Identification of a novel protein binding motif within the T-synthase for the molecular chaperone Cosmc

The biological role of core 1β1-3galactosyltransferase (T-synthase) in mucin-type O-glycosylation in Silkworm, Bombyx mori

O-glycosylation of secreted and membrane-bound proteins is an important post-translational modification that affects recognition of cell surface receptors, protein folding, and stability. However, despite the importance of O-linked glycans, their biological functions have not yet been fully elucidated and the synthetic pathway of O-glycosylation has not been investigated in detail, especially in the silkworm. In this study, we aimed to investigate O-glycosylation in silkworms by analyzing the overall structural profiles of mucin-type O-glycans using LC-MS. We found GalNAc or GlcNAc monosaccharide and core 1 disaccharide (Galβ1-3-GalNAcα1-Ser/Thr) were major components of the O-glycan attached to secreted proteins produced in silkworms. Furthermore, we characterized the 1 b1,3-galactosyltransferase (T-synthase) required for synthesis of the core 1 structure, common to many animals. Five transcriptional variants and four protein isoforms were identified in silkworms, and the biological functions of these isoforms were investigated. We found that BmT-synthase isoforms 1 and 2 were localized in the Golgi apparatus in cultured BmN4 cells and functioned both in cultured cells and silkworms. Additionally, a specific functional domain of T-synthase, called the stem domain, was found to be essential for activity and is presumed to be needed for dimer formation and galactosyltransferase activity. Altogether, our results elucidated the O-glycan profile and function of T-synthase in the silkworm. Our findings allow the practical comprehension of O-glycosylation required for employing silkworms as a productive expression system.

Emerging Roles of the Unique Molecular Chaperone Cosmc in the Regulation of Health and Disease

The core-1 β1-3galactosyltransferase-specific chaperone 1 (Cosmc) is a unique molecular chaperone of core-1 β1-3galactosyltransferase(C1GALT1), which typically functions inside the endoplasmic reticulum (ER). Cosmc helps C1GALT1 to fold correctly and maintain activity. It also participates in the synthesis of the T antigen, O-glycan, together with C1GALT1. Cosmc is a multifaceted molecule with a wide range of roles and functions. It involves platelet production and the regulation of immune cell function. Besides that, the loss of function of Cosmc also facilitates the development of several diseases, such as inflammation diseases, immune-mediated diseases, and cancer. It suggests that Cosmc is a critical control point in diseases and that it should be regarded as a potential target for oncotherapy. It is essential to fully comprehend Cosmc's roles, as they may provide critical information about its involvement in disease development and pathogenesis. In this review, we summarize the recent progress in understanding the role of Cosmc in normal development and diseases.

Structural basis for the synthesis of the core 1 structure by C1GalT1

C1GalT1 is an essential inverting glycosyltransferase responsible for synthesizing the core 1 structure, a common precursor for mucin-type O-glycans found in many glycoproteins. To date, the structure of C1GalT1 and the details of substrate recognition and catalysis remain unknown. Through biophysical and cellular studies, including X-ray crystallography of C1GalT1 complexed to a glycopeptide, we report that C1GalT1 is an obligate GT-A fold dimer that follows a S_N2 mechanism. The binding of the glycopeptides to the enzyme is mainly driven by the GalNAc moiety while the peptide sequence provides optimal kinetic and binding parameters. Interestingly, to achieve glycosylation, C1GalT1 recognizes a high-energy conformation of the α-GalNAc-Thr linkage, negligibly populated in solution. By imposing this 3D-arrangement on that fragment, characteristic of α-GalNAc-Ser peptides, C1GalT1 ensures broad glycosylation of both acceptor substrates. These findings illustrate a structural and mechanistic blueprint to explain glycosylation of multiple acceptor substrates, extending the repertoire of mechanisms adopted by glycosyltransferases.

The glycosyltransferase C1GalT1 directs a key step in protein O-glycosylation important for the expression of the cancer-associated Tn and T antigens. Here, the authors provide molecular insights into the function of C1GalT1 by solving the crystal structure of the Drosophila enzyme-substrate complex.

Expression and Impact of C1GalT1 in Cancer Development and Progression

Simple Summary

C1GalT1 is one of the enzymes that catalyze the addition of sugar residues to proteins (protein glycosylation). It specifically controls the synthesis and formation of a special disaccharide structure Galβ1,3GalNAcα-, which occurs predominately in cancer but rarely in normal cells. Recent studies have shown that C1GalT1 is overexpressed in many common cancers including colon, breast, gastric, lung, head and neck, pancreatic, esophageal, prostate, and hepatocellular cancer. C1GalT1 overexpression is also often associated with poorer prognosis and poorer patient survival. This review summarizes our current understanding of the expression of C1GalT1 in various cancers and discusses the impact of C1GalT change on cancer cell activities in cancer development and progression.

Abstract

C1GalT1 (T-synthase) is one of the key glycosyltransferases in the biosynthesis of O-linked mucin-type glycans of glycoproteins. It controls the formation of Core-1 disaccharide Galβ1,3GalNAcα- (Thomsen–Friedenreich oncofetal antigen, T or TF antigen) and Core-1-associated carbohydrate structures. Recent studies have shown that C1GalT1 is overexpressed in many cancers of epithelial origin including colon, breast, gastric, head and neck, pancreatic, esophageal, prostate, and hepatocellular cancer. Overexpression of C1GalT1 is often seen to also be associated with poorer prognosis and poorer patient survival. Change of C1GalT1 expression causes glycosylation changes of many cell membrane glycoproteins including mucin proteins, growth factor receptors, adhesion molecules, and death receptors. This leads to alteration of the interactions of these cell surface molecules with their binding ligands, resulting in changes of cancer cell activity and behaviors. This review summarizes our current understanding of the expression of C1GalT1 in various cancers and discusses the impact of C1GalT change on cancer cell activities in cancer development and progression.

C1GALT1 in health and disease

O-linked glycosylation (O-glycosylation) and N-linked glycosylation (N-glycosylation) are the two most important forms of protein glycosylation, which is an important post-translational modification. The regulation of protein function involves numerous mechanisms, among which protein glycosylation is one of the most important. Core 1 synthase glycoprotein-N-acetylgalactosamine 3-β-galactosyltransferase 1 (C1GALT1) serves an important role in the regulation of O-glycosylation and is an essential enzyme for synthesizing the core 1 structure of mucin-type O-glycans. Furthermore, C1GALT1 serves a vital role in a number of biological functions, such as angiogenesis, platelet production and kidney development. Impaired C1GALT1 expression activity has been associated with different types of human diseases, including inflammatory or immune-mediated diseases, and cancer. O-glycosylation exists in normal tissues, as well as in tumor tissues. Previous studies have revealed that changes in the level of glycosyltransferase in different types of cancer may be used as potential therapeutic targets. Currently, numerous studies have reported the dual role of C1GALT1 in tumors (carcinogenesis and cancer suppression). The present review reports the role of C1GALT1 in normal development and human diseases. Since the mechanism and regulation of C1GALT1 and O-glycosylation remain elusive, further studies are required to elucidate their effects on development and disease.

A phylogenetic view and functional annotation of the animal β1,3-glycosyltransferases of the GT31 CAZy family

The formation of β1,3-linkages on animal glycoconjugates is catalyzed by a subset of β1,3-glycosyltransferases grouped in the Carbohydrate-Active enZYmes family glycosyltransferase-31 (GT31). This family represents an extremely diverse set of β1,3-N-acetylglucosaminyltransferases [B3GNTs and Fringe β1,3-N-acetylglucosaminyltransferases], β1,3-N-acetylgalactosaminyltransferases (B3GALNTs), β1,3-galactosyltransferases [B3GALTs and core 1 β1,3-galactosyltransferases (C1GALTs)], β1,3-glucosyltransferase (B3GLCT) and β1,3-glucuronyl acid transferases (B3GLCATs or CHs). The mammalian enzymes were particularly well studied and shown to use a large variety of sugar donors and acceptor substrates leading to the formation of β1,3-linkages in various glycosylation pathways. In contrast, there are only a few studies related to other metazoan and lower vertebrates GT31 enzymes and the evolutionary relationships of these divergent sequences remain obscure. In this study, we used bioinformatics approaches to identify more than 920 of putative GT31 sequences in Metazoa, Fungi and Choanoflagellata revealing their deep ancestry. Sequence-based analysis shed light on conserved motifs and structural features that are signatures of all the GT31. We leverage pieces of evidence from gene structure, phylogenetic and sequence-based analyses to identify two major subgroups of GT31 named Fringe-related and B3GALT-related and demonstrate the existence of 10 orthologue groups in the Urmetazoa, the hypothetical last common ancestor of all animals. Finally, synteny and paralogy analysis unveiled the existence of 30 subfamilies in vertebrates, among which 5 are new and were named C1GALT2, C1GALT3, B3GALT8, B3GNT10 and B3GNT11. Altogether, these various approaches enabled us to propose the first comprehensive analysis of the metazoan GT31 disentangling their evolutionary relationships.

COSMC mutations reduce T-synthase activity in advanced Alzheimer's disease

INTRODUCTION

Mutations in brain tissues that cumulate with age may contribute to Alzheimer's disease (AD). Abnormal glycoprotein and Tn antigen expression have been demonstrated in AD. We identified C1GALT1C1/COSMC mutations in AD and age-matched normals without AD. The COSMC coding mutations resulted in a significant reduction in T-synthase activity in advanced AD cases.

METHODS

Identification of COSMC mutations, Real-Time Quantitative Reverse Transcription PCR (Q-RT-PCR), western blotting, and T-synthase activity assays.

RESULTS

COSMC mutations were detected in the promotor, coding region and 3'UTR in AD and normals. COSMC coding mutations demonstrated a correlation with AD progression. T-synthase levels were significantly elevated in advanced AD compared to AD III (P = 0.03) and normals (P = 0.002). T-synthase activity in advanced AD {Braak and Braak (B&B) stages V and VI} with COSMC coding mutations was 3-fold lower than advanced AD without mutations, and 1.3-fold lower than normal (P = 0.001) and AD B&B stage III (P = 0.01) with coding mutations.

DISCUSSION

COSMC coding mutations significantly diminished T-synthase activity in advanced AD, potentially causing defective galactosylation.

C1GALT1C1/COSMC is a novel prognostic biomarker for hepatocellular carcinoma

BACKGROUND/AIMS

The aim of this study is to explore the effects of COSMC on the prognosis of hepatocellular carcinoma (HCC), and establish a novel model with improved predictive capacity.

METHODS

Ninety-two patients diagnosed with HCC from 2006 to 2010 in our hospital were recruited to analyze the correlation between COSMC expression and prognosis. Cellular experiments were performed to verify the anti-tumor effects of COSMC. A predictive model was established based on the risk factors from multiple COX regression analysis. After validation, the novel model was compared with the conventional model in terms of capacity of predicting the prognosis.

RESULTS

The expression of COSMC was lower in tumor tissues than in normal tissues and inhibited HCC migration in cells. Besides the expression of COSMC was significantly negatively correlated with overall survival (OS) in HCC, regression analysis showed that COSMC expression, vascular invasion, and TNM stage were prognostic risk factors. Our novel model comprising these three elements was established and validated. Besides the good fit of the calibration curves, a higher concordance index (C-index) for OS (P=0.011) as well as better decision curve analysis (DCA) and survival curves for both disease-free survival (DFS) and OS suggested the superiority of this novel model compared with conventional TNM staging in predicting the prognosis of HCC patients.

CONCLUSIONS

We established a novel model by integrating the expression of COSMC, vascular invasion, and TNM stage, and found that it was better able to predict survival in patients with HCC compared with conventional TNM staging.

Truncated O-glycans promote epithelial-to-mesenchymal transition and stemness properties of pancreatic cancer cells

Aberrant expression of Sialyl-Tn (STn) antigen correlates with poor prognosis and reduced patient survival. We demonstrated that expression of Tn and STn in pancreatic ductal adenocarcinoma (PDAC) is due to hypermethylation of Core 1 synthase specific molecular chaperone (COSMC) and enhanced the malignant properties of PDAC cells with an unknown mechanism. To explore the mechanism, we have genetically deleted COSMC in PDAC cells to express truncated O-glycans (SimpleCells, SC) which enhanced cell migration and invasion. Since epithelial-to-mesenchymal transition (EMT) play a vital role in metastasis, we have analysed the induction of EMT in SC cells. Expressions of the mesenchymal markers were significantly high in SC cells as compared to WT cells. Equally, we found reduced expressions of the epithelial markers in SC cells. Re-expression of COSMC in SC cells reversed the induction of EMT. In addition to this, we also observed an increased cancer stem cell population in SC cells. Furthermore, orthotopic implantation of T3M4 SC cells into athymic nude mice resulted in significantly larger tumours and reduced animal survival. Altogether, these results suggest that aberrant expression of truncated O-glycans in PDAC cells enhances the tumour aggressiveness through the induction of EMT and stemness properties.

"Stuck on sugars - how carbohydrates regulate cell adhesion, recognition, and signaling"

We have explored the fundamental biological processes by which complex carbohydrates expressed on cellular glycoproteins and glycolipids and in secretions of cells promote cell adhesion and signaling. We have also explored processes by which animal pathogens, such as viruses, bacteria, and parasites adhere to glycans of animal cells and initiate disease. Glycans important in cell signaling and adhesion, such as key O-glycans, are essential for proper animal development and cellular differentiation, but they are also involved in many pathogenic processes, including inflammation, tumorigenesis and metastasis, and microbial and parasitic pathogenesis. The overall hypothesis guiding these studies is that glycoconjugates are recognized and bound by a growing class of proteins called glycan-binding proteins (GBPs or lectins) expressed by all types of cells. There is an incredible variety and diversity of GBPs in animal cells involved in binding N- and O-glycans, glycosphingolipids, and proteoglycan/glycosaminoglycans. We have specifically studied such molecular determinants recognized by selectins, galectins, and many other C-type lectins, involved in leukocyte recruitment to sites of inflammation in human tissues, lymphocyte trafficking, adhesion of human viruses to human cells, structure and immunogenicity of glycoproteins on the surfaces of human parasites. We have also explored the molecular basis of glycoconjugate biosynthesis by exploring the enzymes and molecular chaperones required for correct protein glycosylation. From these studies opportunities for translational biology have arisen, involving production of function-blocking antibodies, anti-glycan specific antibodies, and synthetic glycoconjugates, e.g. glycosulfopeptides, that specifically are recognized by GBPs. This invited short review is based in part on my presentation for the IGO Award 2019 given by the International Glycoconjugate Organization in Milan.

C1GALT1 predicts poor prognosis and is a potential therapeutic target in head and neck cancer

Core 1 β1,3-galactosyltransferase (C1GALT1) controls the crucial step of GalNAc-type O-glycosylation and is overexpressed in various human malignancies. However, its role in head and neck squamous cell carcinoma (HNSCC) remains unclear. Here we demonstrate that C1GALT1 expression is upregulated in HNSCC tumors and is associated with adverse clinicopathologic features. Moreover, high C1GALT1 expression predicts poor disease-free and overall survivals. C1GALT1 overexpression enhances HNSCC cell viability, migration, and invasion, which can be reversed by erlotinib. Silencing of C1GALT1 suppresses the malignant behavior both in vitro and in vivo. Mass spectrometry and lectin pull-down assays demonstrate that C1GALT1 modifies O-glycans on EGFR. Blocking O-glycan elongation on EGFR by C1GALT1 knockdown decreases EGF-EGFR binding affinity and inhibits EGFR signaling, thereby suppressing malignant phenotypes. Using molecular docking simulations, we identify itraconazole as a C1GALT1 inhibitor that directly binds C1GALT1 and promotes its proteasomal degradation, leading to significant blockade of C1GALT1-mediated effects in HNSCC cells in vitro and in vivo. Collectively, our findings demonstrate a critical role of O-glycosylation in HNSCC progression and highlight the therapeutic potential of targeting C1GALT1 in HNSCC treatment.

CD3/CD28 dynabeads induce expression of tn antigen in human t cells accompanied by hypermethylation of the cosmc promoter

Glycosylation is an important protein post-translational modification. In this process, the intermediate product, Tn antigen, arises from somatic mutations in core1β3-galactosyltransferase-specific molecular chaperone (Cosmc), which is required for the formation of active core1β3-galactosyltransferase (T-synthase). As a type of tumor-associated carbohydrate antigen, Tn antigen is mainly expressed in many human tumor cells and is absent in normal cells. Surprisingly, it is also expressed in normal activated T cells after in vitro stimulation, but the mechanism underlying its expression remains unclear. This study demonstrated that Tn antigen was expressed in activated T cells and that the percentage of positive (Tn+) cells increased and subsequently decreased within 72h after stimulation with CD3/CD28 Dynabeads, with peak expression occurring at 48h. During activation, interleukin-4 (IL-4) expression in the T-cell supernatant consistently increased with Tn+ cells, and was inversely correlated with serum interferon gamma (IFN-γ) levels. Compared with unactivated (without CD3/CD28 Dynabead stimulation) T cells, the level of T-synthase transcription in activated T cells did not significantly change, whereas T-synthase activity and Cosmc transcription significantly decreased, accompanied by a further increase in methylation of the Cosmc promoter. The results also showed that Cosmc transcription and translation decreased and then increased, and that Cosmc promoter methylation was a dynamic process during T cell activation. These data suggest that hypermethylation of the Cosmc promoter may induce the expression of Tn antigen in activated T cells.

Biochemical characterization of functional domains of the chaperone Cosmc

Cosmc is an endoplasmic reticulum chaperone necessary for normal protein O-GalNAc glycosylation through regulation of T-synthase, its single client. Loss-of-function of Cosmc results in expression of the Tn antigen, which is associated with multiple human diseases including cancer. Despite intense interest in dysregulated expression of the Tn antigen, little is known about the structure and function of Cosmc, including domain organization, secondary structure, oligomerization, and co-factors. Limited proteolysis experiments show that Cosmc contains a structured N-terminal domain (CosmcΔ256), and biochemical characterization of CosmcΔ256 reveals wild type chaperone activity. Interestingly, CosmcE152K, which shows loss of function in vivo, exhibits wild type-like activity in vitro. Cosmc and CosmcE152K heterogeneously oligomerize and form monomeric, dimeric, trimeric, and tetrameric species, while CosmcΔ256 is predominantly monomeric as characterized by chemical crosslinking and blue native page electrophoresis. Additionally, Cosmc selectively binds divalent cations in thermal shift assays and metal binding is abrogated by the CosmcΔ256 truncation, and perturbed by the E152K mutation. Therefore, the N-terminal domain of Cosmc mediates T-synthase binding and chaperone function, whereas the C-terminal domain is necessary for oligomerization and metal binding. Our results provide new structure-function insight to Cosmc, indicate that Cosmc behaves as a modular protein and suggests points of modulation or regulation of in vivo chaperone function.

Tumor-associated antigens: Tn antigen, sTn antigen, and T antigen

Glycosylation is one of the major posttranslational modifications of proteins. N-glycosylation (Asn-linked) and O-glycosylation (Ser/Thr-linked) are the two main forms. Abnormal O-glycosylation is frequently observed on the surface of tumor cells, and is associated with an adverse outcome and poor prognosis in patients with cancer. O-glycans (Tn, sTn, and T antigen) can be synthesized in the Golgi apparatus with the aid of several glycosyltransferases (such as T-synthase and ST6GalNAc-I) in a suitable environment. The unique molecular chaperone of T-synthase is Cosmc, which helps T-synthase to fold correctly in the endoplasmic reticulum. Dysregulation of these glycosyltransferases, molecular chaperones, or the environment is involved in the dysregulation of O-glycans. Tn, sTn, and T antigen neo- or over-expression occurs in many types of cancer including gastric, colon, breast, lung, esophageal, prostate, and endometrial cancer. This review discusses the major synthetic pathway of O-glycans and the mechanism by which Tn, sTn, and T antigens promote tumor metastasis.

[Association of Cosmc gene mutation with susceptibility to Henoch-Schönlein purpura in children]

OBJECTIVE

To investigate the presence of Cosmc gene mutation in children with Henoch-Schönlein purpura (HSP) and the association between Cosmc gene mutation and the susceptibility to HSP.

MESULTS

Eighty-four children who were diagnosed with HSP between March 2014 and December 2015 were selected as the HSP group. Fifty-eight healthy volunteers matched for age and sex were enrolled as the control group. Fasting venous blood (5 mL) from the two groups was collected in EDTA anticoagulated tubes, followed by the isolation of peripheral blood mononuclear cells (PBMCs) through density gradient centrifugation. Genomic DNA was extracted from PBMCs according to the manufacturer's protocol, and the whole exon region of Cosmc gene was amplified by touch-down polymerase chain reaction (touch-down PCR). The PCR products were identified by 1% agarose gel and sequenced in order to further examine the association between Cosmc gene mutation and the susceptibility to HSP.

RESULTS

Sequencing results showed two mutations (c.393T>A and c.72A>G) of Cosmc gene in children with HSP. There were no significant differences in the genotype and allele frequencies at the two loci between the HSP and control groups, and this distribution was not associated with sex.

CONCLUSIONS

The mutations c.393T>A and c.72A>G in the exon region of Cosmc gene in children with HSP are not associated with the onset of HSP.

[Association of Cosmc gene mutation with susceptibility to Henoch-Schönlein purpura in children]

OBJECTIVE

To investigate the presence of Cosmc gene mutation in children with Henoch-Schönlein purpura (HSP) and the association between Cosmc gene mutation and the susceptibility to HSP.

MESULTS

Eighty-four children who were diagnosed with HSP between March 2014 and December 2015 were selected as the HSP group. Fifty-eight healthy volunteers matched for age and sex were enrolled as the control group. Fasting venous blood (5 mL) from the two groups was collected in EDTA anticoagulated tubes, followed by the isolation of peripheral blood mononuclear cells (PBMCs) through density gradient centrifugation. Genomic DNA was extracted from PBMCs according to the manufacturer's protocol, and the whole exon region of Cosmc gene was amplified by touch-down polymerase chain reaction (touch-down PCR). The PCR products were identified by 1% agarose gel and sequenced in order to further examine the association between Cosmc gene mutation and the susceptibility to HSP.

RESULTS

Sequencing results showed two mutations (c.393T>A and c.72A>G) of Cosmc gene in children with HSP. There were no significant differences in the genotype and allele frequencies at the two loci between the HSP and control groups, and this distribution was not associated with sex.

CONCLUSIONS

The mutations c.393T>A and c.72A>G in the exon region of Cosmc gene in children with HSP are not associated with the onset of HSP.

Glycosyltransferases and non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is the most common form of chronic liver disease and its incidence is increasing worldwide. However, the underlying mechanisms leading to the development of NAFLD are still not fully understood. Glycosyltransferases (GTs) are a diverse class of enzymes involved in catalyzing the transfer of one or multiple sugar residues to a wide range of acceptor molecules. GTs mediate a wide range of functions from structure and storage to signaling, and play a key role in many fundamental biological processes. Therefore, it is anticipated that GTs have a role in the pathogenesis of NAFLD. In this article, we present an overview of the basic information on NAFLD, particularly GTs and glycosylation modification of certain molecules and their association with NAFLD pathogenesis. In addition, the effects and mechanisms of some GTs in the development of NAFLD are summarized.

Protein glycosylation in cancer

Neoplastic transformation results in a wide variety of cellular alterations that impact the growth, survival, and general behavior of affected tissue. Although genetic alterations underpin the development of neoplastic disease, epigenetic changes can exert an equally significant effect on neoplastic transformation. Among neoplasia-associated epigenetic alterations, changes in cellular glycosylation have recently received attention as a key component of neoplastic progression. Alterations in glycosylation appear to not only directly impact cell growth and survival but also facilitate tumor-induced immunomodulation and eventual metastasis. Many of these changes may support neoplastic progression, and unique alterations in tumor-associated glycosylation may also serve as a distinct feature of cancer cells and therefore provide novel diagnostic and even therapeutic targets.

Simple sugars to complex disease--mucin-type O-glycans in cancer

Mucin-type O-glycans are a class of glycans initiated with N-acetylgalactosamine (GalNAc) α-linked primarily to Ser/Thr residues within glycoproteins and often extended or branched by sugars or saccharides. Most secretory and membrane-bound proteins receive this modification, which is important in regulating many biological processes. Alterations in mucin-type O-glycans have been described across tumor types and include expression of relatively small-sized, truncated O-glycans and altered terminal structures, both of which are associated with patient prognosis. New discoveries in the identity and expression of tumor-associated O-glycans are providing new avenues for tumor detection and treatment. This chapter describes mucin-type O-glycan biosynthesis, altered mucin-type O-glycans in primary tumors, including mechanisms for structural changes and contributions to the tumor phenotype, and clinical approaches to detect and target altered O-glycans for cancer treatment and management.

Glycosyltransferases: mechanisms and applications in natural product development

Glycosylation reactions mainly catalyzed by glycosyltransferases (Gts) occur almost everywhere in the biosphere, and always play crucial roles in vital processes.