Heparan sulfate (HS) proteoglycans are essential regulators of cell signaling, development, and extracellular matrix organization. Central to the HS biosynthesis pathway is the exostosin family of enzymes that control commitment to HS assembly and catalyze formation of the polymer backbone in the Golgi apparatus. Recent structural studies have substantially revised our understanding by demonstrating that the HS co-polymerase is a tightly organized EXT1-EXT2 heterodimer, wherein the GT-B domain of EXT1 and the GT-A domain of EXT2 provide the β1,4-GlcA and α1,4-GlcNAc transferase activities, respectively. Parallel work on EXTL3 clarified how selective recognition of the linker-region glycopeptide commits to HS extension, outcompeting the default chondroitin sulfate pathway. In contrast, EXTL2 is best viewed as a regulatory GlcNAc transferase that can cap or divert linker intermediates rather than as a polymerase, whereas EXTL1 remains the least defined family member despite evidence for GlcNAc transferase activity. Genetic studies continue to reveal their importance in human disease, including hereditary multiple exostoses and EXTL3-associated immunodeficiency. Despite this progress, major questions remain regarding chain length control, enzyme coordination, and therapeutic targeting. This short review integrates recent structural, biochemical, and genetic advances to provide an updated narrative of how mammalian EXT proteins govern HS biosynthesis.

Glycosyltransferases (GTs) are the primary enzymes responsible for the biosynthesis of the complex polysaccharides in plant cell walls. Given the important role of GTs in plants, it is necessary to undertake their functional characterization to better understand plant cell wall synthesis pathways to develop improved feedstocks for efficient conversion into fuels and products to support the emerging bioeconomy. The GT47 family in plants represents a unique target for characterization due to the substantial diversity of donor and acceptor substrates observed within a single family. Here, we have carried out the biochemical characterization of MUR3 and XLT2 orthologs from the aquatic monocot Spirodela polyrhiza. Our findings support existing genetic and phylogenetic data classifying these enzymes as regio-specific galactosyltransferases involved in xyloglucan (XyG) sidechain biosynthesis. In addition, we have identified novel characteristics for both enzymes, such as in vitro arabinopyranosyltransferase activity and distinctiveness in xyloglucan reducing end specificity.

β1,3-Glucosyltransferase (B3GLCT) adds a glucose onto O-linked fucose on thrombospondin type 1 repeats (TSRs). Protein O-fucosyltransferase 2 (POFUT2) first transfers a fucose to properly folded TSRs containing a consensus sequence for O-fucosylation. This uncommon O-fucose modification is then extended to a glucose-fucose disaccharide by B3GLCT. B3GLCT is a GT-A fold glycosyltransferase and pathogenic variants cause Peters Plus Syndrome (PTRPLS, OMIM #261540), a Congenital Disorder of Glycosylation (CDG). Most GT-A fold family members have a single GT-A domain, but B3GLCT contains an additional GT-A domain. To assess the function of the additional GT-A domain and binding of TSR substrates, we determined the crystal structure of an O-fucosylated TSR (Fuc-O-TSR3) from Thrombospondin-1 bound to B3GLCT. The additional GT-A domain is essential for substrate binding but is catalytically inactive: it does not bind UDP or Mn2+ and is not a β1,3-glucosyltransferase. It also creates a deep pocket with a vestigial active site located on the opposite face of the Fuc-O-TSR3 substrate binding site. The Fuc-O-TSR3 acceptor substrate binds in a cleft between the two GT-A domains, each of which has evolved hypervariable regions for Fuc-O-TSR recognition. This structure is an unusual example of a glycosyltransferase with a catalytically inactive extra GT-A domain, providing insight into the binding of B3GLCT's diverse Fuc-O-TSR substrates. We also discuss how B3GLCT mimics the two-domain structure of GT-B fold glycosyltransferases. All GT-B fold glycosyltransferases contain two bilobal Rossmann-like fold domains, like B3GLCT. Our data also explains how PTRPLS-associated variants and predicted pathogenic mutations disrupt B3GLCT function.

Heparan sulfate proteoglycans (HSPGs) are essential cell surface and extracellular matrix glycoconjugates that mediate diverse biological processes through interactions between their heparan sulfate (HS) chains and extracellular ligands. While HS sulfation patterning is known to dictate ligand specificity, how cells control HS assembly to regulate these interactions remains incompletely understood. To systematically identify genetic modifiers of HS-protein interactions, we performed genome-wide CRISPR activation (CRISPRa) screens in HEK293T cells using binding of antithrombin (AT), which selectively recognizes 3-O-sulfated HS motifs, or the N-sulfation-specific antibody 10E4 as functional readouts. Strikingly, the screens revealed proteoglycan core proteins as key modulators of HS function. In particular, syndecan-1 (SDC1) emerged as a preferential enhancer of AT binding compared to other syndecan family members. Targeted upregulation of syndecan family members increased total HS levels, but only SDC1 enhanced AT binding. Structural and enzymatic analyses demonstrated that SDC1-associated HS chains contain elevated 6-O-sulfation and serve as superior substrates for 3-O-sulfotransferases relative to SDC2-associated HS chains. Additionally, SDC1 exhibited slower cell surface recovery, which was blocked by cycloheximide treatment, consistent with extended trafficking and biosynthetic processing. Overall, these findings indicate that proteoglycan core protein identity influences HS sulfation patterning and ligand-binding specificity and trafficking kinetics may contribute to core protein-dependent regulation of HS modification.

A covalent catch-and-release approach is described for the chemoenzymatic synthesis of HS oligosaccharides. It exploits that a glycan modified by a hydrazide tag can under mild acidic conditions efficiently be reacted with an aldehyde-containing resin to give an immobilized hydrazone product. The hydrazone is stable under neutral conditions, allowing stringent washing conditions to remove all other components. The product can be released by transiminolysis using aqueous hydroxylamine, which, after evaporation, yields a compound that can be immediately employed in the next cycle of modification. As many as 11 consecutive transformations could be performed to give a homogenous octasaccharide having a complex pattern of sulfation and epimerization. The methodology provides opportunities to prepare convenient collections of biologically important HS oligosaccharides for structure-function studies.

Chondroitin sulfate (CS) proteoglycans are extended (-GlcAβ1,3GalNAcβ1,4-)n co-polymers attached to cell surface and extracellular matrix core proteins that are further modified by extensive sulfation and epimerization. Four homologous proteins contribute to CS backbone synthesis (CHPF1, CHPF2, CHSY1, and CHSY3) and prior data suggests assembly of the proteins into heterocomplexes is required for function. Here we show by sequence alignment and structural modeling that all CHSYs and CHPFs contain an N-terminal CAZy GT31-like domain and a C-terminal GT7-like domain separated by a cystatin-like linker domain. Co-expression of one CHPF and one CHSY is required to form a soluble, functional heterodimeric CS synthase and structural modeling indicates all four potential CHSY-CHPF combinations can form equivalent heterodimeric complexes. Cryo-EM studies on CHSY3-CHPF1 confirm the structure, interface, and active site features predicted by the structural models. Enzymatic analyses of catalytic mutants demonstrate that only the glycosyltransferase domains in the CHSYs are responsible for polymer synthesis: the GT31 domain transfers β1,3-GlcA while the GT7 domain transfers β1,4-GalNAc. The corresponding CHPF domains do not contribute to polymer synthesis but stabilize the corresponding CHSY functional domains. Additional mutagenesis and modeling suggest that the bridging cystatin-like domains may contribute to efficient polymer synthesis.

Adoptive T-cell transfer has revolutionized the treatment of hematological malignancies. However, this approach has had very limited success in treating solid tumors, largely due to inadequate infiltration of vascularly administered T cells at tumor sites. The shear-resistant interaction between endothelial E-selectin and its cognate ligand expressed on leukocytes, sialyl Lewis X (sLeX), is an essential prerequisite for extravasation of circulating leukocytes. Here, we report that enforced E-selectin ligand expression (enforced sLeX display) on antigen-specific T cells can be achieved by fucosylating cells via cell surface treatment with the human α1-3-fucosyltransferase, FUT6 ("exofucosylation"), or via Golgi-targeted FUT6 overexpression ("Golgi-fucosylation"). However, despite comparable E-selectin binding, only sLeX-modified T cells engendered by exofucosylation, not by Golgi-fucosylation, exhibited enhanced parenchymal infiltration of target malignant sites. This heightened homing yielded significantly improved therapeutic efficacy in various murine syngeneic and xenograft cancer models, including subcutaneous solid tumors, lymphoma and leukemia, as well as lung and bone marrow metastases. Therefore, exofucosylation represents a promising strategy to improve the efficacy of adoptive T-cell therapy, particularly in the treatment of solid tumors and metastatic disease.

O-glycosylation is a ubiquitous post-translational modification essential for protein stability, cell signaling, and tissue organization, yet how distinct O-glycan subclasses coordinate tissue development remains unclear. Here, we identify functional crosstalk between extended mucin-type O-glycans and heparan sulfate proteoglycans (HSPGs). Genetic ablation of β-1,3-galactosyltransferase 1 (C1GALT1) or its chaperone COSMC in human chondrocytes reduced cell surface HSPGs and fibroblast growth factor (FGF) binding, leading to impaired MAPK/ERK signaling, which was recapitulated in hypomorphic COSMC-CDG patient cells. Transcriptomic and secretome analyses revealed selective loss of proteoglycan expression and broader extracellular matrix remodeling in COSMC- and C1GALT1-deficient cells. Mechanistically, truncation of O-GalNAc glycans reduced Syndecan-1 levels at the cell surface via enhancing its lysosome-dependent degradation and diminished CD44v3-mediated FGF1 binding. Functionally, loss of complex O-GalNAc glycans disrupted chondrogenesis of growth plate-like chondroprogenitors. Overall, these findings reveal previously unrecognized roles for mucin-type O-glycans in maintaining proteoglycan stability and function, highlighting cross-regulatory mechanisms of glycosylation crucial for normal development that likely contribute to the pathophysiology of glycosylation-related disorders.

Xylan, the most abundant non-cellulosic polymer in plant cell walls, is structurally diverse, especially in grasses where it is heavily substituted with arabinofuranose and further modified by various residues. Common substitutions across species include glucuronic and 4- O -methyl-glucuronic acid. Arabinose and xylose sidechains are synthesized by glycosyltransferase family 61 (GT61) proteins, many of which remain uncharacterized in plants, with limited structural and mechanistic understanding. In this study, we identified two novel GT61 enzymes in Sorghum bicolor , functioning as xylan arabinosyltransferase (SbXAT) and xylan xylosyltransferase (SbXXT). We resolved the crystal structure of SbXAT, which exhibits a GT-B fold with two Rossmann-like domains linked by a cleft that accommodates the catalytic site. Structural comparison with a predicted SbXXT model revealed a substrate-binding residue critical for sugar donor specificity, validated through site-directed mutagenesis and enzymatic assays. These findings enhance understanding of xylan biosynthesis and provide a foundation for engineering glycosyltransferases and predicting their functions.

Sulfated N-glycans are present in many glycoproteins, which are implicated in playing important roles in biological recognition processes. Here, we report the systematic chemoenzymatic synthesis of a library of sulfated and sialylated biantennary N-glycans and assess their binding to Siglecs and glycan-specific antibodies that recognize them as glycan ligands. The combined use of three human sulfotransferases, GlcNAc-6-O-sulfotransferase (CHST2), Gal-3-O-sulfotransferase (Gal3ST1), and keratan sulfate Gal-6-O-sulfotransferase (CHST1), resulted in asymmetric and symmetric branch-selective sulfation of the GlcNAc and/or Gal moieties of N-glycans. The extension of the sugar chain using alpha-2,3- and alpha-2,6-sialyltransferases afforded the sulfated and sialylated N-glycans. These synthetic glycans with different patterns of sulfation and sialylation were evaluated for binding to selected Siglecs and sulfoglycan-specific antibodies using glycan microarrays. The results confirm previously documented glycan-recognizing properties and further reveal novel specificities for these glycan-binding proteins, demonstrating the utility of the library for assessing the specificity of glycan-binding proteins recognizing sulfated and sialylated glycans.

Keratan sulfate (KS) is a proteoglycan that is widely expressed in the extracellular matrix of various tissue types where it performs multiple biological functions. KS is the least understood proteoglycan, which in part is due to a lack of panels of well-defined KS oligosaccharides that are needed for structure-binding studies, as analytical standards, to examine substrate specificities of keratinases and for drug development. Here, we report a biomimetic approach that makes it possible to install, in a regioselective manner, sulfates and fucosides on oligo-N-acetyllactosamine (LacNAc) chains to provide any structural element of KS by using specific enzyme modules. It is based on the observation that 1,3-fucosides, 2,6-sialosides and C-6 sulfation of galactose (Gal6S) are mutually exclusive and cannot occur on the same LacNAc moiety. As a result, the pattern of sulfation on galactosides can be controlled by installing1,3-fucosides or 2,6-sialosides to temporarily block certain LacNAc moieties from sulfation by keratan sulfate galactose 6-sulfotransferase (CHST1). The pattern of 1,3-fucosylation and 2,6-sialylation can be controlled by exploiting the mutual exclusivity of these modifications, which in turn controls the sites of sulfation by CHST1. Late-stage treatment with a fucosidase or sialidase to remove blocking fucosides or sialosides provides selectively sulfated KS oligosaccharides. These treatments also unmasked specific galactosides for further controlled modification by CHST1. To showcase the potential of the enzymatic strategy, we have prepared a range of poly-LacNAc derivatives having different patterns of fucosylation and sulfation and several N-glycans decorated by specific arrangements of sulfates.

Keratan sulfate (KS) is a highly complex proteoglycan that has a poly-LacNAc chain that can be modified by diverse patterns of sulfate esters at C-6 positions of galactoside (Gal) and N-acetylglucosamine (GlcNAc) residues. Here, a chemo-enzymatic methodology is described that can control the pattern of sulfation at Gal using UDP-Gal-aldehyde as a donor for poly-LacNAc assembly to temporarily block specific sites from sulfation by galactose 6-sulfotransferase (CHST1).

Ganglioside glycans are ubiquitous and complex biomolecules that are involved in a wide range of biological functions and disease processes. Variations in sialylation and sulfation render the structural complexity and diversity of ganglioside glycans, and influence protein-carbohydrate interactions. Structural and functional insights into the biological roles of these glycans are impeded due to the limited accessibility of well-defined structures. Here we report an integrated chemoenzymatic strategy for expeditious and systematic synthesis of a comprehensive 65-membered ganglioside glycan library covering all possible patterns of sulfation and sialylation. This strategy relies on the streamlined modular assembly of three common sialylated precursors by highly stereoselective iterative sialylation, modular site-specific sulfation through flexible orthogonal protecting-group manipulations and enzymatic-catalysed diversification using three sialyltransferase modules and a galactosidase module. These diverse ganglioside glycans enable exploration into their structure-function relationships using high-throughput glycan microarray technology, which reveals that different patterns of sulfation and sialylation on these glycans mediate their unique binding specificities. Deciphering the sulfation and sialylation codes of ganglioside glycans is impeded by the limited accessibility of well-defined structures. Now, an integrated chemoenzymatic strategy has been developed for efficient synthesis of a comprehensive 65-membered ganglioside glycan library, enabling an extensive exploration into their structure-function relationships using glycan microarray technology.

Pectins are enriched in primary cell walls and the middle lamella, determining cell-to-cell adhesion. O-acetylation of pectin backbones influences their physicochemical properties and plays a role in plant development and interactions with the environment. Here we report the isolation and mapping of two trichome birefringence (tbr) mutants from an ozone-sensitivity screen and show that TBR is required for leaf epidermal cell adhesion. We further demonstrate that TBR is an RG-I rhamnose O-acetyltransferase with an acidic pH optimum, and, through structure-function modelling coupled with biochemical analysis determine the significance of mutated amino acid residues. Notably, TBR utilizes multiple O-acetyl donors, and is able to form acyl-enzyme intermediates in the presence of acetylated pectins, suggesting transacetylase activity. We propose that, in tbr, loss of cell adhesion stems from cell wall modifications triggered by decreased RG-I acetylation during leaf cell proliferation. Together, our data suggest that RG-I O-acetylation affects cell adhesion.

Suppression of immune response is a phenomenon that enables biological processes such as gamete fertilization, cell growth, cell proliferation, endophyte recruitment, parasitism, and pathogenesis. Here, we show for the first time that the Plasminogen-Apple-Nematode (PAN) domain present in G-type lectin receptor-like kinases is essential for immunosuppression in plants. Defense pathways involving jasmonic acid and ethylene are critical for plant immunity against microbes, necrotrophic pathogens, parasites, and insects. Using two Salix purpurea G-type lectin receptor kinases, we demonstrated that intact PAN domains suppress jasmonic acid and ethylene signaling in Arabidopsis and tobacco. Variants of the same receptors with mutated residues in this domain could trigger induction of both defense pathways. Assessment of signaling processes revealed significant differences between receptors with intact and mutated PAN domain in MAPK phosphorylation, global transcriptional reprogramming, induction of downstream signaling components, hormone biosynthesis and resistance to Botrytis cinerea . Further, we demonstrated that the domain is required for oligomerization, ubiquitination, and proteolytic degradation of these receptors. These processes were completely disrupted when conserved residues in the domain were mutated. Additionally, we have tested the hypothesis in recently characterized Arabidopsis mutant which has predicted PAN domain and negatively regulates plant immunity against root nematodes. ern1.1 mutant complemented with mutated PAN shows triggered immune response with elevated WRKY33 expression, hyperphosphorylation of MAPK and resistant to necrotrophic fungus Botrytis cinerea . Collectively, our results suggest that ubiquitination and proteolytic degradation mediated by the PAN domain plays a role in receptor turn-over to suppress jasmonic acid and ethylene defense signaling in plants.

Keratan sulfate (KS) is a glycosaminoglycan that is widely expressed in the extracellular matrix of various tissue types, where it is involved in many biological processes. Herein, we describe a chemo-enzymatic approach to preparing well-defined KS oligosaccharides by exploiting the known and newly discovered substrate specificities of relevant sulfotransferases. The premise of the approach is that recombinant GlcNAc-6-O-sulfotransferases (CHST2) only sulfate terminal GlcNAc moieties to give GlcNAc6S that can be galactosylated by B4GalT4. Furthermore, CHST1 can modify the internal galactosides of a poly-LacNAc chain; however, it was found that a GlcNAc6S residue greatly increases the reactivity of CHST1 of a neighboring and internal galactoside. The presence of a 2,3-linked sialoside further modulates the site of modification by CHST1, and a galactoside flanked by 2,3-Neu5Ac and GlcNAc6S is preferentially sulfated over the other Gal residues. The substrate specificities of CHST1 and 2 were exploited to prepare a panel of KS oligosaccharides, including selectively sulfated N-glycans. The compounds and several other reference derivatives were used to construct a microarray that was probed for binding by several plant lectins, Siglec proteins, and hemagglutinins of influenza viruses. It was found that not only the sulfation pattern but also the presentation of epitopes as part of an O- or N-glycan determines binding properties.

Glycosyltransferases (GTs) are enzymes that catalyze the formation of glycosidic bonds and hundreds of GTs have been identified so far in humans. Glycosyltransferase 8 domain-containing protein 1 (GLT8D1) has been associated with central nervous system diseases and cancer. However, evidence on its enzymatic properties, including its substrates, has been scarcely described. In this paper, we have produced and purified recombinant secretory GLT8D1. The enzyme was found to be N -glycosylated. Differential scanning fluorimetry was employed to analyze the stabilization of GLT8D1 by Mn 2+ and nucleotides, revealing UDP as the most stabilizing nucleotide scaffold. GLT8D1 displayed glycosyltransferase activity from UDP-galactose onto N -acetylgalactosamine but with a low efficiency. Modeling of the structure revealed similarities with other GT-A fold enzymes in CAZy family GT8 and glycosyltransferases in other families with galactosyl-, glucosyl-, and xylosyltransferase activities, each with retaining catalytic mechanisms. Our study provides novel structural and functional insights into the properties of GLT8D1 with implications in pathological processes.