Glycobiology and Extracellular Matrices
Sequential in vitro enzymatic N-glycoprotein modification reveals site-specific rates of glycoenzyme processing
N-glycosylation is an essential eukaryotic posttranslational modification that affects various glycoprotein properties, including folding, solubility, protein–protein interactions, and half-life. N-glycans are processed in the secretory pathway to form varied ensembles of structures, and diversity at a single site on a glycoprotein is termed ‘microheterogeneity’. To understand the factors that influence glycan microheterogeneity, we hypothesized that local steric and electrostatic factors surrounding each site influence glycan availability for enzymatic modification.Examination of differential glycoprotein preferences of N-acetylglucosaminyltransferase-IV isozymes a and b
The N-glycans attached to proteins contain various GlcNAc branches, the aberrant formation of which correlates with various diseases. N-Acetylglucosaminyltransferase-IVa (GnT-IVa or MGAT4A) and Gnt-IVb (or MGAT4B) are isoenzymes that catalyze the formation of the β1,4-GlcNAc branch in N-glycans. However, the functional differences between these isozymes remain unresolved. Here, using cellular and UDP-Glo enzyme assays, we discovered that GnT-IVa and GnT-IVb have distinct glycoprotein preferences both in cells and in vitro.Incorporation of fucose into glycans independent of the GDP-fucose transporter SLC35C1 preferentially utilizes salvaged over de novo GDP-fucose
Mutations in the SLC35C1 gene encoding the Golgi GDP-fucose transporter are known to cause leukocyte adhesion deficiency II. However, improvement of fucosylation in leukocyte adhesion deficiency II patients treated with exogenous fucose suggests the existence of an SLC35C1-independent route of GDP-fucose transport, which remains a mystery. To investigate this phenomenon, we developed and characterized a human cell–based model deficient in SLC35C1 activity. The resulting cells were cultured in the presence/absence of exogenous fucose and mannose, followed by examination of fucosylation potential and nucleotide sugar levels.Substrate specificities and reaction kinetics of the yeast oligosaccharyltransferase isoforms
Oligosaccharyltransferase (OST) catalyzes the central step in N-linked protein glycosylation, the transfer of a preassembled oligosaccharide from its lipid carrier onto asparagine residues of secretory proteins. The prototypic hetero-octameric OST complex from the yeast Saccharomyces cerevisiae exists as two isoforms that contain either Ost3p or Ost6p, both noncatalytic subunits. These two OST complexes have different protein substrate specificities in vivo. However, their detailed biochemical mechanisms and the basis for their different specificities are not clear.Characterizing human α-1,6-fucosyltransferase (FUT8) substrate specificity and structural similarities with related fucosyltransferases
Mammalian Asn-linked glycans are extensively processed as they transit the secretory pathway to generate diverse glycans on cell surface and secreted glycoproteins. Additional modification of the glycan core by α-1,6-fucose addition to the innermost GlcNAc residue (core fucosylation) is catalyzed by an α-1,6-fucosyltransferase (FUT8). The importance of core fucosylation can be seen in the complex pathological phenotypes of FUT8 null mice, which display defects in cellular signaling, development, and subsequent neonatal lethality.Biosynthesis of GlcNAc-rich N- and O-glycans in the Golgi apparatus does not require the nucleotide sugar transporter SLC35A3
Nucleotide sugar transporters, encoded by the SLC35 gene family, deliver nucleotide sugars throughout the cell for various glycosyltransferase-catalyzed glycosylation reactions. GlcNAc, in the form of UDP-GlcNAc, and galactose, as UDP-Gal, are delivered into the Golgi apparatus by SLC35A3 and SLC35A2 transporters, respectively. However, although the UDP-Gal transporting activity of SLC35A2 has been clearly demonstrated, UDP-GlcNAc delivery by SLC35A3 is not fully understood. Therefore, we analyzed a panel of CHO, HEK293T, and HepG2 cell lines including WT cells, SLC35A2 knockouts, SLC35A3 knockouts, and double-knockout cells.Uncoupling the hydrolysis of lipid-linked oligosaccharide from the oligosaccharyl transfer reaction by point mutations in yeast oligosaccharyltransferase
Oligosaccharyltransferase (OST) is responsible for the first step in the N-linked glycosylation, transferring an oligosaccharide chain onto asparagine residues to create glycoproteins. In the absence of an acceptor asparagine, OST hydrolyzes the oligosaccharide donor, releasing free N-glycans (FNGs) into the lumen of the endoplasmic reticulum (ER). Here, we established a purification method for mutated OSTs using a high-affinity epitope tag attached to the catalytic subunit Stt3, from yeast cells co-expressing the WT OST to support growth.Golgi-localized exo-β1,3-galactosidases involved in cell expansion and root growth in Arabidopsis
Plant arabinogalactan proteins (AGPs) are a diverse group of cell surface– and wall–associated glycoproteins. Functionally important AGP glycans are synthesized in the Golgi apparatus, but the relationships among their glycosylation levels, processing, and functionalities are poorly understood. Here, we report the identification and functional characterization of two Golgi-localized exo-β-1,3-galactosidases from the glycosyl hydrolase 43 (GH43) family in Arabidopsis thaliana. GH43 loss-of-function mutants exhibited root cell expansion defects in sugar-containing growth media.Conserved Glu-47 and Lys-50 residues are critical for UDP-N-acetylglucosamine/UMP antiport activity of the mouse Golgi-associated transporter Slc35a3
Nucleotide sugar transporters (NSTs) regulate the flux of activated sugars from the cytosol into the lumen of the Golgi apparatus where glycosyltransferases use them for the modification of proteins, lipids, and proteoglycans. It has been well-established that NSTs are antiporters that exchange nucleotide sugars with the respective nucleoside monophosphate. Nevertheless, information about the molecular basis of ligand recognition and transport is scarce. Here, using topology predictors, cysteine-scanning mutagenesis, expression of GFP-tagged protein variants, and phenotypic complementation of the yeast strain Kl3, we identified residues involved in the activity of a mouse UDP-GlcNAc transporter, murine solute carrier family 35 member A3 (mSlc35a3).Biosynthesis of O-N-acetylgalactosamine glycans in the human cell nucleus
Biological functions of nuclear proteins are regulated by post-translational modifications (PTMs) that modulate gene expression and cellular physiology. However, the role of O-linked glycosylation (O-GalNAc) as a PTM of nuclear proteins in the human cell has not been previously reported. Here, we examined in detail the initiation of O-GalNAc glycan biosynthesis, representing a novel PTM of nuclear proteins in the nucleus of human cells, with an emphasis on HeLa cells. Using soluble nuclear fractions from purified nuclei, enzymatic assays, fluorescence microscopy, affinity chromatography, MS, and FRET analyses, we identified all factors required for biosynthesis of O-GalNAc glycans in nuclei: the donor substrate (UDP-GalNAc), nuclear polypeptide GalNAc -transferase activity, and a GalNAc transferase (polypeptide GalNAc-T3).My journey in the discovery of nucleotide sugar transporters of the Golgi apparatus
Defects in protein glycosylation can have a dramatic impact on eukaryotic cells and is associated with mental and developmental pathologies in humans. The studies outlined below illustrate how a basic biochemical problem in the mechanisms of protein glycosylation, specifically substrate transporters of nucleotide sugars, including ATP and 3′-phosphoadenyl-5′-phosphosulfate (PAPS), in the membrane of the Golgi apparatus and endoplasmic reticulum, expanded into diverse biological systems from mammals, including humans, to yeast, roundworms, and protozoa.Genetic disruption of multiple α1,2-mannosidases generates mammalian cells producing recombinant proteins with high-mannose–type N-glycans
Recombinant therapeutic proteins are becoming very important pharmaceutical agents for treating intractable diseases. Most biopharmaceutical proteins are produced in mammalian cells because this ensures correct folding and glycosylation for protein stability and function. However, protein production in mammalian cells has several drawbacks, including heterogeneity of glycans attached to the produced protein. In this study, we established cell lines with high-mannose–type N-linked, low-complexity glycans.Autopolysialylation of polysialyltransferases is required for polysialylation and polysialic acid chain elongation on select glycoprotein substrates
Polysialic acid (polySia) is a large glycan polymer that is added to some glycoproteins by two polysialyltransferases (polySTs), ST8Sia-II and ST8Sia-IV. As polySia modulates cell adhesion and signaling, immune cell function, and tumor metastasis, it is of interest to determine how the polySTs recognize their select substrates. We have recently identified residues within the ST8Sia-IV polybasic region (PBR) that are required for neural cell adhesion molecule (NCAM) recognition and subsequent polysialylation.Single-subunit oligosaccharyltransferases of Trypanosoma brucei display different and predictable peptide acceptor specificities
Trypanosoma brucei causes African trypanosomiasis and contains three full-length oligosaccharyltransferase (OST) genes; two of which, TbSTT3A and TbSTT3B, are expressed in the bloodstream form of the parasite. These OSTs have different peptide acceptor and lipid-linked oligosaccharide donor specificities, and trypanosomes do not follow many of the canonical rules developed for other eukaryotic N-glycosylation pathways, raising questions as to the basic architecture and detailed function of trypanosome OSTs.Extrinsic Functions of Lectin Domains in O-N-Acetylgalactosamine Glycan Biosynthesis
Glycan biosynthesis occurs mainly in Golgi. Molecular organization and functional regulation of this process are not well understood. We evaluated the extrinsic effect of lectin domains (β-trefoil fold) of polypeptide GalNAc-transferases (ppGalNAc-Ts) on catalytic activity of glycosyltransferases during O-GalNAc glycan biosynthesis. The presence of lectin domain T3lec or T4lec during ppGalNAc-T2 and ppGalNAc-T3 catalytic reaction had a clear inhibitory effect on GalNAc-T activity. Interaction of T3lec or T4lec with ppGalNAc-T2 catalytic domain was not mediated by carbohydrate.Engineering and Dissecting the Glycosylation Pathway of a Streptococcal Serine-rich Repeat Adhesin
Serine-rich repeat glycoproteins (SRRPs) are conserved in Gram-positive bacteria. They are crucial for modulating biofilm formation and bacterial-host interactions. Glycosylation of SRRPs plays a pivotal role in the process; thus understanding the glycosyltransferases involved is key to identifying new therapeutic drug targets. The glycosylation of Fap1, an SRRP of Streptococcus parasanguinis, is mediated by a gene cluster consisting of six genes: gtf1, gtf2, gly, gtf3, dGT1, and galT2. Mature Fap1 glycan possesses the sequence of Rha1–3Glc1-(Glc1–3GlcNAc1)-2,6-Glc1–6GlcNAc.Mammalian α-1,6-Fucosyltransferase (FUT8) Is the Sole Enzyme Responsible for the N-Acetylglucosaminyltransferase I-independent Core Fucosylation of High-mannose N-Glycans
Understanding the biosynthetic pathway of protein glycosylation in various expression cell lines is important for controlling and modulating the glycosylation profiles of recombinant glycoproteins. We found that expression of erythropoietin (EPO) in a HEK293S N-acetylglucosaminyltransferase I (GnT I)−/− cell line resulted in production of the Man5GlcNAc2 glycoforms, in which more than 50% were core-fucosylated, implicating a clear GnT I-independent core fucosylation pathway. Expression of GM-CSF and the ectodomain of FcγIIIA receptor led to ∼30% and 3% core fucosylation, suggesting that the level of core fucosylation also depends on the nature of the recombinant proteins.ERManI (Endoplasmic Reticulum Class I α-Mannosidase) Is Required for HIV-1 Envelope Glycoprotein Degradation via Endoplasmic Reticulum-associated Protein Degradation Pathway
Background: HIV-1 envelope (Env) glycoprotein is targeted to endoplasmic reticulum (ER)-associated protein degradation (ERAD) pathway for degradation after infecting cells.Results: ER class I α-mannosidase (ERManI) interacts with Env and initiates this degradation process.Conclusion: ERManI is essential for the Env degradation.Significance: These findings define a novel endogenous and potential therapeutically applicable antiretroviral mechanism by targeting Env for degradation.
