The transmembrane and flanking sequences of beta 1,2-N-acetylglucosaminyltransferase I specify medial-Golgi localization

The antibody-binding Fc gamma receptor IIIa / CD16a is N-glycosylated with high occupancy at all five sites

The antibody-binding Fc γ receptors (FcγRs) trigger life-saving immune responses and many therapeutic monoclonal antibodies require FcγR engagement for full effect. One proven strategy to improve the efficacy of antibody therapies is to increase receptor binding affinity, in particular binding to FcγRIIIa/CD16a. Currently, affinities are measured using recombinantly-expressed soluble extracellular FcγR domains and CD16a-mediated antibody-dependent immune responses are characterized using cultured cells. It is notable that CD16a is highly processed with multiple N-glycosylation sites, and preventing individual N-glycan modifications affects affinity. Furthermore, multiple groups have demonstrated that CD16a N-glycan composition is variable and composition impacts antibody binding affinity. The level of N-glycosylation at each site is not known though computational prediction indicates low to moderate potential at each site based on primary sequence (40-70%). Here we quantify occupancy of the extracellular domains using complementary mass spectrometry-based methods. All five sites of the tighter-binding CD16a V158 allotype showed 65-100% N-glycan occupancy in proteomics-based experiments. These observations were confirmed using intact protein mass spectrometry that demonstrated the predominant species corresponded to CD16a V158 with five N-glycans, with a smaller contribution from CD16a with four N-glycans. Occupancy was likewise high for the membrane-bound receptor at all detected N-glycosylation sites using CD16a purified from cultured human natural killer cells. Occupancy of the N162 site, critical for antibody binding, appeared independent of N169 occupancy based on analysis of the T171A mutant protein. The weaker-binding CD16a F158 allotype showed higher occupancy of >93% at each site.

GOLPH3 and GOLPH3L are broad-spectrum COPI adaptors for sorting into intra-Golgi transport vesicles

The fidelity of Golgi glycosylation is, in part, ensured by compartmentalization of enzymes within the stack. The COPI adaptor GOLPH3 has been shown to interact with the cytoplasmic tails of a subset of Golgi enzymes and direct their retention. However, other mechanisms of retention, and other roles for GOLPH3, have been proposed, and a comprehensive characterization of the clientele of GOLPH3 and its paralogue GOLPH3L is lacking. GOLPH3's role is of particular interest as it is frequently amplified in several solid tumor types. Here, we apply two orthogonal proteomic methods to identify GOLPH3+3L clients and find that they act in diverse glycosylation pathways or have other roles in the Golgi. Binding studies, bioinformatics, and a Golgi retention assay show that GOLPH3+3L bind the cytoplasmic tails of their clients through membrane-proximal positively charged residues. Furthermore, deletion of GOLPH3+3L causes multiple defects in glycosylation. Thus, GOLPH3+3L are major COPI adaptors that impinge on most, if not all, of the glycosylation pathways of the Golgi.

A versatile system for USER cloning-based assembly of expression vectors for mammalian cell engineering

A new versatile mammalian vector system for protein production, cell biology analyses, and cell factory engineering was developed. The vector system applies the ligation-free uracil-excision based technique – USER cloning – to rapidly construct mammalian expression vectors of multiple DNA fragments and with maximum flexibility, both for choice of vector backbone and cargo. The vector system includes a set of basic vectors and a toolbox containing a multitude of DNA building blocks including promoters, terminators, selectable marker- and reporter genes, and sequences encoding an internal ribosome entry site, cellular localization signals and epitope- and purification tags. Building blocks in the toolbox can be easily combined as they contain defined and tested Flexible Assembly Sequence Tags, FASTs. USER cloning with FASTs allows rapid swaps of gene, promoter or selection marker in existing plasmids and simple construction of vectors encoding proteins, which are fused to fluorescence-, purification-, localization-, or epitope tags. The mammalian expression vector assembly platform currently allows for the assembly of up to seven fragments in a single cloning step with correct directionality and with a cloning efficiency above 90%. The functionality of basic vectors for FAST assembly was tested and validated by transient expression of fluorescent model proteins in CHO, U-2-OS and HEK293 cell lines. In this test, we included many of the most common vector elements for heterologous gene expression in mammalian cells, in addition the system is fully extendable by other users. The vector system is designed to facilitate high-throughput genome-scale studies of mammalian cells, such as the newly sequenced CHO cell lines, through the ability to rapidly generate high-fidelity assembly of customizable gene expression vectors.

Substrate specificities and intracellular distributions of three N-glycan processing enzymes functioning at a key branch point in the insect N-glycosylation pathway

Man(α1-6)[GlcNAc(β1-2)Man(α1-3)]ManGlcNAc(2) is a key branch point intermediate in the insect N-glycosylation pathway because it can be either trimmed by a processing β-N-acetylglucosaminidase (FDL) to produce paucimannosidic N-glycans or elongated by N-acetylglucosaminyltransferase II (GNT-II) to produce complex N-glycans. N-acetylglucosaminyltransferase I (GNT-I) contributes to branch point intermediate production and can potentially reverse the FDL trimming reaction. However, there has been no concerted effort to evaluate the relationships among these three enzymes in any single insect system. Hence, we extended our previous studies on Spodoptera frugiperda (Sf) FDL to include GNT-I and -II. Sf-GNT-I and -II cDNAs were isolated, the predicted protein sequences were analyzed, and both gene products were expressed and their acceptor substrate specificities and intracellular localizations were determined. Sf-GNT-I transferred N-acetylglucosamine to Man(5)GlcNAc(2), Man(3)GlcNAc(2), and GlcNAc(β1-2)Man(α1-6)[Man(α1-3)]ManGlcNAc(2), demonstrating its role in branch point intermediate production and its ability to reverse FDL trimming. Sf-GNT-II only transferred N-acetylglucosamine to Man(α1-6)[GlcNAc(β1-2)Man(α1-3)]ManGlcNAc(2), demonstrating that it initiates complex N-glycan production, but cannot use Man(3)GlcNAc(2) to produce hybrid or complex structures. Fluorescently tagged Sf-GNT-I and -II co-localized with an endogenous Sf Golgi marker and Sf-FDL co-localized with Sf-GNT-I and -II, indicating that all three enzymes are Golgi resident proteins. Unexpectedly, fluorescently tagged Drosophila melanogaster FDL also co-localized with Sf-GNT-I and an endogenous Drosophila Golgi marker, indicating that it is a Golgi resident enzyme in insect cells. Thus, the substrate specificities and physical juxtapositioning of GNT-I, GNT-II, and FDL support the idea that these enzymes function at the N-glycan processing branch point and are major factors determining the net outcome of the insect cell N-glycosylation pathway.

MODIFIED VACUOLE PHENOTYPE1 is an Arabidopsis myrosinase-associated protein involved in endomembrane protein trafficking

We identified an Arabidopsis (Arabidopsis thaliana) ethyl methanesulfonate mutant, modified vacuole phenotype1-1 (mvp1-1), in a fluorescent confocal microscopy screen for plants with mislocalization of a green fluorescent protein-delta tonoplast intrinsic protein fusion. The mvp1-1 mutant displayed static perinuclear aggregates of the reporter protein. mvp1 mutants also exhibited a number of vacuole-related phenotypes, as demonstrated by defects in growth, utilization of stored carbon, gravitropic response, salt sensitivity, and specific susceptibility to the fungal necrotroph Alternaria brassicicola. Similarly, crosses with other endomembrane marker fusions identified mislocalization to aggregate structures, indicating a general defect in protein trafficking. Map-based cloning showed that the mvp1-1 mutation altered a gene encoding a putative myrosinase-associated protein, and glutathione S-transferase pull-down assays demonstrated that MVP1 interacted specifically with the Arabidopsis myrosinase protein, THIOGLUCOSIDE GLUCOHYDROLASE2 (TGG2), but not TGG1. Moreover, the mvp1-1 mutant showed increased nitrile production during glucosinolate hydrolysis, suggesting that MVP1 may play a role in modulation of myrosinase activity. We propose that MVP1 is a myrosinase-associated protein that functions, in part, to correctly localize the myrosinase TGG2 and prevent inappropriate glucosinolate hydrolysis that could generate cytotoxic molecules.

Arginine/lysine residues in the cytoplasmic tail promote ER export of plant glycosylation enzymes

Plant N-glycan processing enzymes are arranged along the early secretory pathway, forming an assembly line to facilitate the step-by-step modification of oligosaccharides on glycoproteins. Thus, these enzymes provide excellent tools to study signals and mechanisms, promoting their localization and retention in the endoplasmic reticulum (ER) and Golgi apparatus. Herein, we focused on a detailed investigation of amino acid sequence motifs present in their short cytoplasmic tails in respect to ER export. Using site-directed mutagenesis, we determined that single arginine/lysine residues within the cytoplasmic tail are sufficient to promote rapid Golgi targeting of Golgi-resident N-acetylglucosaminyltransferase I (GnTI) and alpha-mannosidase II (GMII). Furthermore, we reveal that an intact ER export motif is essential for proper in vivo function of GnTI. Coexpression studies with Sar1p provided evidence for COPII-dependent transport of GnTI to the Golgi. Our data provide evidence that efficient ER export of Golgi-resident plant N-glycan processing enzymes occurs through a selective mechanism based on recognition of single basic amino acids present in their cytoplasmic tails.

Signal-mediated dynamic retention of glycosyltransferases in the Golgi

Golgi-resident glycosyltransferases are a family of enzymes that sequentially modify glycoproteins in a subcompartment-specific manner. These type II integral membrane proteins are characterized by a short cytoplasmically exposed amino-terminal tail and a luminal enzymatic domain. The cytoplasmic tails play a role in the localization of glycosyltransferases, and coat protein complex I (COPI) vesicle-mediated retrograde transport is also involved in their Golgi localization. However, the tails of these enzymes lack known COPI-binding motifs. Here, we found that Vps74p bound to a pentameric motif present in the cytoplasmic tails of the majority of yeast Golgi-localized glycosyltransferases, as well as to COPI. We propose that Vps74p maintains the steady-state localization of Golgi glycosyltransferases dynamically, by promoting their incorporation into COPI-coated vesicles.

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.

Commuting between Golgi cisternae—Mind the GAP!

Intracellular transport has remained central to cell biology now for more than 40 years. Despite this, we still lack an overall mechanistic framework that describes transport in different parts of the cell. In the secretory pathway, basic questions, such as how biosynthetic cargo traverses the pathway, are still debated. Historically, emphasis was first put on interpreting function from morphology at the ultrastructural level revealing membrane structures such as the transitional ER, vesicular carriers, vesicular tubular clusters, Golgi cisternae, Golgi stacks and the Golgi ribbon. This emphasis on morphology later switched to biochemistry and yeast genetics yielding many of the key molecular players and their associated functions that we know today. More recently, microscopy studies of living cells incorporating biophysics and system analysis has proven useful and is often used to readdress earlier findings, sometimes with surprising outcomes.

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.

Intracellular sorting and transport of proteins

The secretory and endocytic pathways of eukaryotic organelles consist of multiple compartments, each with a unique set of proteins and lipids. Specific transport mechanisms are required to direct molecules to defined locations and to ensure that the identity, and hence function, of individual compartments are maintained. The localisation of proteins to specific membranes is complex and involves multiple interactions. The recent dramatic advances in understanding the molecular mechanisms of membrane transport has been due to the application of a multi-disciplinary approach, integrating membrane biology, genetics, imaging, protein and lipid biochemistry and structural biology. The aim of this review is to summarise the general principles of protein sorting in the secretory and endocytic pathways and to highlight the dynamic nature of these processes. The molecular mechanisms involved in this transport along the secretory and endocytic pathways are discussed along with the signals responsible for targeting proteins to different intracellular locations.

Golgi secretion is not required for marking the preprophase band site in cultured tobacco cells

The preprophase band predicts the future cell division site. However, the mechanism of how a transient preprophase band fulfils this function is unknown. We have investigated the possibility that Golgi secretion might be involved in marking the preprophase band site. Observations on living BY-2 cells labeled for microtubules and Golgi stacks indicated an increased Golgi stack frequency at the preprophase band site. However, inhibition of Golgi secretion by brefeldin A during preprophase band formation did not prevent accurate phragmoplast fusion, and subsequent cell plate formation, at the preprophase band site. The results show that Golgi secretion does not mark the preprophase band site and thus does not play an active role in determination of the cell division site.

Tissues of the clawed frog Xenopus laevis contain two closely related forms of UDP-GlcNAc:alpha3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I

UDP-GlcNAc:alpha3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnTI; EC 2.4.1.101) is a medial-Golgi enzyme that is essential for the processing of oligomannose to hybrid and complex N-glycans. On the basis of highly conserved sequences obtained from previously cloned mammalian GnTI genes, cDNAs for two closely related GnTI isoenzymes were isolated from a Xenopus laevis ovary cDNA library. As typical for glycosyltransferases, both proteins exhibit a type II transmembrane protein topology with a short N-terminal cytoplasmic tail (4 amino acids); a transmembrane domain of 22 residues; a stem region with a length of 81 (isoenzyme A) and 77 (isoenzyme B) amino acids, respectively; and a catalytic domain consisting of 341 residues. The two proteins differ not only in length but also at 13 (stem) and 18 (catalytic domain) positions, respectively. The overall identity of the catalytic domains of the X. laevis GnTI isoenzymes with their mammalian and plant orthologues ranges from 30% (Nicotiana tabacum) to 67% (humans). Isoenzymes A and B are encoded by two separate genes that were both found to be expressed in all tissues examined, albeit in varying amounts and ratios. On expression of the cDNAs in the baculovirus/insect cell system, both isoenzymes were found to exhibit enzymatic activity. Isoenzyme B is less efficiently folded in vivo and thus appears of lower physiological relevance than isoenzyme A. However, substitution of threonine at position 223 with alanine was sufficient to confer isoenzyme B with properties similar to those observed for isoenzyme A.

The transmembrane domain of murine alpha-mannosidase IB is a major determinant of Golgi localization

Murine alpha1,2-mannosidase IB is a type II transmembrane protein localized to the Golgi apparatus where it is involved in the biogenesis of complex and hybrid N-glycans. This enzyme consists of a cytoplasmic tail, a transmembrane domain followed by a "stem" region and a large C-terminal catalytic domain. To analyze the determinants of targeting, we constructed various deletion mutants of murine alpha1,2-mannosidase IB as well as alpha1,2-mannosidase IB/yeast alpha1,2-mannosidase and alpha1,2-mannosidase IB/GFP chimeras and localized these proteins by fluorescence microscopy, when expressed transiently in COS7 cells. Replacing the catalytic domain of alpha1,2-mannosidase IB with that of the homologous yeast alpha1,2-mannosidase and deleting the "stem" region in this chimera had no effect on Golgi targeting, but caused increased cell surface localization. The N-terminal tagged protein lacking a catalytic domain was also localized to the Golgi. In the latter case, when the stem region was partially or completely removed, the protein was found in both the ER and the Golgi. A chimera consisting of the alpha1,2-mannosidase IB N-terminal region (cytoplasmic and transmembrane domains plus 10 amino acids of the "stem" region) and GFP was localized mainly to the Golgi. Deletion of 30 out of 35 amino acids in the cytoplasmic tail had no effect on Golgi localization. A GFP chimera lacking the entire cytoplasmic tail was found in both the ER and the Golgi. These results indicate that the transmembrane domain of alpha1,2-mannosidase IB is a major determinant of Golgi localization.

Endoplasmic reticulum retention determinants in the transmembrane and linker domains of cytochrome P450 2C1

The cytochrome P450 2C1 N-terminal signal anchor sequence mediates direct retention of the protein in the endoplasmic reticulum and consists of a hydrophobic transmembrane domain, residues 3-20, followed by a hydrophilic linker, residues 21-28. Fusions of the N-terminal 21 or 28 amino acids of P450 2C1 to green fluorescent protein resulted in endoplasmic reticulum localization of the chimera in transfected cells. Disruption of microtubules by nocodazole treatment resulted in redistribution into a punctate pattern for the 1-21, but not for the 1-28, chimera indicating that the linker was preventing transport from the endoplasmic reticulum but was not required for retrieval to the endoplasmic reticulum from the pre-Golgi compartment. In the 1-28 chimera, mutations of residues 21-23 (KQS) in the linker resulted in redistribution of the chimera after nocodazole treatment. Mutations in the transmembrane domain affected both direct retention in the endoplasmic reticulum and retrieval from the pre-Golgi compartment, and although structural requirements for each process are distinct, in both cases the arrangement of amino acids and distribution of hydrophobicity are critical. In contrast, the linker region exhibits a sequence-specific requirement for direct retention in the endoplasmic reticulum.

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.

Protein retention and localization in the endoplasmic reticulum and the golgi apparatus

Protein transport along the secretory pathway is supported by a noria of vesicles that bud and fuse, load and unload their cargo from one compartment into the other. However, despite this constant flow-through of proteins and lipids the various compartments of the secretory pathway are able to maintain their own specific composition. Here, we discuss recent insights into mechanisms of protein retention and localization that are necessary for the maintenance of endoplasmic reticulum (ER)- and Golgi-associated typical functions such as protein folding and glycosylation in plant cells.

beta1,6 N-acetylglucosaminyltransferase (core 2 GlcNAc-T) expression in normal rat tissues and different cell lines: evidence for complex mechanisms of regulation

The distribution of the Golgi enzyme beta1, 6-N-acetylglucosaminyltransferase (core 2 GlcNAc-T for short) has been investigated in several tissue and cell systems by combining the potentials of a polyclonal antibody and a novel, sensitive fluorescent enzyme assay. In normal rat tissues, levels of the protein were found to vary and as a general trend did not correlate with enzyme activities. Additionally, we observed tissue-specific core 2 GlcNAc-T forms of various size: 75 kDa (liver), 70 kDa (spleen), 60 kDA (heart), and 50 kDa (heart and lung). These forms might arise from differential protein modifications; alternatively, the smaller form may be a product of proteolytic cleavage, given the presence of a catalytically inactive 50 kDa species in rat serum. Chinese hamster ovary (CHO), MDAY-D2, PSA-5E, and PYS-2 cell lines consistently displayed a 70 kDa enzyme. When induced to retrodifferentiate in the presence of butyrate + cholera toxin, CHO cells exhibited a 21-fold increase in enzyme activity, while protein levels remained constant. A similar trend was observed in the embryonal endoderm cell lines PSA-5E and PYS-2, where an approximately 100-fold difference in core 2 GlcNAc-T activity was found notwithstanding unchanged amounts of the protein and identical mRNA levels, as evidenced by RT-PCR. In contrast, levels of core 2 GlcNAc-T activity in MDAY-D2 cells correlated well with protein expression. Taken together, these observations demonstrate that core 2 GlcNAc-T expression may be subjected to multiple mechanisms of regulation and suggest that in at least some instances (i.e., PSA-5E and PYS-2 cells) expression may be regulated exclusively via posttranslational mechanism(s) of control.

Molecular cloning and expression of a novel human trans-Golgi network glycoprotein, TGN51, that contains multiple tyrosine-containing motifs

Previously, it has been shown that glycoproteins with approximately 130-kDa molecular mass react with antisera from patients with renal vasculitis (Kain, R., Matsui, K., Exner, M., Binder, S., Schaffner, G., Sommer, E. M., and Kerjaschki, D. (1995) J. Exp. Med. 181, 585-597). To search for a molecule that reacts with the antibodies, we screened a lambdagt11 human placental cDNA library. Two of the isolated clones were found to encode a putative counterpart of the rodent trans-Golgi network (TGN) glycoprotein 38, hTGN46, which has the tyrosine containing motif YQRL shared by mouse and rat TGN38. Moreover, reverse transcription-polymerase chain reaction analysis of hTGN46 transcripts and genomic analysis of a cDNA deposited as an expressed sequence tag in dbEST Data Base revealed that additional cDNAs exist that are produced by alternate usage of 3'-splice sites of intron III. Alternative splicing results in frame shifts and leads to novel larger translation products with one (for hTGN48) or two (for hTGN51) additional tyrosine-containing motifs. hTGN51 expressed in Chinese hamster ovary cells were localized to the trans-Golgi network, overlapping with beta-1,4-galactosyltransferase even after mutating the tyrosine-containing motif common to hTGN46. In contrast, mutated hTGN48 and hTGN46 are no longer retrieved to the TGN. These results strongly suggest that hTGN51 may have a unique function compared with hTGN46 or hTGN48 in shuttling between the cell surface and the TGN.

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

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

A retention signal necessary and sufficient for Golgi localization maps to the cytoplasmic tail of a Bunyaviridae (Uukuniemi virus) membrane glycoprotein

Members of the Bunyaviridae family mature by a budding process in the Golgi complex. The site of maturation is thought to be largely determined by the accumulation of the two spike glycoproteins, G1 and G2, in this organelle. Here we show that the signal for localizing the Uukuniemi virus (a phlebovirus) spike protein complex to the Golgi complex resides in the cytoplasmic tail of G1. We constructed chimeric proteins in which the ectodomain, transmembrane domain (TMD), and cytoplasmic tail (CT) of Uukuniemi virus G1 were exchanged with the corresponding domains of either vesicular stomatitis virus G protein (VSV G), chicken lysozyme, or CD4, all proteins readily transported to the plasma membrane. The chimeras were expressed in HeLa or BHK-21 cells by using either the T7 RNA polymerase-driven vaccinia virus system or the Semliki Forest virus system. The fate of the chimeric proteins was monitored by indirect immunofluorescence, and their localizations were compared by double labeling with markers specific for the Golgi complex. The results showed that the ectodomain and TMD (including the 10 flanking residues on either side of the membrane) of G1 played no apparent role in targeting chimeric proteins to the Golgi complex. Instead, all chimeras containing the CT of G1 were efficiently targeted to the Golgi complex and colocalized with mannosidase II, a Golgi-specific enzyme. Conversely, replacing the CT of G1 with that from VSV G resulted in the efficient transport of the chimeric protein to the cell surface. Progressive deletions of the G1 tail suggested that the Golgi retention signal maps to a region encompassing approximately residues 10 to 50, counting from the proposed border between the TMD and the tail. Both G1 and G2 were found to be acylated, as shown by incorporation of [3H]palmitate into the viral proteins. By mutational analyses of CD4-G1 chimeras, the sites for palmitylation were mapped to two closely spaced cysteine residues in the G1 tail. Changing either or both of these cysteines to alanine had no effect on the targeting of the chimeric protein to the Golgi complex.

Glycobiotechnology: enzymes for the synthesis of nucleotide sugars

Complex carbohydrates, as constituting part of glycoconjugates such as glycoproteins, glycolipids, hormones, antibiotics and other secondary metabolites, play an active role in inter- and intracellular communication. The aim of "glycobiotechnology" as an upcoming interdisciplinary research field is to develop highly efficient synthesis strategies, including in vivo and in vitro approaches, in order to bring such complex molecules into analytical and therapeutic studies. The enzymatic synthesis of glycosidic bonds by Leloir-glycosyltransferases is an efficient strategy for obtaining saccharides with absolute stereo- and regioselectivity in high yields and under mild conditions. There are, however, two obstacles hindering the realization of this process on a biotechnological scale, namely the production of recombinant Leloir-glycosyltransferases and the availability of enzymes for the synthesis of nucleotide sugars (the glycosyltransferase donor substrates). The present review surveys some synthetic targets which have attracted the interest of glycobiologists as well as recombinant expression systems which give Leloir-glycosyltransferase activities in the mU and U range. The main part summarizes publications concerned with the complex pathways of primary and secondary nucleotide sugars and the availability and use of these enzymes for synthesis applications. In this context, a survey of our work will demonstrate how enzymes from different sources and pathways can be combined for the synthesis of nucleotide deoxysugars and oligosaccharides.

Switching amino-terminal cytoplasmic domains of alpha(1,2)fucosyltransferase and alpha(1,3)galactosyltransferase alters the expression of H substance and Galalpha(1,3)Gal

When alpha(1,2)fucosyltransferase cDNA is expressed in cells that normally express large amounts of the terminal carbohydrate Galalpha(1,3)Gal, and therefore the alpha(1,3)galactosyltransferase (GT), the Galalpha(1,3)Gal almost disappears, indicating that the presence of the alpha(1,2)fucosyltransferase (HT) gene/enzyme alters the synthesis of Galalpha(1,3)Gal. A possible mechanism to account for these findings is enzyme location within the Golgi apparatus. We examined the effect of Golgi localization by exchanging the cytoplasmic tails of HT and GT; if Golgi targeting signals are contained within the cytoplasmic tail sequences of these enzymes then a "tail switch" would permit GT first access to the substrate and thereby reverse the observed dominance of HT. Two chimeric glycosyltransferase proteins were constructed and compared with the normal glycosyltransferases after transfection into COS cells. The chimeric enzymes showed Km values and cell surface carbohydrate expression comparable with normal glycosyltransferases. Co-expression of the two chimeric glycosyltransferases resulted in cell surface expression of Galalpha(1,3)Gal, and virtually no HT product was expressed. Thus the cytoplasmic tail of HT determines the temporal order of action, and therefore dominance, of these two enzymes.

Glycosylation defect in Lec1 Chinese hamster ovary mutant is due to a point mutation in N-acetylglucosaminyltransferase I gene

The Lec1 Chinese hamster ovary (CHO) mutant is a leuco-phytohemagglutinin resistant cell line unable to synthesize complex and hybrid N-glycans due to the lack of N-acetylglucosaminyltransferase I (GnTI) activity. Here we have identified the lec1 mutation. Using specific antibodies to GnTI we demonstrate that Lec1 cells synthesize an inactive GnTI protein identical in size to the wild-type CHO enzyme. We have cloned and sequenced the gene coding GnTI from parental CHO and Lec1 mutant cells. Comparison of GnTI sequences detected three mutations within the luminal domain of Lec1 GnTI, each resulting in an amino acid substitution. The effect of each mutation on enzyme activity was analyzed by site-directed mutagenesis of wild-type rabbit GnTI and transient expression in COS cells. One of the three mutations (Cys123 --> Arg123) resulted in complete loss of activity, whereas the other two mutations had no apparent effect on enzyme activity. This conclusion was confirmed by expression of GnTI mutants in the GnTI null background of Saccharomyces cerevisiae. Both Lec1 GnTI and the GnTI mutant (Cys123 --> Arg123) are correctly localized to the Golgi apparatus, indicating that the inactive GnTI molecules are sufficiently well folded for efficient transport from the endoplasmic reticulum. These results demonstrate that the lec1 mutation is a point mutation and that Cys123 is a critical residue for GnTI activity.

The role of the membrane-spanning domain and stalk region of N-acetylglucosaminyltransferase I in retention, kin recognition and structural maintenance of the Golgi apparatus in HeLa cells

Using a series of chimeric and truncated N-acetylglucosaminyltransferase I (NAGT I) molecules we have shown that part of the lumenal stalk region is both necessary and sufficient for kin recognition of mannosidase II and retention in the Golgi stack. The membrane-spanning domain was not required for retention, but replacing part or all of this domain with leucine residues did have a dramatic effect on Golgi morphology. In stable cell lines, stacked cisternae were replaced by tubulo-vesicular clusters containing the mutated NAGT I. The loss of stacked cisternae was proportional to the number of leucines used to replace the membrane-spanning domain.

Identification of a region required for subtype-specific agonist-induced sequestration of the m2 muscarinic acetylcholine receptor

When the m1 and m2 muscarinic acetylcholine receptors are transiently expressed in JEG-3 cells, the m2, but not the m1, receptor undergoes agonist-induced sequestration. Both receptors exhibit internalization when expressed in Y1 cells. These results suggest that the m1 and m2 receptors use distinct cellular mechanisms or pathways for agonist-induced internalization and that JEG-3 cells are deficient in the mechanism or pathway used by the m1 receptor. Transfection experiments with chimeric receptors indicate that the specificity for agonist-induced internalization for the m2 receptor lies in the carboxyl-terminal fifth of the receptor. The intracellular carboxyl-terminal tail of the m2 receptor is neither sufficient nor required for the m2-specific sequestration. Site-directed mutagenesis demonstrates that two amino acids in the carboxyl-terminal end of the third cytoplasmic loop of the m2 receptor are required for sequestration in JEG-3 cells. In addition, the sixth transmembrane domain, which is adjacent to this cytoplasmic domain, is also required. Thus, m2-specific agonist-induced sequestration requires sequences both in the carboxyl-terminal end of the third cytoplasmic loop and the adjacent transmembrane domain.

Recent progress in the molecular biology of the cloned N-acetylglucosaminyltransferases

Several genes which code for the N-acetylglucosaminyltransferases have been cloned and characterized. Physiological and pathophysiological roles of the genes still remain to be elucidated but accumulated evidence suggests that the N-acetylglucosaminyltransferase genes are implicated in differentiation, morphogenesis and cancer metastasis.

Insertion into Aspergillus nidulans of functional UDP-GlcNAc: alpha 3-D- mannoside beta-1,2-N-acetylglucosaminyl-transferase I, the enzyme catalysing the first committed step from oligomannose to hybrid and complex N-glycans

Filamentous fungi are capable of secreting relatively large amounts of heterologous recombinant proteins. Recombinant human glycoproteins expressed in this system, however, carry only carbohydrates of the oligomannose type limiting their potential use in humans. One approach to the problem is genetic engineering of the fungal host to permit production of complex and hybrid N-glycans. UDP-GlcNAc:alpha 3-D-mannoside beta- 1,2-N-acetylglucosaminyltransferase I (GnT I) is essential for the conversion of oligomannose to hybrid and complex N-glycans in higher eukaryotic cells. Since GnT I is not produced by fungi, we have introduced into the genome of Aspergillus nidulans the gene encoding full-length rabbit GnT I and demonstrated the expression of GnT I enzyme activity at levels appreciably higher than occurs in most mammalian tissues. All the GnT I activity in the Aspergillus transformants remains intracellular suggesting that the rabbit trans-membrane sequence may be capable of targeting GnT I to the fungal Golgi apparatus.

Golgi retention mechanism of beta-1,4-galactosyltransferase. Membrane-spanning domain-dependent homodimerization and association with alpha- and beta-tubulins

Recent studies on proteins residing in the Golgi complex revealed that the membrane-spanning domain of these proteins are largely responsible for their retention in the Golgi complex. We show here that beta-1,4-galactosyltransferase (GT) forms homodimers and large oligomers in vivo, and the formation of the homodimers is dependent on cysteine and histidine residues within the transmembrane domain. Double mutations of these residues, Cys29-->Ser and His32-->Leu, abolish homodimerization and simultaneously reduce the Golgi retention. Co-immunoprecipitation of GT and various GT chimeras with anti-GT and anti-reporter molecule antibodies revealed that large aggregates of GT are associated with alpha- and beta-tubulins and also with other cellular proteins. This association between tubulins and GT suggests a supportive role of the cytoskeleton in the Golgi retention mechanism.

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

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

MG-160, a membrane sialoglycoprotein of the medial cisternae of the rat Golgi apparatus, binds basic fibroblast growth factor and exhibits a high level of sequence identity to a chicken fibroblast growth factor receptor

We report the primary structure of MG-160, a 160 kDa membrane sialoglycoprotein residing in the medial cisternae of the Golgi apparatus of rat neurons, pheochromocytoma (PC-12), and several other cells. The cDNA encodes a polypeptide of 1,171 amino acids with an M(r) of 133,403. An intralumenal cleavable signal peptide is followed by a Pro-Gln-rich segment and 16 contiguous, approx. 60-residue-long, regularly spaced cysteine-rich segments showing sequence identities ranging from 15 to 35%. The lumenal domain is followed by a single membrane spanning domain and a short carboxy-terminal cytoplasmic tail. The protein contains 5 potential NXT glycosylation sites. The sequence of MG-160 shows no homologies with enzymes and other membrane proteins of the Golgi apparatus. MG-160 displays a so far unique feature for a membrane protein of the Golgi apparatus: namely, an upstream, open reading frame (uORF), encoding 58 amino acids, located in front of the major open reading frame (ORF). Most vertebrate mRNAs containing uORF or AUG codons in front of the major ORF encode growth factors and cell surface receptors (Geballe and Morris 1994). In that regard a 90% identity between the primary structure of MG-160 and a receptor for acidic and basic fibroblast growth factors (CFR), isolated from chicken embryos (Burrus et. al., 1992), may be relevant. Immunoreactivity for MG-160 has been detected in the Golgi apparatus of neural and other cells of 2-day-old chicken embryos and adult chicken; furthermore, recombinant human basic fibroblast growth factor (bFGF) binds MG-160 purified from rat brain. MG-160 shows no sequence similarity with members of the family of fibroblast growth factor receptors (FGFR) involved in signal transduction. These findings are consistent with the hypothesis that MG-160 is involved in the traffic and processing of endogenous or autocrine FGFs. This is the first example of an intrinsic membrane protein of the Golgi apparatus which binds a growth factor and may be involved in its regulation.

Targeting of proteins to the Golgi apparatus

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

Expression of recombinant rabbit UDP-GlcNAc: alpha 3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I catalytic domain in Sf9 insect cells

UDP-GlcNAc: alpha 3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnT I; EC 2.4.1.101) catalyses a key reaction in the conversion of oligomannose to complex and hybrid N-glycans. The cytoplasmic tail and transmembrane segment of rabbit GnT I cDNA were replaced with an in-frame cleavable signal sequence and the hybrid construct was inserted into the genome of Autographa californica nuclear polyhedrosis virus (AcMNPV) under the control of the polyhedrin promoter. Sf9 insect cells were infected with the recombinant baculovirus and the enzymatically active and soluble catalytic domain of GnT I was purified from the medium (1-5 mg l-1) in two steps to a specific activity of about 2 mumol min-1 mg-1 protein. Recombinant GnT I has been used for the chemical-enzymatic synthesis of analogues of Man alpha 1-6]GlcNAc beta 1-2Man alpha 1-3]Man beta-O-octyl.

Specificity and promiscuity in membrane helix interactions

The membrane-spanning portions of many integral membrane proteins consist of one or a number of transmembrane α-helices, which are expected to be independently stable on thermodynamic grounds. Side-by-side interactions between these transmembrane α-helices are important in the folding and assembly of such integral membrane proteins and their complexes. In considering the contribution of these helix–helix interactions to membrane protein folding and oligomerization, a distinction between the energetics and specificity should be recognized. A number of contributions to the energetics of transmembrane helix association within the lipid bilayer will be relatively non-specific, including those resulting from charge–charge interactions and lipid–packing effects. Specificity (and part of the energy) in transmembrane α-helix association, however, appears to rely mainly upon a detailed stereochemical fit between sets of dynamically accessible states of particular helices. In some cases, these interactions are mediated in part by prosthetic groups.

Targeting and retention of Golgi membrane proteins

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