The cytoplasmic tail of alpha 1,3-galactosyltransferase inhibits Golgi localization of the full-length enzyme

Engineering protein glycosylation in CHO cells to be highly similar to murine host cells

Since 2015 more than 34 biosimilars have been approved by the FDA. This new era of biosimilar competition has stimulated renewed technology development focused on therapeutic protein or biologic manufacturing. One challenge in biosimilar development is the genetic differences in the host cell lines used to manufacture the biologics. For example, many biologics approved between 1994 and 2011 were expressed in murine NS0 and SP2/0 cell lines. Chinese Hamster ovary (CHO) cells, however, have since become the preferred hosts for production due to their increased productivity, ease of use, and stability. Differences between murine and hamster glycosylation have been identified in biologics produced using murine and CHO cells. In the case of monoclonal antibodies (mAbs), glycan structure can significantly affect critical antibody effector function, binding activity, stability, efficacy, and in vivo half-life. In an attempt to leverage the intrinsic advantages of the CHO expression system and match the reference biologic murine glycosylation, we engineered a CHO cell expressing an antibody that was originally produced in a murine cell line to produce murine-like glycans. Specifically, we overexpressed cytidine monophospho-N-acetylneuraminic acid hydroxylase (CMAH) and N-acetyllactosaminide alpha-1,3-galactosyltransferase (GGTA) to obtain glycans with N-glycolylneuraminic acid (Neu5Gc) and galactose-α-1,3-galactose (alpha gal). The resulting CHO cells were shown to produce mAbs with murine glycans, and they were then analyzed by the spectrum of analytical methods typically used to demonstrate analytical similarity as a part of demonstrating biosimilarity. This included high-resolution mass spectrometry, biochemical, as well as cell-based assays. Through selection and optimization in fed-batch cultures, two CHO cell clones were identified with similar growth and productivity criteria to the original cell line. They maintained stable production for 65 population doubling times while matching the glycosylation profile and function of the reference product expressed in murine cells. This study demonstrates the feasibility of engineering CHO cells to express mAbs with murine glycans to facilitate the development of biosimilars that are highly similar to marketed reference products expressed in murine cells. Furthermore, this technology can potentially reduce the residual uncertainty regarding biosimilarity, resulting in a higher probability of regulatory approval and potentially reduced costs and time in development.

Translation of genome to glycome: role of the Golgi apparatus

Glycans are one of the four biopolymers of the cell and they play important roles in cellular and organismal physiology. They consist of both linear and branched structures and are synthesized in a nontemplated manner in the secretory pathway of mammalian cells with the Golgi apparatus playing a key role in the process. In spite of the absence of a template, the glycans synthesized by a cell are not a random collection of possible glycan structures but a distribution of specific glycans in defined quantities that is unique to each cell type (Cell type here refers to distinct cell forms present in an organism that can be distinguished based on morphological, phenotypic and/or molecular criteria.) While information to produce cell type-specific glycans is encoded in the genome, how this information is translated into cell type-specific glycome (Glycome refers to the quantitative distribution of all glycan structures present in a given cell type.) is not completely understood. We summarize here the factors that are known to influence the fidelity of glycan biosynthesis and integrate them into known glycosylation pathways so as to rationalize the translation of genetic information to cell type-specific glycome.

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.

Widespread intronic polyadenylation inactivates tumour suppressor genes in leukaemia

DNA mutations are known cancer drivers. Here we investigated whether mRNA events that are upregulated in cancer can functionally mimic the outcome of genetic alterations. RNA sequencing or 3'-end sequencing techniques were applied to normal and malignant B cells from 59 patients with chronic lymphocytic leukaemia (CLL)^1-3. We discovered widespread upregulation of truncated mRNAs and proteins in primary CLL cells that were not generated by genetic alterations but instead occurred by intronic polyadenylation. Truncated mRNAs caused by intronic polyadenylation were recurrent (n = 330) and predominantly affected genes with tumour-suppressive functions. The truncated proteins generated by intronic polyadenylation often lack the tumour-suppressive functions of the corresponding full-length proteins (such as DICER and FOXN3), and several even acted in an oncogenic manner (such as CARD11, MGA and CHST11). In CLL, the inactivation of tumour-suppressor genes by aberrant mRNA processing is substantially more prevalent than the functional loss of such genes through genetic events. We further identified new candidate tumour-suppressor genes that are inactivated by intronic polyadenylation in leukaemia and by truncating DNA mutations in solid tumours^4,5. These genes are understudied in cancer, as their overall mutation rates are lower than those of well-known tumour-suppressor genes. Our findings show the need to go beyond genomic analyses in cancer diagnostics, as mRNA events that are silent at the DNA level are widespread contributors to cancer pathogenesis through the inactivation of tumour-suppressor genes.

How Golgi glycosylation meets and needs trafficking: the case of the COG complex

Protein glycosylation is one of the major biosynthetic functions occurring in the endoplasmic reticulum and Golgi compartments. It requires an amazing number of enzymes, chaperones, lectins and transporters whose actions delicately secure the fidelity of glycan structures. Over the past 30 years, glycobiologists hammered that glycan structures are not mere decorative elements but serve crucial cellular functions. This becomes dramatically illustrated by a group of mostly severe, inherited human disorders named congenital disorders of glycosylation (CDG). To date, many types of CDG have been defined genetically and most of the time the defects impair the biosynthesis, transfer and remodeling of N-glycans. Recently, the identification of the several types of CDG caused by deficiencies in the conserved oligomeric Golgi (COG) complex, a complex involved in vesicular Golgi trafficking, expanded the field of CDG but also brought novel insights in glycosylation. The molecular mechanisms underlying the complex pathway of N-glycosylation in the Golgi are far from understood. The availability of COG-deficient CDG patients and patients' cells offered a new way to study how COG, and its different subunits, could influence the Golgi N-glycosylation machinery and localization. This review summarizes the recent findings on the implication of COG in Golgi glycosylation. It highlights the need for a dynamic, finely tuned balance between anterograde and retrograde trafficking for the correct localization of Golgi enzymes to assure the stepwise maturation of N-glycan chains.

The cytoplasmic and transmembrane domains of secretor type alpha1,2fucosyltransferase confer atypical cellular localisation

Carbohydrate structures influence many aspects of cell biology. Manipulating the glycosyltransferase enzymes, that sequentially add carbohydrate moieties to proteins and lipids as they pass through the Golgi and secretory pathway, can alter these carbohydrate epitopes. We previously demonstrated that the eight amino acid cytoplasmic tail of alpha1,2fucosyltransferase (FT) contained a sequence for Golgi localisation. In this study, we examined the localisation of the closely related secretor type alpha1,2fucosyltransferase (Sec) which has a smaller, yet apparently unrelated, five amino acid cytoplasmic tail. In contrast to the Golgi localisation of FT, Sec displayed atypical cytoplasmic vesicular-like staining. However, replacing just the five amino acid tail of Sec with FT was sufficient to relocalise the enzyme to a perinuclear region with Golgi-like staining. The biological significance of this relocalisation was this chimaeric enzyme was more effective than FT at competing for N-Acetyl-lactosamine and thus was superior in reducing expression of the Galalpha(1,3)Gal xenoepitope.

Association of beta-1,3-N-acetylglucosaminyltransferase 1 and beta-1,4-galactosyltransferase 1, trans-Golgi enzymes involved in coupled poly-N-acetyllactosamine synthesis

Poly-N-acetyllactosamine (polyLacNAc) is a linear carbohydrate polymer composed of alternating N-acetylglucosamine and galactose residues involved in cellular functions ranging from differentiation to metastasis. PolyLacNAc also serves as a scaffold on which other oligosaccharides such as sialyl Lewis X are displayed. The polymerization of the alternating N-acetylglucosamine and galactose residues is catalyzed by the successive action of UDP-GlcNAc:betaGal beta-1,3-N-acetylglucosaminyltransferase 1 (B3GNT1) and UDP-Gal:betaGlcNAc beta-1,4-galactosyltransferase, polypeptide 1 (B4GALT1), respectively. The functional association between these two glycosyltransferases led us to investigate whether the enzymes also associate physically. We show that B3GNT1 and B4GALT1 colocalize by immunofluorescence microscopy, interact by coimmunoprecipitation, and affect each other's subcellular localization when one of the two proteins is artificially retained in the endoplasmic reticulum. These results demonstrate that B3GNT1 and B4GALT1 physically associate in vitro and in cultured cells, providing insight into possible mechanisms for regulation of polyLacNAc production.

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.

Subcellular localization of mammalian type II membrane proteins

Application of a computational membrane organization prediction pipeline, MemO, identified putative type II membrane proteins as proteins predicted to encode a single alpha-helical transmembrane domain (TMD) and no signal peptides. MemO was applied to RIKEN's mouse isoform protein set to identify 1436 non-overlapping genomic regions or transcriptional units (TUs), which encode exclusively type II membrane proteins. Proteins with overlapping predicted InterPro and TMDs were reviewed to discard false positive predictions resulting in a dataset comprised of 1831 transcripts in 1408 TUs. This dataset was used to develop a systematic protocol to document subcellular localization of type II membrane proteins. This approach combines mining of published literature to identify subcellular localization data and a high-throughput, polymerase chain reaction (PCR)-based approach to experimentally characterize subcellular localization. These approaches have provided localization data for 244 and 169 proteins. Type II membrane proteins are localized to all major organelle compartments; however, some biases were observed towards the early secretory pathway and punctate structures. Collectively, this study reports the subcellular localization of 26% of the defined dataset. All reported localization data are presented in the LOCATE database (http://www.locate.imb.uq.edu.au).

Functional involvement of TMF/ARA160 in Rab6-dependent retrograde membrane traffic

The small GTPase Rab6 regulates retrograde membrane traffic from endosomes to the Golgi apparatus and from the Golgi to the endoplasmic reticulum (ER). We examined the role of a Rab6-binding protein, TMF/ARA160 (TATA element modulatory factor/androgen receptor-coactivator of 160 kDa), in this process. High-resolution immunofluorescence imaging revealed that TMF signal surrounded Rab6-positive Golgi structures and immunoelectron microscopy revealed that TMF is concentrated at the budding structures localized at the tips of cisternae. The knockdown of either TMF or Rab6 by RNA interference blocked retrograde transport of endocytosed Shiga toxin from early/recycling endosomes to the trans-Golgi network, causing missorting of the toxin to late endosomes/lysosomes. However, the TMF knockdown caused Rab6-dependent displacement of N-acetylgalactosaminyltransferase-2 (GalNAc-T2), but not beta1,4-galactosyltransferase (GalT), from the Golgi. Analyses using chimeric proteins, in which the cytoplasmic regions of GalNAc-T2 and GalT were exchanged, revealed that the cytoplasmic region of GalNAc-T2 plays a crucial role in its TMF-dependent Golgi retention. These observations suggest critical roles for TMF in two Rab6-dependent retrograde transport processes: one from endosomes to the Golgi and the other from the Golgi to the ER.

ABO blood group and related antigens, natural antibodies and transplantation

The current success rate of transplant surgery and immunosuppression has led to a demand for organs that has outstripped the supply. This has required investigation of alternate strategies. Therefore, allotransplantation across the ABO blood group barrier has commenced, and pig-to-human xenotransplantation is under consideration. The first immunological barrier to both these types of transplantation is the prevention of the antibody-mediated rejection. This rejection is a result of natural preformed antibodies circulating in the serum of the recipient binding to either ABO (for allo) or alpha-galactose (alpha-Gal) (for xeno) antigens expressed on the donor tissue. These antibodies recognise antigens that are, in both cases, carbohydrate molecules with the characteristic feature that the nonreducing terminal carbohydrate is either a Gal or N-acetlygalactosamine residue in an alpha1,3 linkage. These epitopes are synthesised by closely related members of a single family of glycosyltransferases. This review discusses the carbohydrate antigens, the enzymes involved in their synthesis and the consequences of natural antibodies binding these antigens.

Regulating cell surface glycosylation with a small-molecule switch

Correct localization of Golgi-resident enzymes is essential for the formation of specific glycan epitopes. In this chapter, we describe a method to control the localization, and thus the activity, of an individual glycosyltransferase by administration of a small molecule. Our method takes advantage of the modularity of most Golgi-resident enzymes, which are composed of localization and catalytic domains. These domains can be physically separated and fused to the small molecule binding proteins FRB and FKBP, which dimerize in the presence of rapamycin. In this way, rapamycin serves as a "switch" for enzyme activity.

Plant N-glycan processing enzymes employ different targeting mechanisms for their spatial arrangement along the secretory pathway

The processing of N-linked oligosaccharides in the secretory pathway requires the sequential action of a number of glycosidases and glycosyltransferases. We studied the spatial distribution of several type II membrane-bound enzymes from Glycine max, Arabidopsis thaliana, and Nicotiana tabacum. Glucosidase I (GCSI) localized to the endoplasmic reticulum (ER), alpha-1,2 mannosidase I (ManI) and N-acetylglucosaminyltransferase I (GNTI) both targeted to the ER and Golgi, and beta-1,2 xylosyltransferase localized exclusively to Golgi stacks, corresponding to the order of expected function. ManI deletion constructs revealed that the ManI transmembrane domain (TMD) contains all necessary targeting information. Likewise, GNTI truncations showed that this could apply to other type II enzymes. A green fluorescent protein chimera with ManI TMD, lengthened by duplicating its last seven amino acids, localized exclusively to the Golgi and colocalized with a trans-Golgi marker (ST52-mRFP), suggesting roles for protein-lipid interactions in ManI targeting. However, the TMD lengths of other plant glycosylation enzymes indicate that this mechanism cannot apply to all enzymes in the pathway. In fact, removal of the first 11 amino acids of the GCSI cytoplasmic tail resulted in relocalization from the ER to the Golgi, suggesting a targeting mechanism relying on protein-protein interactions. We conclude that the localization of N-glycan processing enzymes corresponds to an assembly line in the early secretory pathway and depends on both TMD length and signals in the cytoplasmic tail.

Modulation of GalT1 and SialT1 sub-Golgi localization by SialT2 expression reveals an organellar level of glycolipid synthesis control

Ganglioside glycosyltransferases organize as multienzyme complexes that localize in different sub-Golgi compartments. Here we studied whether in CHO-K1 cells lacking CMP-NeuAc: GM3 sialyltransferase (SialT2), the sub-Golgi localization of UDP-Gal:glucosylceramide beta-1,4-galactosyltransferase (GalT1) and CMP-NeuAc:lactosylceramide sialyltransferase (SialT1) complex is affected when SialT2, another member of this complex, is coexpressed. GalT1 and SialT1 sub-Golgi localization was determined by studying the effect of brefeldin A (BFA) and monensin on the synthesis of glycolipids and on the sub-Golgi localization of GalT1(1-52)-CFP (cyan fluorescent protein) and SialT1(1-54)-YFP (yellow fluorescent protein) chimeras by single cell fluorescence microscopy and by isopycnic subfractionation. We found that BFA, and also monensin, impair the synthesis of glycolipids beyond GM3 ganglioside in wild type (WT) cells but beyond GlcCer in SialT2(+) cells. Although BFA redistributed GalT1-CFP and SialT1-YFP to the endoplasmic reticulum in WT cells, a fraction of these chimeras remained associated with a distal Golgi compartment, enriched in trans Golgi network, and recycling endosome markers in SialT2(+) cells. In BFA-treated cells, the percentage of GalT1-CFP and SialT1-YFP associated with Golgi-like membrane fractions separated by isopycnic subfractionation was higher in SialT2(+) cells than in WT cells. These effects were reverted by knocking down the expression of SialT2 with specific siRNA. Results indicate that sub-Golgi localization of glycosyltransferase complexes may change according to the relative levels of the expression of participating enzymes and reveal a capacity of the organelle to adapt the topology of the glycolipid synthesis machinery to functional states of the cell.

Cloning and recombinant expression of active full-length xylosyltransferase I (XT-I) and characterization of subcellular localization of XT-I and XT-II

Xylosyltransferase I (XT-I) catalyzes the transfer of xylose from UDP-xylose to serine residues in proteoglycan core proteins. This is the first and apparently rate-limiting step in the biosynthesis of the tetrasaccharide linkage region in glycosaminoglycan-containing proteoglycans. The XYLT-II gene codes for a highly homologous protein, but its physiological function is not yet known. Here we present for the first time the construction of a vector encoding the full-length GFP-tagged human XT-I and the recombinant expression of the active enzyme in mammalian cells. We expressed XT-I-GFP and various GFP-tagged XT-I and XT-II mutants with C-terminal truncations and deletions in HEK-293 and SaOS-2 cells in order to investigate the intracellular localization of XT-I and XT-II. Immunofluorescence analysis showed a distinct perinuclear pattern of XT-I-GFP and XT-II-GFP similar to that of alpha-mannosidase II, which is a known enzyme of the Golgi cisternae. Furthermore, a co-localization of native human XT-I and alpha-mannosidase II could also be demonstrated in untransfected cells. Using brefeldin A, we could also show that both xylosyltransferases are resident in the early cisternae of the Golgi apparatus. For its complete Golgi retention, XT-I requires the N-terminal 214 amino acids. Unlike XT-I, for XT-II, the first 45 amino acids are sufficient to target and retain the GFP reporter in the Golgi compartment. Here we show evidence that the stem regions were indispensable for Golgi localization of XT-I and XT-II.

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.

The Molecular Basis for Galα(1,3)Gal Expression in Animals with a Deletion of the α1,3Galactosyltransferase Gene

The production of homozygous pigs with a disruption in the GGTA1 gene, which encodes alpha1,3galactosyltransferase (alpha1,3GT), represented a critical step toward the clinical reality of xenotransplantation. Unexpectedly, the predicted complete elimination of the immunogenic Galalpha(1,3)Gal carbohydrate epitope was not observed as Galalpha(1,3)Gal staining was still present in tissues from GGTA1(-/-) animals. This shows that, contrary to previous dogma, alpha1,3GT is not the only enzyme able to synthesize Galalpha(1,3)Gal. As iGb3 synthase (iGb3S) is a candidate glycosyltransferase, we cloned iGb3S cDNA from GGTA1(-/-) mouse thymus and confirmed mRNA expression in both mouse and pig tissues. The mouse iGb3S gene exhibits alternative splicing of exons that results in a markedly different cytoplasmic tail compared with the rat gene. Transfection of iGb3S cDNA resulted in high levels of cell surface Galalpha(1,3)Gal synthesized via the isoglobo series pathway, thus demonstrating that mouse iGb3S is an additional enzyme capable of synthesizing the xenoreactive Galalpha(1,3)Gal epitope. Galalpha(1,3)Gal synthesized by iGb3S, in contrast to alpha1,3GT, was resistant to down-regulation by competition with alpha1,2fucosyltransferase. Moreover, Galalpha(1,3)Gal synthesized by iGb3S was immunogenic and elicited Abs in GGTA1 (-/-) mice. Galalpha(1,3)Gal synthesized by iGb3S may affect survival of pig transplants in humans, and deletion of this gene, or modification of its product, warrants consideration.

α1,3‐Galactosyltransferase knockout pigs are available for xenotransplantation: Are glycosyltransferases still relevant?

In the early 1990s, the Galalpha(1,3)Gal carbohydrate linkage was found to be the major xenoepitope causing hyperacute rejection. This carbohydrate, the antibodies that bind to it, and the enzyme that produces it (alpha1,3-galactosyltransferase) were the foci of research by many groups. Nearly a decade later, alpha1,3-galactosyltransferase knockout pigs were finally produced; hyperacute rejection could be avoided in these pigs. Having achieved this goal, enthusiasm declined for the study of glycosyltransferases and their carbohydrate products. To examine whether this decline was premature, we evaluate whether gene deletion has indeed solved the initial rejection problem or, in fact, created new problems. This review addresses this by examining the impact of the gene deletion on cell surface carbohydrate. Surprisingly, Galalpha(1,3)Gal is still present in alpha1,3-galactosyltransferase knockout animals: it is possibly synthesized on lipid by iGb3 synthase. Furthermore, removal of alphaGal resulted in the exposure of the N-acetyllactosamine epitope. This exposed epitope can bind natural antibodies and perhaps should be capped by transgenic expression of another transferase. We believe the continued study of glycosyltransferases is essential to examine the new issues raised by the deletion of alpha1,3-galactosyltransferase.

Missense mutations outside the catalytic domain of the ABO glycosyltransferase can cause weak blood group A and B phenotypes

BACKGROUND

Only little is known about the impact of amino acid substitutions outside an enzyme's active site on A and B transferase activity.

STUDY DESIGN AND METHODS

A panel of blood group A- and B-specific plasmids containing the six known missense mutations of the coding sequence upstream of exon 6 of the ABO gene were constructed. HeLa cells were used to transfect ABO expression plasmids.

RESULTS

Expression of ABO variants containing single or multiple missense mutations in HeLa cells resulted in a significant decrease in the percentage of antigen-expressing cells (up to 29%) and in mean fluorescence intensity (MFI; up to 50%) compared to transfection with ABO*A101 or ABO*B101. Coexpression of the respective antithetical wild-type construct (ABO*A101 and ABO*B101, respectively) further reduced cell surface expression of variant ABO constructs in regard to the percentage of expressing cells (up to 53% decrease) and MFI (up to 59% decrease).

CONCLUSION

Weak A and B subgroups can arise from transferases with amino acid changes in the N-terminal domain, particularly in AB phenotypes, where normal A1 or B1 glycosyltransferases compete for the same substrates.

Identification of a region within the cytoplasmic domain of the subtype B Vpu protein of human immunodeficiency virus type 1 (HIV-1) that is responsible for retention in the golgi complex and its absence in the Vpu protein from a subtype C HIV-1

The structure of the Vpu protein of human immunodeficiency virus type 1 (HIV-1) is composed of a short Nterminal domain (NTD), a transmembrane domain (TM), and a cytoplasmic domain (CD). Previous studies have shown that the Vpu protein from subtype B HIV-1 is transported predominantly to the rough endoplasmic reticulum (RER)/Golgi complex compartments of the cell and is not incorporated into virions. Using a previously described VpuEGFP reporter system in which the Vpu protein was fused to the gene for enhanced green fluorescent protein (EGFP), we showed that the subtype B Vpu fusion protein was localized to the RER/Golgi region of the cell, similar to the native protein. In the present study, we show that fusion of the subtype C Vpu to EGFP results in a fusion protein that is transported to the cell surface. Using this reporter system, chimeric Vpu proteins in which the CD of the subtype B and C proteins were exchanged showed that the CD was sufficient for targeting the subtype B protein to the Golgi complex of the cell. Following identification of the cytoplasmic domain as being responsible for intracellular targeting, we then generated a series of mutants in which 13, 23, 31, 38, 51, and 56 amino acids were deleted from the cytoplasmic domain of subtype B Vpu. These deletion mutants were analyzed by SDS-PAGE for size, for membrane localization, and intracellular localization by confocal fluorescence microscopy. Our results indicate that the mutant with the carboxyl-terminal 13 amino acids deleted was still localized to the Golgi complex but mutants with 23, 31, 38, 51, and 56 amino acids from the carboxyl-terminus of the subtype B Vpu were transported to the cell surface. These results suggest that a signal for the retention of the subtype B Vpu within the Golgi complex resides in the second alpha-helical domain.

Stem domains of heparan sulfate 6-O-sulfotransferase are required for Golgi localization, oligomer formation and enzyme activity

Heparan sulfate O-sulfotransferases catalyze the O-sulfation of the glucosamine and uronic acid residues of heparan sulfate, thereby determining the binding sites for ligands necessary for important biological functions such as the formation of morphogen gradients and growth factor signaling. Here we investigated the localization of the three heparan sulfate 6-O-sulfotransferase (HS6ST) isoforms and the mechanism of their localization. All three GFP-tagged HS6STs localized in the Golgi apparatus. C-5 epimerase and HS2ST have been shown to form complexes that facilitate their localization in the Golgi but we found that the absence of HS2ST did not alter the localization of any of the HS6STs. Neither the forced expression of HS2ST in the rough endoplasmic reticulum (ER), the deletion of most of the lumenal domain nor increasing the length of the transmembrane domain had any effect on the localization of HS6STs. However, deletions in the stem region did affect the Golgi localization of the HS6STs and also reduced their sulfotransferase activity and oligomer formation. These findings suggest that the stem region of HS6ST plays an important role in normal functioning, including the transit of HS6ST to the Golgi apparatus and maintaining the active conformation essential for enzyme activity.

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.

Deregulation of the carbohydrate (chondroitin 4) sulfotransferase 11 (CHST11) gene in a B-cell chronic lymphocytic leukemia with a t(12;14)(q23;q32)

The t(12;14)(q23;q32) breakpoints in a case of B-cell chronic lymphocytic leukemia (B-CLL) were mapped by fluorescence in situ hybridization (FISH) and Southern blot analysis and cloned using an IGH switch-gamma probe. The translocation affected a productively rearranged IGH allele and the carbohydrate (chondroitin 4) sulfotransferase 11 (CHST11) locus at 12q23, with a reciprocal break in intron 2 of the CHST11 gene. CHST11 belongs to the HNK1 family of Golgi-associated sulfotransferases, a group of glycosaminoglycan-modifying enzymes, and is expressed mainly in the hematopoietic lineage. Northern Blot analysis of tumor RNA using CHST11-specific probes showed expression of two CHST11 forms of abnormal size. 5'- and 3'-Rapid Amplification of cDNA Ends (RACE) revealed IGH/CHST11 as well as CHST11/IGH fusion RNAs expressed from the der(14) and der(12) chromosomes. Both fusion species contained open reading frames making possible the translation of two truncated forms of CHST11 protein. The biological consequence of t(12;14)(q23;q32) in this case presumably is a disturbance of the cellular distribution of CHST11 leading to deregulation of a chondroitin-sulfate-dependent pathway specific to the hematopoietic lineage.

Regulating cell surface glycosylation by small molecule control of enzyme localization

Cell surface carbohydrates mediate interactions between the cell and its environment. Glycosyltransferases responsible for synthesis of cell surface oligosaccharides are therefore essential administrators of cellular communication. These enzymes often comprise large families. Redundancy of related family members and embryonic lethality both complicate genetic methods for deconvoluting functions of glycosyltransferases. We report a chemical method in which the activity of an individual glycosyltransferase is controlled by a small molecule. The approach exploits the requirement of Golgi localization, a common feature of glycosyltransferase superfamily members. In our approach, the glycosyltransferase is separated into two domains, one that determines localization and one responsible for catalysis. Control of enzyme activity is achieved using a small molecule to regulate association of the two domains. We used this method to regulate production of sialyl Lewis x by alpha1,3-fucosyltransferase VII in living cells.

Ligand binding determines whether CD46 is internalized by clathrin-coated pits or macropinocytosis

CD46 is a ubiquitous human cell surface receptor for the complement components C3b and C4b and for various pathogens, including the measles virus and human herpes virus 6. Ligand binding to CD46 affects (i) protection of autologous cells from complement attack by breakdown of complement components, (ii) intracellular signals that affect the regulation of immune cell function, (iii) antigen presentation, and (iv) down-regulation of cell surface CD46. Recent evidence indicates that CD46 signaling can link innate and acquired immune function. The molecular mechanisms for these processes and the importance of intracellular trafficking of the receptor have not yet been elucidated. We demonstrate here that, in nonlymphoid cells, CD46 is constitutively internalized via clathrin-coated pits, traffics to multivesicular bodies, and is recycled to the cell surface. However, cross-linking of CD46 at the cell surface, by either multivalent antibody or by measles virus, induces pseudopodia that engulf the ligand in a process similar to macropinocytosis, and leads to the degradation of cell surface CD46. Thus, we have elucidated two pathways for CD46 internalization, which are regulated by the valence of cross-linking of CD46 and which utilize either clathrin-coated pits or pseudopodial extension. This has important implications for CD46 signaling, antigen presentation, CD46 down-regulation, and engulfment of pathogens.

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.

Transcription starting from an alternative promoter leads to the expression of the human ABO histo-blood group antigen

BACKGROUND

Using the 5'-rapid amplification of cDNA ends technique with the ex vivo culture of AC133-CD34+ cells, a transcription start site was recently identified approximately 0.7 kb upstream from the transcription start sites previously determined. The transcripts from the alternative starting exon 1a were demonstrated in the cells of both erythroid and epithelial lineages. Because the nucleotide sequence of exon 1a does not contain an ATG codon, we examined whether transcription starting from exon 1a leads to production of a functional glycosyltransferase.

STUDY DESIGN AND METHODS

Stable transfection experiments into the human gastric cancer MKN28 cells were performed using the various A transferase expression plasmids.

RESULTS

Large amounts of A antigens were demonstrated on the cells transfected with the A transferase expression plasmid containing the entire cDNA from exon 1a or the 5'-truncated cDNA leading to the production of the N-truncated protein with deletion of the cytoplasmic tail and a portion of the transmembrane domain. However, negligible amounts of A antigens were observed on the cells transfected with the A transferase expression plasmids containing the 5'-truncated cDNA leading to the production of the N-truncated proteins without the cytoplasmic tail and the transmembrane domain.

CONCLUSION

This study suggests that a functional A transferase could be produced by the transcription from exon 1a.

Importance of Cys, Gln, and Tyr from the transmembrane domain of human alpha 3/4 fucosyltransferase III for its localization and sorting in the Golgi of baby hamster kidney cells

Human fucosyltransferase III (EC ) (FT3wt) is localized in the Golgi of baby hamster kidney cells and synthesizes Lewis determinants associated with cell adhesion events. Replacement of the amino acid residues from the transmembrane domain (TM) Cys-16, Gln-23, Cys-29, and Tyr-33 by Leu (FT3np) caused a shift in enzyme localization to the plasma membrane. The mislocalization caused a dramatic decrease in the amount of biosynthetic products of FT3wt, the Lewis determinants. Determination of the expression levels on the surface with mutants of the enzyme, where one, two, or three of these residues were replaced by Leu, suggested that Cys from the TM was required for the localization of FT3 in the Golgi. Furthermore, Cys-23 and Cys-29 mediated the formation of disulfide-bonded dimers but not higher molecular weight oligomers. In vitro reconstitution of intra-Golgi transport showed that FT3wt was incorporated into coatomer protein (COP) I vesicles, contrary to FT3np. These data suggested that Cys, Gln, and Tyr residues are important for FT3wt sorting into the transport vesicles possibly due to interactions with other membrane proteins.