelets are megakaryocyte subfragments that participate in hemostatic and host defense reactions and deliver pro- and antiangiogenic factors throughout the vascular system. Although they are anucleated cells that lack a complex secretory apparatus with distinct Golgi/endoplasmic reticulum compartments, past studies have shown that platelets have glycosyltransferase activities. In the present study, we show that members of 3 distinct glycosyltransferase families are found within and on the surface of platelets. Immunocytology and flow cytometry results indicated that megakaryocytes package these Golgi-derived glycosyltransferases into vesicles that are sent via proplatelets to nascent platelets, where they accumulate. These glycosyltransferases are active, and intact platelets glycosylate large exogenous substrates. Furthermore, we show that activation of platelets results in the release of soluble glycosyltransferase activities and that platelets contain sufficient levels of sugar nucleotides for detection of glycosylation of exogenously added substrates. Therefore, the results of the present study show that blood platelets are a rich source of both glycosyltransferases and donor sugar substrates that can be released to function in the extracellular space. This platelet-glycosylation machinery offers a pathway to a simple glycoengineering strategy improving storage of platelets and may serve hitherto unknown biologic functions.

Vertebrate protein glycosylation: diversity, synthesis and function

Protein glycosylation is a ubiquitous post-translational modification found in all domains of life. Despite their significant complexity in animal systems, glycan structures have crucial biological and physiological roles, from contributions in protein folding and quality control to involvement in a large number of biological recognition events. As a result, they impart an additional level of 'information content' to underlying polypeptide structures. Improvements in analytical methodologies for dissecting glycan structural diversity, along with recent developments in biochemical and genetic approaches for studying glycan biosynthesis and catabolism, have provided a greater understanding of the biological contributions of these complex structures in vertebrates.

GO-PROMTO illuminates protein membrane topologies of glycan biosynthetic enzymes in the Golgi apparatus of living tissues

The Golgi apparatus is the main site of glycan biosynthesis in eukaryotes. Better understanding of the membrane topology of the proteins and enzymes involved can impart new mechanistic insights into these processes. Publically available bioinformatic tools provide highly variable predictions of membrane topologies for given proteins. Therefore we devised a non-invasive experimental method by which the membrane topologies of Golgi-resident proteins can be determined in the Golgi apparatus in living tissues. A Golgi marker was used to construct a series of reporters based on the principle of bimolecular fluorescence complementation. The reporters and proteins of interest were recombinantly fused to split halves of yellow fluorescent protein (YFP) and transiently co-expressed with the reporters in the Nicotiana benthamiana leaf tissue. Output signals were binary, showing either the presence or absence of fluorescence with signal morphologies characteristic of the Golgi apparatus and endoplasmic reticulum (ER). The method allows prompt and robust determinations of membrane topologies of Golgi-resident proteins and is termed GO-PROMTO (for GOlgi PROtein Membrane TOpology). We applied GO-PROMTO to examine the topologies of proteins involved in the biosynthesis of plant cell wall polysaccharides including xyloglucan and arabinan. The results suggest the existence of novel biosynthetic mechanisms involving transports of intermediates across Golgi membranes.

Molecular insights into beta-galactoside alpha2,6-sialyltransferase secretion in vivo

Beta-galactoside alpha2,6-sialyltransferase (ST6Gal I), which is highly expressed in the liver, is mainly cleaved by Alzheimer's beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1) and secreted into the serum. During our studies to elucidate the molecular mechanism underlying the cleavage and secretion of ST6Gal I, we hypothesized that plasma ST6Gal I may represent a sensitive biomarker for hepatopathological situations. In the present study, we used recently developed sandwich ELISA systems that specifically detect the soluble cleaved form of ST6Gal I in plasma. We found that the level of plasma ST6Gal I was increased in two different types of liver injury models. In zone 1 hepatocyte-injured rats, the level of plasma ST6Gal I was increased together with acute phase reactions. Meanwhile, in zone 3 hepatocyte-injured rats, ST6Gal I secretion was most likely triggered by oxidative stress. Taken together, we propose two possible mechanisms for the upregulation of plasma ST6Gal I in hepatopathological situations: one accompanied by acute phase reactions to increase hepatic ST6Gal I expression and the other triggered by oxidative stress in the liver. We also found that the serum level of ST6Gal I in hepatitis C patients was correlated with the activity of hepatic inflammation.

Glycosylation of glycolipids in the Golgi complex

Gangliosides are a family of glycolipids characterized by containing a variable number of sialic acid residues. Nearly, all animal cells contain at least some class of ganglioside in their membranes, but membranes from the CNS are characterized by their high content of these lipids. The synthesis of the oligosaccharide moiety of glycolipids is carried out in the Golgi complex. In this study, I will discuss the cellular and molecular basis of the organization of the glycosylating machinery in the Golgi complex, with particular attention to the mutual relationships, sub-Golgi localization, and intracellular trafficking of glycolipid glycosyltransferases, and to their relationships with the corresponding glycolipid acceptors and sugar nucleotide donors. I will also discuss how the organization of the glycosylating machinery in the Golgi may adapt to events controlling glycolipid expression.

FOG-2 attenuates endothelial-to-mesenchymal transformation in the endocardial cushions of the developing heart

Development of the heart valves is a complex process that relies on the successful remodeling of endocardial cushions. This process is dependent on a number of transcriptional regulators, including GATA4 and its interacting partner FOG-2. We have previously shown that the endocardial cushions in FOG-2 deficient mice are hyperplastic and fail to remodel appropriately, suggesting a defect late in endocardial cushion development. To elucidate this defect, we examined the later steps in endocardial cushion development including mesenchymal cell proliferation, differentiation, and apoptosis. We also measured myocardialization and endothelial-to-mesenchymal transformation (EMT) using previously described in vitro assays. We found no difference in the ability of the endocardial cushions to undergo myocardialization or in the rates of mesenchymal cell proliferation, differentiation, or apoptosis in the FOG-2 deficient cushions when compared to wild-type controls. However, using a collagen gel invasion assay, we found a 78% increase in outflow tract cushion EMT and a 35% increase in atrioventricular cushion EMT in the FOG-2 deficient mice when compared with wild-type mice. Taken together with GATA4's known role in promoting EMT, these results suggest that FOG-2 functions in cardiac valve formation as an attenuator of EMT by repressing GATA4 activity within the developing endocardial cushions.

Cytoplasmic Tails of SialT2 and GalNAcT Impose Their Respective Proximal and Distal Golgi Localization

Complex glycolipid synthesis is catalyzed by different glycosyltransferases resident of the Golgi complex. Most of them are type II membrane proteins comprising a lumenal, C-terminal domain linked to an N-terminal domain (Ntd) constituted by a short cytoplasmic tail (ct), a transmembrane, and a lumenal stem regions. They concentrate selectively in different sub-Golgi compartments, in an overlapped manner, acting in succession in the addition of sugars to acceptor glycolipids. The Ntds are sufficient to localize glycosyltransferases in the Golgi complex, but it is not clear whether they also confer selective concentration in sub-Golgi compartments. Here, we studied whether the Ntd of SialT2, localized in the proximal Golgi, and the one of GalNAcT, a trans/TGN Golgi-concentrated enzyme, concentrate reporter proteins in the corresponding sub-Golgi compartment. The sub-Golgi concentration of the Ntds fused to spectral variants of the GFP was determined in CHO-K1 cells from their behavior upon addition of brefeldin A. Fluorescence microscopy and subcellular fractionation showed that the SialT2 Ntd concentrates in a proximal sub-Golgi compartment - and that of GalNAcT in TGN elements. Exchanging the transmembrane region and the cts of SialT2 and GalNAcT indicates that information for proximal or distal Golgi concentration is associated with the cts.

Cell-type specific trafficking of expressed mutant COMP in a cell culture model for PSACH

Pseudoachondroplasia (PSACH) is an autosomal dominant disease that mainly affects cartilage, resulting in skeletal dysplasias and early onset osteoarthritis. PSACH is caused by mutations in the cartilage oligomeric matrix protein (COMP) gene. PSACH chondrocytes accumulate unique COMP-containing lamellar structures in an expanded rough endoplasmic reticulum (rER). Although COMP is also present in tendon extracellular matrix (ECM), it does not accumulate in PSACH tendon cells, suggesting the disease involves a chondrocyte-specific trafficking problem. To investigate putative cell-specific trafficking differences, we generated a cell culture model utilizing expression of the common DeltaD469 COMP mutation. In rat chondrosarcoma (RCS) cells, we find delayed secretion and ER accumulation of DeltaD469 COMP, paralleling the altered trafficking defect in PSACH chondrocytes. Non-chondrocytic COS-1 cells, in contrast, efficiently trafficked and secreted both mutant and wild-type COMP. In chondrocytic cells, expression of DeltaD469 COMP led to ER accumulation of type IX collagen, but did not affect aggrecan trafficking. Endogenous rat COMP accumulated in the ER along with expressed DeltaD469 COMP in a stably expressing RCS clone, consistent with the dominant negative effect of PSACH. When these stably expressing cells were cultured to promote ECM deposition, the small amount of secreted mutant COMP disrupted assembly of the normal fibrillar meshwork and caused irregular aggregates of COMP and type IX collagen to form. Thus, in a new model that reflects the cellular pathology of PSACH, we establish trafficking differences for mutant COMP in chondrocytic and non-chondrocytic cells and demonstrate that mutant COMP interferes with assembly of a normal ECM.

In Vivo Cleavage of α2,6-Sialyltransferase by Alzheimer β-Secretase

beta-Site amyloid precursor protein-cleaving enzyme 1 (BACE1) is a membrane-bound aspartic protease that cleaves amyloid precursor protein to produce a neurotoxic peptide, Abeta, and is implicated in triggering the pathogenesis of Alzheimer disease. We previously reported that BACE1 cleaved rat beta-galactoside alpha2,6-sialyltransferase (ST6Gal I) that was overexpressed in COS cells and that the NH(2) terminus of ST6Gal I secreted from the cells (E41 form) was Glu(41). Here we report that BACE1 gene knock-out mice have one third as much plasma ST6Gal I as control mice, indicating that BACE1 is a major protease which is responsible for cleaving ST6Gal I in vivo. We also found that BACE1-transgenic mice have increased level of ST6Gal I in plasma. Secretion of ST6Gal I from the liver into the plasma is known to be up-regulated during the acute-phase response. To investigate the role of BACE1 in ST6Gal I secretion in vivo, we analyzed the levels of BACE1 mRNA in the liver, as well as the plasma levels of ST6Gal I, in a hepatopathological model, i.e. Long-Evans Cinnamon (LEC) rats. This rat is a mutant that spontaneously accumulates copper in the liver and incurs hepatic damage. LEC rats exhibited simultaneous increases in BACE1 mRNA in the liver and in the E41 form of the ST6Gal I protein, the BACE1 product, in plasma as early as 6 weeks of age, again suggesting that BACE1 cleaves ST6Gal I in vivo and controls the secretion of the E41 form.

Multiple signals are required for alpha2,6-sialyltransferase (ST6Gal I) oligomerization and Golgi localization

A single amino acid difference in the catalytic domain of two isoforms of the alpha2,6-sialyltransferase (ST6Gal I) leads to differences in their trafficking, processing, and oligomerization. The STtyr isoform is transiently localized in the Golgi and is ultimately cleaved and secreted, whereas the STcys isoform is stably localized in the Golgi and is not cleaved and secreted. The stable localization of STcys is correlated with its enhanced ability to oligomerize. To test the hypothesis that multiple signals can mediate Golgi localization and further evaluate the role of oligomerization in the localization process, we evaluated the effects of individually and simultaneously altering the cytosolic tail and transmembrane region of the STcys isoform. We found that the localization, processing, and oligomerization of STcys were not substantially changed when either the core amino acids of the cytosolic tail were deleted or the sequence and length of the transmembrane region were altered. In contrast, when these changes were made simultaneously, the STcys isoform was converted into a form that was processed, secreted, and weakly oligomerized like STtyr. We propose that STcys oligomerization is a secondary event resulting from its concentration in the Golgi via mechanisms independently mediated by its cytosolic tail and transmembrane region.

Deletion of the cytoplasmic domain of human α3/4 fucosyltransferase III causes the shift of the enzyme to early Golgi compartments

The transmembrane domain (TM) and flanking regions of glycosyltransferases (GTs) have been implicated in the localization of these proteins in the Golgi apparatus (GA). alpha3/4 Fucosyltransferase III (FT3wt) (EC 2.4.1.65) is localized in the trans-Golgi and trans-Golgi network (TGN) of baby hamster kidney (BHK) cells and synthesizes Lewis determinants associated with cell adhesion events. We have evaluated the effect of removing the cytosolic domain on the localization of the enzyme and its capacity for synthesizing the Lewis A (Le A) determinant. The mutant where the cytoplasmic domain (Asp-2 to Trp-13) of FT3wt has been deleted (FT3dc) was localized in the Golgi but it was shifted to earlier compartments than FT3wt. The mutant was not detected on the plasma membrane (PM) and glycosylation analysis indicated that FT3dc was transported beyond the endoplasmic reticulum (ER) since complex type glycosylation was observed. Cells expressing FT3dc showed a significantly lower efficiency to synthesize Le A when compared with cells expressing FT3wt, in vivo. This reduction was not due to lower specific activity because both enzyme forms had a similar specific activity in vitro. Therefore, removal of FT3 cytosolic tail caused a shift in enzyme distribution to earlier Golgi compartments concomitant to the decrease of its biosynthetic capacity.

The stem region of the sulfotransferase GlcNAc6ST-1 is a determinant of substrate specificity

The GlcNAc-6-sulfotransferases are a family of Golgi-resident enzymes that modulate glycan function. Two members of this family, GlcNAc6ST-1 and -2, collaborate in the biosynthesis of ligands for the leukocyte adhesion molecule L-selectin. Although their biochemical properties are similar in vitro, the enzymes have distinct glycoprotein substrate preferences in vivo. The sulfotransferases share similar overall architecture with the exception of an extended stem region in GlcNAc6ST-1 that is absent in GlcNAc6ST-2. In this study we probed the importance of the stem region with respect to substrate preference, localization, and oligomerization. Analysis of truncation mutants demonstrated that perturbation of the stem region of GlcNAc6ST-1 affects the cellular substrate preference of the enzyme without altering its retention within the Golgi. A chimeric enzyme comprising the stem region of GlcNAc6ST-1 inserted between the catalytic and transmembrane domains of GlcNAc6ST-2 had the same substrate preference as native GlcNAc6ST-1. In cells, GlcNAc6ST-1 exists as a dimer; two cysteine residues within the stem and transmembrane domain were found to be critical for dimerization. However, disruption of the dimer by mutagenesis did not affect either localization or substrate preference. Collectively, these results indicate that the stem region of GlcNAc6ST-1 influences substrate specificity, independent of its role in dimerization or Golgi retention.

Effects of Neprilysin Chimeric Proteins Targeted to Subcellular Compartments on Amyloid β Peptide Clearance in Primary Neurons

Neprilysin (NEP) is a rate-limiting amyloid beta peptide (Abeta)-degrading enzyme in the brain. We demonstrated previously that overexpression of neprilysin in primary cortical neurons remarkably decreased not only extracellular but also intracellular Abeta levels. To investigate the subcellular compartments where neprilysin degrades Abeta most efficiently, we expressed neprilysin chimeric proteins containing various subcellular compartment-targeting domains in neurons. Sec12-NEP, beta-galactoside alpha2,6-sialyltransferase-NEP, transferrin receptor-NEP, and growth-associated protein 43-NEP were successfully sorted to the endoplasmic reticulum, trans-Golgi network, early/recycling endosomes, and lipid rafts, respectively. We found that intracellularly, wild-type neprilysin and all the chimeras showed equivalent Abeta40-degrading activities. Abeta40 was more effectively cleared than Abeta42, and this tendency was greater for intracellular Abeta than for extracellular Abeta. Wild-type and trans-Golgi network-targeted ST-NEP cleared more intracellular Abeta42 than the other chimeras. Wild-type neprilysin cleared extracellular Abeta more effectively than any of the chimeras, among which endoplasmic reticulum-targeted Sec12-NEP was the least effective. These observations indicate that different intracellular compartments may be involved in the metabolism of distinct pools of Abeta (Abeta40 and Abeta42) to be retained or recycled intracellularly and to be secreted extracellularly, and that the endogenous targeting signal in wild-type neprilysin is well optimized for the overall neuronal clearance of Abeta.

Endoplasmic reticulum export of glycosyltransferases depends on interaction of a cytoplasmic dibasic motif with Sar1

Membrane proteins exit the endoplasmic reticulum (ER) in COPII-transport vesicles. ER export is a selective process in which transport signals present in the cytoplasmic tail (CT) of cargo membrane proteins must be recognized by coatomer proteins for incorporation in COPII vesicles. Two classes of ER export signals have been described for type I membrane proteins, the diacidic and the dihydrophobic motifs. Both motifs participate in the Sar1-dependent binding of Sec23p-Sec24p complex to the CTs during early steps of cargo selection. However, information concerning the amino acids in the CTs that interact with Sar1 is lacking. Herein, we describe a third class of ER export motif, [RK](X)[RK], at the CT of Golgi resident glycosyltransferases that is required for these type II membrane proteins to exit the ER. The dibasic motif is located proximal to the transmembrane border, and experiments of cross-linking in microsomal membranes and of binding to immobilized peptides showed that it directly interacts with the COPII component Sar1. Sar1GTP-bound to immobilized peptides binds Sec23p. Collectively, the present data suggest that interaction of the dibasic motif with Sar1 participates in early steps of selection of Golgi resident glycosyltransferases for transport in COPII vesicles.

The minimal structural domains required for neural cell adhesion molecule polysialylation by PST/ST8Sia IV and STX/ST8Sia II

A limited number of mammalian proteins are modified by polysialic acid, with the neural cell adhesion molecule (NCAM) being the most abundant of these. We hypothesize that polysialylation is a protein-specific glycosylation event and that an initial protein-protein interaction between polysialyltransferases and glycoprotein substrates mediates this specificity. To evaluate the regions of NCAM required for recognition and polysialylation by PST/ST8Sia IV and STX/ST8Sia II, a series of domain deletion proteins were generated, co-expressed with each enzyme, and their polysialylation analyzed. A protein consisting of the fifth immunoglobulin-like domain (Ig5), which contains the reported sites of polysialylation, and the first fibronectin type III repeat (FN1) was polysialylated by both enzymes, whereas a protein consisting of Ig5 alone was not polysialylated by either enzyme. This demonstrates that the Ig5 domain of NCAM and FN1 are sufficient for polysialylation, and suggests that the FN1 may constitute an enzyme recognition and docking site. Two other NCAM mutants, NCAM-6 (Ig1-5) and NCAM-7 (FN1-FN2), were weakly polysialylated by PST/ST8Sia IV, suggesting that a weaker enzyme recognition site may exist within the Ig domains, and that glycans in the FN region are polysialylated. Further analysis indicated that O-linked oligosaccharides in NCAM-7, and O-linked and N-linked glycans in full-length NCAM, are polysialylated when these proteins are co-expressed with the polysialyltransferases in COS-1 cells. Our data support a model in which the polysialyltransferases bind to the FN1 of NCAM to polymerize polysialic acid chains on appropriately presented glycans in adjacent regions.

Structural requirements for Arabidopsis beta1,2-xylosyltransferase activity and targeting to the Golgi

Characterization of a beta1,2-xylosyltransferase from Arabidopsis thaliana (AtXylT) was carried out by expression in Sf9 insect cells using a baculovirus vector system. Serial deletions at both the N- and C-terminal ends proved that integrity of a large domain located between amino acid 31 and the C-terminal lumenal region is required for AtXylT activity expression. The influence of N-glycosylation on AtXylT activity has been evaluated using either tunicamycin or mutagenesis of potential N-glycosylation sites. AtXylT is glycosylated on two of its three potential N-glycosylation sites (Asn51, Asn301, Asn478) and the occupancy of at least one of these two sites (Asn51 and Asn301) is necessary for AtXylT stability and activity. Contribution of the N-terminal part of AtXylT in targeting and intracellular distribution of this protein was studied by expression of variably truncated, GFP-tagged AtXylT forms in tobacco cells using confocal and electron microscopy. These studies have shown that the transmembrane domain of AtXylT and its short flanking amino acid sequences are sufficient to specifically localize a reporter protein to the medial Golgi cisternae in tobacco cells. This study is the first detailed characterization of a plant glycosyltransferase at the molecular level.

Biosynthesis and cellular trafficking of the convertase SKI-1/S1P: ectodomain shedding requires SKI-1 activity

Subtilisin kexin isozyme-1 (SKI-1)/site 1 protease is a mammalian subtilase composed of distinct functional domains. Among the major substrates of SKI-1 are the sterol regulatory element-binding proteins, regulating cholesterol and fatty acid homeostasis. Other substrates include the stress response factor activating transcription factor-6, the brain-derived neurotrophic factor, and the surface glycoproteins of highly infectious viruses belonging to the family of Arenaviridae. Domain deletion and/or point mutants were used to gauge the role of the various domains of SKI-1. Biosynthesis, cellular trafficking, and sterol regulatory element-binding protein-2 cleavage activity were used as diagnostic tools. Results revealed that Arg(130) and Arg(134) are critical for the autocatalytic primary processing of the prosegment and for the subsequent efficient exit of SKI-1 from the endoplasmic reticulum. Functional mapping of the growth factor cytokine receptor motif suggested a folding role within the endoplasmic reticulum. Microsequencing of the remaining membrane-bound stub following ectodomain shedding of SKI-1 localized the shedding site to KHQKLL(953) downward arrow. Site-directed mutagenesis, in vitro cleavage of a synthetic peptide containing the shedding site, and inhibitor studies favor an autocatalytic event occurring at a non-canonical SKI-1 recognition sequence, with P2 and P1 Leu being very critical. In conclusion, multiple domains ensuring optimal functional characteristics control SKI-1 activity and cellular trafficking.

Alzheimer's beta-secretase, beta-site amyloid precursor protein-cleaving enzyme, is responsible for cleavage secretion of a Golgi-resident sialyltransferase

The deposition of amyloid beta-peptide (A beta) in the brain is closely associated with the development of Alzheimer's disease. A beta is generated from the amyloid precursor protein (APP) by sequential action of beta-secretase (BACE1) and gamma-secretase. Although BACE1 is distributed among various other tissues, its physiological substrates other than APP have yet to be identified. ST6Gal I is a sialyltransferase that produces a sialyl alpha 2,6galactose residue, and the enzyme is secreted out of the cell after proteolytic cleavage. We report here that BACE1 is involved in the proteolytic cleavage of ST6Gal I, on the basis of the following observations. ST6Gal I was colocalized with BACE1 in the Golgi apparatus by immunofluorescence microscopy, suggesting that BACE1 acts on ST6Gal I within the same intracellular compartment. When BACE1 was overexpressed with ST6Gal I in COS cells, the secretion of ST6Gal I markedly increased. When APP(SW) (Swedish familial Alzheimer's disease mutation), a preferable substrate for BACE1, was coexpressed with ST6Gal I in COS cells, the secretion of ST6Gal I significantly decreased, suggesting that that the beta-cleavage of overexpressed APP(SW) competes with ST6Gal I processing. In addition, BACE1-Fc (Fc, the hinge and constant region of IgG) chimera cleaved protein A-ST6Gal I fusion protein in vitro. Thus, we conclude that BACE1 is responsible for the cleavage and secretion of ST6Gal I.

The polysialyltransferase ST8Sia II/STX: posttranslational processing and role of autopolysialylation in the polysialylation of neural cell adhesion molecule

The presence of alpha2,8-linked polysialic acid on the neural cell adhesion molecule (NCAM) is known to modulate cell interactions during development and oncogenesis. Two enzymes, the alpha2,8-polysialyltransferases ST8Sia IV()/PST and ST8Sia II()/STX are responsible for the polysialylation of NCAM. We previously reported that both ST8Sia IV/PST and ST8Sia II/STX enzymes are themselves modified by alpha2,8-linked polysialic acid chains, a process called autopolysialylation. In the case of ST8Sia IV/PST, autopolysialylation is not required for enzymatic activity. However, whether the autopolysialylation of ST8Sia II/STX is required for its ability to polysialylate NCAM is unknown. To understand how autopolysialylation impacts ST8Sia II/STX enzymatic activity, we employed a mutagenesis approach. We found that ST8Sia II/STX is modified by six Asn-linked oligosaccharides and that polysialic acid is distributed among the oligosaccharides modifying Asn 89, 219, and 234. Coexpression of a nonautopolysialylated ST8Sia II/STX mutant with NCAM demonstrated that autopolysialylation is not required for ST8Sia II/STX polysialyltransferase activity. In addition, catalytically active, nonautopolysialylated ST8Sia II/STX does not polysialylate any endogenous COS-1 cell proteins, highlighting the protein specificity of polysialylation. Furthermore, immunoblot analysis of NCAM polysialylation by autopolysialylated and nonautopolysialylated ST8Sia II/STX suggests that the NCAM is polysialylated to a higher degree by autopolysialylated ST8Sia II/STX. Therefore, we conclude that autopolysialylation of ST8Sia II/STX, like that of ST8Sia IV/PST, is not required for, but does enhance, NCAM polysialylation.

Location and mechanism of alpha 2,6-sialyltransferase dimer formation. Role of cysteine residues in enzyme dimerization, localization, activity, and processing

A significant proportion of the alpha2,6-sialyltransferase of protein Asn-linked glycosylation (ST6Gal I) forms disulfide-bonded dimers that exhibit decreased activity, but retain the ability to bind asialoglycoprotein substrates. Here, we have investigated the subcellular location and mechanism of ST6Gal I dimer formation, as well as the role of Cys residues in the enzyme's trafficking, localization, and catalytic activity. Pulse-chase analysis demonstrated that the ST6Gal I disulfide-bonded dimer forms in the endoplasmic reticulum. Mutagenesis experiments showed that Cys-24 in the transmembrane region is required for dimerization, while catalytic domain Cys residues are required for trafficking and catalytic activity. Replacement of Cys-181 and Cys-332 generated proteins that are largely retained in the endoplasmic reticulum and minimally active or inactive, respectively. Replacement of Cys-350 or Cys-361 inactivated the enzyme without compromising its localization or processing, suggesting that these amino acids are part of the enzyme's active site. Replacement of Cys-139 or Cys-403 generated proteins that are catalytically active and appear to be more stably localized in the Golgi, since they exhibited decreased cleavage and secretion. The Cys-139 mutant also exhibited increased dimer formation suggesting that ST6Gal I dimers may be critical in the oligomerization process involved in stable ST6Gal I Golgi localization.

Conserved cysteines in the sialyltransferase sialylmotifs form an essential disulfide bond

The sialyltransferase gene family is comprised of 16 cloned enzymes. All members contain two conserved protein domains, termed the S- and L-sialylmotifs, that participate in substrate binding. Of only six invariant amino acids, two are cysteines, with one found in each sialylmotif. Although the recombinant soluble form of ST6Gal I has six cysteines, quantitative analysis indicated the presence of only one disulfide linkage, and thiol reducing agents dithiothreitol and beta-mercaptoethanol inactivated the enzyme. Analysis of site-directed mutants showed that alanine or serine mutants of invariant Cys(181) or Cys(332) exhibit no detectable activity, either by direct assay or by staining of the transfected cells with Sambucus nigra agglutinin, which recognizes the product NeuAcalpha2,6Galbeta1,4GlcNAc on glycoproteins. In contrast, alanine mutations of charged residues adjacent to either cysteine showed little or no effect on enzyme activity. Immunofluorescence microscopy showed that although the wild type sialyltransferase is properly localized in the Golgi apparatus, the inactive cysteine mutants are retained in the endoplasmic reticulum. The results suggest that the invariant cysteine residues in the L- and S-sialylmotifs participate in the formation of an intradisulfide linkage that is essential for proper conformation and activity of ST6Gal I.

Aggrecan domains expected to traffic through the exocytic pathway are misdirected to the nucleus

In this article, we report the misdirected targeting of expressed aggrecan domains. Aggrecan, the chondroitin sulfate (CS) proteoglycan of cartilage, normally progresses through the exocytic pathway. Proteins expressed from constructs containing the putative aggrecan signal sequence (i.e., the first 23 N-terminal amino acids), specified globular (G) domains G1 and/or G3, and a segment of the CS domain were detected in the endoplasmic reticulum (ER) and Golgi complex. Although proteins expressed from constructs containing the putative signal and G3, but lacking G1, were detected to a limited extent in the secretory pathway, they primarily accumulated in nuclei. Discrete nuclear inclusions were seen when G3 was expressed. Immunoelectron microscopic characterization of the inclusions suggested the association of nuclear G3 with other proteins. When signal-free G3 constructs and those with G3 immediately following the N-terminal signal were expressed, abundant dispersed accumulations filled the nucleoplasm. The data suggest first, that signal-free and signal-containing G3 proteins enter the nucleus from the cytosol, and second, that the entry of signal-containing G3 proteins into the ER lumen is inefficient. Hsp25, Hsp70, and ubiquitin were colocalized with nuclear G3, indicating the involvement of chaperones and the degradative machinery in the formation and/or attempted disposal of the abnormal nuclear inclusions. Overall, the results focus attention on (1) intracellular protein trafficking at the ER membrane and the nuclear envelope and (2) chaperone interactions and mechanisms leading to abnormal protein deposition in the nucleus.

Rab11 regulates the compartmentalization of early endosomes required for efficient transport from early endosomes to the trans-golgi network

Several GTPases of the Rab family, known to be regulators of membrane traffic between organelles, have been described and localized to various intracellular compartments. Rab11 has previously been reported to be associated with the pericentriolar recycling compartment, post-Golgi vesicles, and the trans-Golgi network (TGN). We compared the effect of overexpression of wild-type and mutant forms of Rab11 on the different intracellular transport steps in the endocytic/degradative and the biosynthetic/exocytic pathways in HeLa cells. We also studied transport from endosomes to the Golgi apparatus using the Shiga toxin B subunit (STxB) and TGN38 as reporter molecules. Overexpression of both Rab11 wild-type (Rab11wt) and mutants altered the localization of the transferrrin receptor (TfR), internalized Tf, the STxB, and TGN38. In cells overexpressing Rab11wt and in a GTPase-deficient Rab11 mutant (Rab11Q70L), these proteins were found in vesicles showing characteristics of sorting endosomes lacking cellubrevin (Cb). In contrast, they were redistributed into an extended tubular network, together with Cb, in cells overexpressing a dominant negative mutant of Rab11 (Rab11S25N). This tubularized compartment was not accessible to Tf internalized at temperatures <20 degrees C, suggesting that it is of recycling endosomal origin. Overexpression of Rab11wt, Rab11Q70L, and Rab11S25N also inhibited STxB and TGN38 transport from endosomes to the TGN. These results suggest that Rab11 influences endosome to TGN trafficking primarily by regulating membrane distribution inside the early endosomal pathway.

Distinct Influences of Carboxyl Terminal Segment Structure on Function in the Two Isoforms of Prostaglandin H Synthase

The cyclooxygenase activity of the two prostaglandin H synthase (PGHS) isoforms, PGHS-1 and -2, is a major control element in prostanoid biosynthesis. The two PGHS isoforms have 60% amino acid identity, with prominent differences near the C-terminus, where PGHS-2 has an additional 18-residue insert. Some mutations of the C-terminal residue in PGHS-1 and -2 have been found to disrupt catalytic activity and/or intracellular targeting of the proteins, but the relationship between C-terminal structure and function in the two isoforms has been poorly defined. Crystallographic data indicate the PGHS-1 and -2 C-termini are positioned to interact with the endoplasmic reticulum (ER) membrane, although the C-terminal segment structure was not resolved for either isoform. We constructed a series of C-terminal substitution, deletion, and insertion mutants of human PGHS-1 and -2 and evaluated the effects on cyclooxygenase activity and intracellular targeting in transfected COS-1 cells expressing the recombinant proteins. PGHS-1 cyclooxygenase activity was strongly disrupted by C-terminal substitutions and deletions, but not by elongation of the C-terminal segment, even when the ultimate residue was altered. Similar alterations to PGHS-2 had markedly less effect on cyclooxygenase activity. The results indicate that the functioning of the longer C-terminal segment in PGHS-2 is distinctly more tolerant of structural change than the shorter PGHS-1 C-terminal segment. C-Terminal substitutions or deletions did not change the subcellular localization of either isoform, even at short times after transfection, indicating that neither C-terminal segment contains indispensable intracellular targeting signals.

Structure/function of the human Ga1beta1,3-glucuronosyltransferase. Dimerization and functional activity are mediated by two crucial cysteine residues

Galbeta1,3-glucuronosyltransferase (GlcAT-I) that catalyzes the transfer of a glucuronic acid residue onto the trisaccharide primer of the glycosaminoglycan-protein linkage region plays an essential role in the early steps of the biosynthesis of glycosaminoglycans. In order to gain insight into the structure/function of the enzyme, the human recombinant GlcAT-I was successfully expressed in the yeast Pichia pastoris, with an apparent molecular mass of 43 kDa. Analysis of the electrophoretic mobility of the membrane-bound protein in nonreducing and reducing conditions, together with cross-linking studies, indicated that the membrane-bound GlcAT-I formed active disulfide-linked dimers. GlcAT-I expressed without the predicted N-terminal cytoplasmic tail or secreted as a polypeptide lacking the cytoplasmic tail and transmembrane domain was similarly organized as dimers, suggesting that the structural determinants for the dimerization state are localized in the luminal domain of the protein. In addition, the role of Cys(33) and Cys(301) in that process was investigated by site-directed mutagenesis combined with chemical modification of GlcAT-I by N-phenylmaleimide. Replacement of Cys(33) with alanine abolished the formation of dimers with a concomitant decrease in the catalytic efficiency mainly due to a decrease in apparent maximal velocity and in affinity for UDP-glucuronic acid. On the other hand, N-phenylmaleimide treatment or alanine substitution of the Cys(301) residue inactivated the enzyme. Our study demonstrates that GlcAT-I is organized as a homodimer as a result of disulfide bond formation mediated by Cys(33) localized in the stem region, whereas the residue Cys(301) localized in a conserved C-terminal domain is strictly required for the functional integrity of the enzyme.

Formation of insoluble oligomers correlates with ST6Gal I stable localization in the golgi

The ST6Gal I is a sialyltransferase that functions in the late Golgi to modify the N-linked oligosaccharides of glycoproteins. The ST6Gal I is expressed as two isoforms with a single amino acid difference in their catalytic domains. The STcys isoform is stably retained in the cell and is predominantly found in the Golgi, whereas the STtyr isoform is only transiently localized in the Golgi and is cleaved and secreted from a post-Golgi compartment. These two ST6Gal I isoforms were used to explore the role of the bilayer thickness mechanism and oligomerization in Golgi localization. Analysis of STcys and STtyr proteins with longer transmembrane regions suggested that the bilayer thickness mechanism is not the predominant mechanism used for ST6Gal I Golgi localization. In contrast, the formation and quantity of Triton X-100-insoluble oligomers was correlated with the stable or transient localization of the ST6Gal I isoforms in the Golgi. Nearly 100% of the STcys and only 13% of the STtyr were found as Triton-insoluble oligomers when Golgi membranes of COS-1 cells expressing these proteins were solubilized at pH 6.3, the pH of the late Golgi. In contrast, both proteins were found in the soluble fraction when these membranes were solubilized at pH 8.0. Analysis of other mutants suggested that a conformational change in the catalytic domain rather than increased disulfide bond-based cross-linking is the basis for the increased ability of STcys protein to form oligomers and the stable localization of STcys protein in the Golgi.

Minimal structural and glycosylation requirements for ST6Gal I activity and trafficking

The influence of N-linked glycosylation on the activity and trafficking of membrane associated and soluble forms of the STtyr isoform of the ST6Gal I has been evaluated. We have demonstrated that the enzyme is glycosylated on Asn 146 and Asn 158 and that glycosylation is not required for the endoplasmic reticulum to Golgi transport of the membrane-associated form of the STtyr isoform. In addition, N-linked glycosylation may stabilize the protein but is not absolutely required for catalytic activity in vivo. In contrast, soluble forms of the protein consisting of amino acids 64-403, 89-403, and 97-403 are efficiently secreted and active in their fully glycosylated forms, but retained in the endoplasmic reticulum and inactive in their unglycosylated forms. These results suggest that membrane associated and soluble forms of the STtyr protein have different requirements for N-linked glycosylation. Elimination of the oligosaccharide attached to Asn 158 in the full length STtyr single and double glycosylation mutants generates proteins that are not cleaved and secreted but stably localized in the Golgi, like the STcys isoform of the ST6Gal I. This stable Golgi localization is correlated with the observation that these two mutants are active in in vivo assays but inactive in in vitro assays of membrane lysates. We predict that removal of N-linked oligosaccharides leads to an increased ability of the STtyr protein to self-associate or oligomerize which subsequently allows more stable retention in the Golgi and increased aggregation and inactivity when membranes are lysed in the in vitro activity assays.

Localization of ribophorin II to the endoplasmic reticulum involves both its transmembrane and cytoplasmic domains

Proteins that are concentrated in specific compartments of the endomembrane system in order to exert their organelle-specific function must possess specific localization signals that prevent their transport to distal regions of the exocytic pathway. Some resident proteins of the endoplasmic reticulum (ER) that are known to escape with low efficiency from this organelle to a post ER compartment are recognized by a recycling receptor and brought back to their site of residence. Other ER proteins, however, appear to be retained in the ER by mechanisms that operate in the organelle itself. The mammalian oligosaccharyltransferase (OST) is a protein complex that effects the cotranslational N-glycosylation of newly synthesized polypeptides, and is composed of at least four rough ER-specific membrane proteins: ribophorins I and II (RI and RII), OST48, and Dadl. The mechanism(s) by which the subunits of this complex are retained in the ER are not well understood. In an effort to identify the domains within RII responsible for its ER localization we have studied the fate of chimeric proteins in which one or more RII domains were replaced by the corresponding ones of the Tac antigen, the latter being a well characterized plasma membrane protein that lacks intrinsic ER retention signals and serves to provide a neutral framework for the identification of retention signals in other proteins. We found that the luminal domain of RII by itself does not contain retention information, while the cytoplasmic and transmembrane domains contain independent ER localization signals. We also show that the retention function of the transmembrane domain is strengthened by the presence of a flanking luminal region consisting of 15 amino acids.

Retention of subunits of the oligosaccharyltransferase complex in the endoplasmic reticulum

Membrane proteins of the endoplasmic reticulum (ER) may be localized to this organelle by mechanisms that involve retention, retrieval, or a combination of both. For luminal ER proteins, which contain a KDEL domain, and for type I transmembrane proteins carrying a dilysine motif, specific retrieval mechanisms have been identified. However, most ER membrane proteins do not contain easily identifiable retrieval motifs. ER localization information has been found in cytoplasmic, transmembrane, or luminal domains. In this study, we have identified ER localization domains within the three type I transmembrane proteins, ribophorin I (RI), ribophorin II (RII), and OST48. Together with DAD1, these membrane proteins form an oligomeric complex that has oligosaccharyltransferase (OST) activity. We have previously shown that ER retention information is independently contained within the transmembrane and the cytoplasmic domain of RII, and in the case of RI, a truncated form consisting of the luminal domain was retained in the ER. To determine whether other domains of RI carry additional retention information, we have generated chimeras by exchanging individual domains of the Tac antigen with the corresponding ones of RI. We demonstrate here that only the luminal domain of RI contains ER retention information. We also show that the dilysine motif in OST48 functions as an ER localization motif because OST48 in which the two lysine residues are replaced by serine (OST48ss) is no longer retained in the ER and is found instead also at the plasma membrane. OST48ss is, however, retained in the ER when coexpressed with RI, RII, or chimeras, which by themselves do not exit from the ER, indicating that they may form partial oligomeric complexes by interacting with the luminal domain of OST48. In the case of the Tac chimera containing only the luminal domain of RII, which by itself exits from the ER and is rapidly degraded, it is retained in the ER and becomes stabilized when coexpressed with OST48.

Polysialyltransferase-1 autopolysialylation is not requisite for polysialylation of neural cell adhesion molecule

Polysialyltransferase-1 (PST; ST8Sia IV) is one of the alpha2, 8-polysialyltransferases responsible for the polysialylation of the neural cell adhesion molecule (NCAM). The presence of polysialic acid on NCAM has been shown to modulate cell-cell and cell-matrix interactions. We previously reported that the PST enzyme itself is modified by alpha2,8-linked polysialic acid chains in vivo. To understand the role of autopolysialylation in PST enzymatic activity, we employed a mutagenesis approach. We found that PST is modified by five Asn-linked oligosaccharides and that the vast majority of the polysialic acid is found on the oligosaccharide modifying Asn-74. In addition, the presence of the oligosaccharide on Asn-119 appeared to be required for folding of PST into an active enzyme. Co-expression of the PST Asn mutants with NCAM demonstrated that autopolysialylation is not required for PST polysialyltransferase activity. Notably, catalytically active, non-autopolysialylated PST does not polysialylate any endogenous COS-1 cell proteins, highlighting the protein specificity of polysialylation. Immunoblot analyses of NCAM polysialylation by polysialylated and non-autopolysialylated PST suggests that the NCAM is polysialylated to a higher degree by autopolysialylated PST. We conclude that autopolysialylation of PST is not required for, but does enhance, NCAM polysialylation.

alpha(1,2)-fucosylation prevents sialyl Lewis x expression and E-selectin-mediated adhesion of fucosyltransferase VII-transfected cells

E-selectin is a cytokine-inducible, calcium-dependent endothelial cell adhesion molecule that plays a critical role in the leucocyte-endothelium interaction during inflammation and is thought to contribute to the metastatic dissemination of tumour cells. Like the other selectins, E-selectin binds to ligands carrying the tetrasaccharide sialyl-Lewis x (NeuAcalpha2,3Galbeta1,4[Fucalpha1, 3]GlcNAc)1 or its isomer sialyl-Lewis a (NeuAcalpha2, 3Galbeta1, 3[Fucalpha1,4]GlcNAc). We examined the effect of expressing the H-type alpha(1,2)-fucosyltransferase or the alpha(2, 6)-sialyltransferase on the synthesis of sialyl-Lewis x by alpha(1, 3)fucosyltransferase. We found that H-type alpha(1, 2)-fucosyltransferase but not alpha(2,6)-sialyltransferase, strongly inhibited sialyl-Lewis x expression and E-selectin adhesion. We assume that H-type alpha(1,2)-fucosyltransferase competes with the endogenous alpha(2,3)-sialyltransferase for the N-acetyllactosamine structures assigned to further serve as acceptors for alpha(1, 3)fucosyltransferase.

The cytoplasmic, transmembrane, and stem regions of glycosyltransferases specify their in vivo functional sublocalization and stability in the Golgi

We provide evidence for the presence of targeting signals in the cytoplasmic, transmembrane, and stem (CTS) regions of Golgi glycosyltransferases that mediate sorting of their intracellular catalytic activity into different functional subcompartmental areas of the Golgi. We have constructed chimeras of human alpha1, 3-fucosyltransferase VI (FT6) by replacement of its CTS region with those of late and early acting Golgi glycosyltransferases and have stably coexpressed these constructs in BHK-21 cells together with the secretory reporter glycoprotein human beta-trace protein. The sialyl Lewis X:Lewis X ratios detected in beta-trace protein indicate that the CTS regions of the early acting GlcNAc-transferases I (GnT-I) and III (GnT-III) specify backward targeting of the FT6 catalytic domain, whereas the CTS region of the late acting human alpha1,3-fucosyltransferase VII (FT7) causes forward targeting of the FT6 in vivo activity in the biosynthetic glycosylation pathway. The analysis of the in vivo functional activity of nine different CTS chimeras toward beta-trace protein allowed for a mapping of the CTS donor glycosyltransferases within the Golgi/trans-Golgi network: GnT-I < (ST6Gal I, ST3Gal III) < GnT-III < ST8Sia IV < GalT-I < (FT3, FT6) < ST3Gal IV < FT7. The sensitivity or resistance of the donor glycosyltransferases toward intracellular proteolysis is transferred to the chimeric enzymes together with their CTS regions. Apparently, there are at least three different signals contained in the CTS regions of glycosyltransferases mediating: first, their Golgi retention; second, their targeting to specific in vivo functional areas; and third, their susceptibility toward intracellular proteolysis as a tool for the regulation of the intracellular turnover.

The relationship between ST6Gal I Golgi retention and its cleavage-secretion

The ST6Gal I is a sialyltransferase that modifies N-linked oligosaccharides of glycoproteins. Previous results suggested a role for luminal stem and active domain sequences in the efficiency of ST6Gal I Golgi retention. Characterization of a series of STtyr isoform deletion mutants demonstrated that the stem is sensitive to proteases and that preventing cleavage in this region leads to increased cell surface expression. A mutant lacking amino acids 32-104 (STDelta4) is not active or cleaved and secreted like the wild type STtyr, but does exhibit increased cell surface expression. It is probable that the STDelta4 mutant lacks the stem region and some amino acids of the active domain because the STDelta5 mutant lacking amino acids 86-104 is also not active but is cleaved and secreted. In contrast, deletion of stem amino acids between residues 32 and 86 in the STDelta1, STDelta2, and STDelta3 mutants does not inactive these enzyme forms, eliminate their cleavage and secretion, or increase their cell surface expression. Surprisingly, cleavage occurs even though the previously identified Asn63-Ser 64 cleavage site is missing. Further evaluation demonstrated that a cleavage site between Lys 40 and Glu 41 is used in COS cells. Mutagenesis of Lys 40 significantly decreased, but did not eliminate cleavage, suggesting that there are additional secondary sites of cleavage in the ST6Gal I stem.

Sialyltransferase isoforms are phosphorylated in the cis-medial Golgi on serine and threonine residues in their luminal sequences

ST6Gal-I (alpha2,6-sialyltransferase) is expressed as two isoforms, STTyr and STCys, which exhibit differences in catalytic activity, trafficking through the secretory pathway, and proteolytic processing and secretion. We have found that the ST6Gal-I isoforms are phosphorylated on luminal Ser and Thr residues. Immunoprecipitation of 35S- and 32P-labeled proteins expressed in COS-1 cells suggests that the STTyr isoform is phosphorylated to a greater extent than the STCys isoform. Analysis of domain deletion mutants revealed that STTyr is phosphorylated on stem and catalytic domain amino acids, whereas STCys is phosphorylated on catalytic domain amino acids. An endoplasmic reticulum retained/retrieved chimeric Iip33-ST protein demonstrates drastically lower phosphorylation than does the wild type STTyr isoform. This suggests that the bulk of the ST6Gal-I phosphorylation is occurring in the Golgi. Treatment of cells with the ionophore monensin does not significantly block phosphorylation of the STTyr isoform, suggesting that phosphorylation is occurring in the cis-medial Golgi prior to the monensin block. This study demonstrates the presence of kinase activities in the cis-medial Golgi and the substantial phosphorylation of the luminal sequences of a glycosyltransferase.

In vivo autopolysialylation and localization of the polysialyltransferases PST and STX

A select group of mammalian proteins have been shown to possess alpha2,8-polysialylated oligosaccharide chains. The best studied of these proteins is the neural cell adhesion molecule (NCAM). Polysialylation of NCAM has been shown to decrease NCAM-dependent and independent cell adhesion. PST (ST8Sia IV) and STX (ST8Sia II) are the two polysialyltransferases responsible for NCAM polysialylation. Recent studies revealed that PST itself is autopolysialylated in vitro (Muhlenhoff, M., Eckhardt, M., Bethe, A., Frosch, M., and Gerardy-Schahn, R. (1996) EMBO J. 15, 6943-6950). Here we report studies on the biosynthesis and localization of the PST and STX polysialyltransferases. Both PST and STX are expressed as high molecular mass, polydisperse forms that are associated with the cell and found soluble in the medium. Analysis of these high molecular mass forms by glycosidase digestion and serial immunoprecipitation/immunoblot experiments demonstrated that PST and STX are autopolysialylated in vivo. Indirect immunofluorescence microscopy and immunoprecipitation analyses demonstrated that autopolysialylated PST and STX are localized in the Golgi, on the cell surface, and in the extracellular space. The cell surface and extracellular localization of these polysialylated polysialyltransferases suggest that their polysialic acid chains, like those of NCAM, may modulate cell interactions.

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

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

Two soluble glycosyltransferases glycosylate less efficiently in vivo than their membrane bound counterparts

Many Golgi glycosyltransferases are type II membrane proteins which are cleaved to produce soluble forms that are released from cells. Cho and Cummings recently reported that a soluble form of alpha1, 3-galactosyltransferase was comparable to its membrane bound counterpart in its ability to galactosylate newly synthesized glycoproteins (Cho,S.K. and Cummings,R.D. (1997) J. Biol. Chem., 272, 13622-13628). To test the generality of their findings, we compared the activities of the full length and soluble forms of two such glycosyltransferases, ss1,4 N-Acetylgalactosaminyltransferase (GM2/GD2/ GA2 synthase; GalNAcT) and beta galactoside alpha2,6 sialyltransferase (alpha2,6-ST; ST6Gal I), for production of their glycoconjugate products in vivo . Unlike the full length form of GalNAcT which produced ganglioside GM2 in transfected cells, soluble GalNAcT did not produce detectable GM2 in vivo even though it possessed in vitro GalNAcT activity comparable to that of full length GalNAcT. When compared with cells expressing full length alpha2,6-ST, cells expressing a soluble form of alpha2,6-ST contained 3-fold higher alpha2,6-ST mRNA levels and secreted 7-fold greater alpha2,6-ST activity as measured in vitro , but in striking contrast contained 2- to 4-fold less of the alpha2,6-linked sialic acid moiety in cellular glycoproteins in vivo . In summary these results suggest that unlike alpha1,3-galactosyltransferase the soluble forms of these two glycosyltransferases are less efficient at glycosylation of membrane proteins and lipids in vivo than their membrane bound counterparts.

A role for the lumenal domain in Golgi localization of the Saccharomyces cerevisiae guanosine diphosphatase

Integral membrane proteins (IMPs) contain localization signals necessary for targeting to their resident subcellular compartments. To define signals that mediate localization to the Golgi complex, we have analyzed a resident IMP of the Saccharomyces cerevisiae Golgi complex, guanosine diphosphatase (GDPase). GDPase, which is necessary for Golgi-specific glycosylation reactions, is a type II IMP with a short amino-terminal cytoplasmic domain, a single transmembrane domain (TMD), and a large catalytic lumenal domain. Regions specifying Golgi localization were identified by analyzing recombinant proteins either lacking GDPase domains or containing corresponding domains from type II vacuolar IMPs. Neither deletion nor substitution of the GDPase cytoplasmic domain perturbed Golgi localization. Exchanging the GDPase TMD with vacuolar protein TMDs only marginally affected Golgi localization. Replacement of the lumenal domain resulted in mislocalization of the chimeric protein from the Golgi to the vacuole, but a similar substitution leaving 34 amino acids of the GDPase lumenal domain intact was properly localized. These results identify a major Golgi localization determinant in the membrane-adjacent lumenal region (stem) of GDPase. Although necessary, the stem domain is not sufficient to mediate localization; in addition, a membrane-anchoring domain and either the cytoplasmic or full-length lumenal domain must be present to maintain Golgi residence. The importance of lumenal domain sequences in GDPase Golgi localization and the requirement for multiple hydrophilic protein domains support a model for Golgi localization invoking protein-protein interactions rather than interactions between the TMD and the lipid bilayer.