One of the two N-glycans on the human Gb3/CD77 synthase is essential for its activity and allosterically regulates its function

Abstract

N-glycosylation is a posttranslational modification that influences many protein properties, such as bioactivity, folding or solubility. The same principles apply to key enzymes in glycosylation pathways, including glycosyltransferases, that also undergoing N-glycosylation, changes in which may affect their activity. Human Gb3/CD77 synthase (encoded by A4GALT) is a Golgi-resident glycosyltransferase, which catalyzes the synthesis of Galα1→4Gal disaccharide on glycosphingolipid- and glycoprotein-derived acceptors, creating Gb3 or P1 antigens and P1 glycotopes (Galα1→4Galβ1→4GlcNAc-R), respectively. The molecules that contain Galα1→4Gal serve as receptors for pathogens and Shiga toxins, which are the major virulence factors of Shiga toxin-producing Escherichia coli (STEC). Human Gb3/CD77 synthase contains two N-glycosylation sites at positions N₁₂₁ and N₂₀₃. Using the recombinant soluble glycovariants of human Gb3/CD77 synthase with mutated N-glycosylation sequons expressed in HEK293E cells, we show that the glycovariants devoid of N-glycan at position N₂₀₃ or simultaneously at N₁₂₁ and N₂₀₃ sites reveal no enzymatic activity. In contrast, the N-glycan at position N₁₂₁ plays a negligible role, whereas the presence of both N-glycans is required for efficient secretion of the enzyme. Moreover, utilizing specific glycosidases, we have found that the fully N-glycosylated enzyme contains one complex and one hybrid/oligomannose N-glycan, while single mutants contain only the complex type. Finally, in silico analysis using the AlphaFold enzyme model showed that N-glycan attached to N₂₀₃ sequon is located in a protein motif near the active site and may allosterically influence the activity. All these findings highlight the prerequisite role of N-glycosylation in human Gb3/CD77 synthase activity (N₂₀₃ sequon) and solubility (both N₁₂₁ and N₂₀₃), with a particularly prominent role of N-glycan at position N₂₀₃ in the regulation of enzyme activity.

Keywords: Gb3/CD77 synthase; Glycosyltransferase; HEK293; N-glycan; P1PK blood group; Protein modeling.