Control of mucin-type O-glycosylation: A classification of the polypeptide GalNAc-transferase gene family

Abstract

Glycosylation of proteins is an essential process in all eukaryotes and a great diversity in types of protein glycosylation exists in animals, plants and microorganisms. Mucin-type O-glycosylation, consisting of glycans attached via O-linked N-acetylgalactosamine (GalNAc) to serine and threonine residues, is one of the most abundant forms of protein glycosylation in animals. Although most protein glycosylation is controlled by one or two genes encoding the enzymes responsible for the initiation of glycosylation, i.e. the step where the first glycan is attached to the relevant amino acid residue in the protein, mucin-type O-glycosylation is controlled by a large family of up to 20 homologous genes encoding UDP-GalNAc:polypeptide GalNAc-transferases (GalNAc-Ts) (EC 2.4.1.41). Therefore, mucin-type O-glycosylation has the greatest potential for differential regulation in cells and tissues. The GalNAc-T family is the largest glycosyltransferase enzyme family covering a single known glycosidic linkage and it is highly conserved throughout animal evolution, although absent in bacteria, yeast and plants. Emerging studies have shown that the large number of genes (GALNTs) in the GalNAc-T family do not provide full functional redundancy and single GalNAc-T genes have been shown to be important in both animals and human. Here, we present an overview of the GalNAc-T gene family in animals and propose a classification of the genes into subfamilies, which appear to be conserved in evolution structurally as well as functionally.

O-Glycosylation regulates polarized secretion by modulating Tango1 stability

“…Here we demonstrate that the expression of PGANT4 specifically in the secretory PR cells of the digestive tract confers increased stability to Tango1 by protecting it from Dfur2-mediated cleavage, thereby allowing the formation of large mucin-containing secretory vesicles within these cells. As the enzymes controlling the initiation of O-glycosylation are typically abundantly expressed in cells under high secretory burden (8,24), it raises the possibility that O-glycosylation may modulate the stability of Tango1 in other tissues, ensuring that Tango1 activity is commensurate with the secretory demands of the cell.…”

“…These family members are expressed in unique spatial and temporal patterns during development and in adult tissues (8,9). Additionally, these enzymes display unique substrate specificities, with some members preferring to add the initial GalNAc (peptide transferases) and others preferring to add GalNAc to previously glycosylated substrates (glycopeptide transferases).…”

“…The concerted activity of these many members is thought to result in the elaborate glycosylation patterns typically seen in mucin-like molecules. As members of this family are abundantly expressed in many secretory cells and tissues (8,24), it raises the possibility that the yin/yang provided by the opposing forces of O-glycosylation and proteolytic cleavage may serve as a more widespread, dynamic system to regulate the stability and bioactivity of many proteins. In support of this theory, recent glycoproteomic studies performed in mammalian cell culture have mapped sites of O-glycosylation to be in close proximity to potential furin cleavage sites on many proteins (26).…”

“…O-glycosylation is an essential, evolutionarily conserved protein modification (8,9) that has direct medical relevance, as aberrations are responsible for the human diseases familial tumoral calcinosis (10,11) and Tn syndrome (12). Loss of this protein modification affected constitutive secretion and Golgi apparatus structure in Drosophila cell culture (7,13) and secretion in the developing respiratory system in vivo (14).…”

A Contra Capture Protein Array Platform for Studying Post-translationally Modified (PTM) Auto-antigenomes

“…7A, green boxes)). On these arrays, we have spotted genes encoding specific substrates for the three different GALNTs based on studies on synthetic peptides, an NRP2 fragment for GalNac-T2, a POMC fragment for GalNac-T11, an APOE and a CD55 fragment for GalNac-T3 (44,67,68). Besides wild type APOE, we have also spotted a gene encoding an APOE mutant with the glycosylation sites Thr and Ser by GALNT3 mutated to Ala that cannot be glycosylated by GalNac-T3.…”