Mechanistic insights into Schizosaccharomyces pombe GT-A family protein Pvg3 in the biosynthesis of pyruvylated β1,3-galactose of N-linked oligosaccharides.

N-linked oligosaccharides in the fission yeast Schizosaccharomyces pombe contain large amounts of d-galactose (Gal), which mainly comprises α1,2- and α1,3-linked Gal except for pyruvylated β1,3-linked Gal (PvGalβ) at the non-reducing end. The PvGalβ unit of N-glycans is important for regulating nonsexual flocculation and invasive growth, but the mechanistic basis for β-galactosylation in fission yeast is poorly understood. To gain insight into this mechanism, we have characterized three genes previously identified to be involved in PvGalβ biosynthesis (pvg2, pvg3, and pvg5), with a focus on pvg3, which is predicted to contain a domain conserved in galactosyltransferase family 31 (GT31) proteins. Fluorescent microscopy revealed that Pvg3 is stably localized at the Golgi membrane, regardless of the presence of pvg2 or pvg5, suggesting that Pvg2 and Pvg5 are essential for the function of Pvg3 as a β1,3-galactosyltransferase, and not for its localization to the Golgi. Mutation of the GT31 family DXD motif and GT-A fold in Pvg3 resulted in loss of catalytic activity in vivo, supporting the idea that Pvg3 is a GT-A type β1,3-galactosyltransferase. Docking simulations further indicated that Pvg3 can recognize donor and acceptor substrates suitable for β-(1→3) bond formation. Yeast two-hybrid assay showed that Pvg5 physically interacts with Pvg3 and the pyruvyltransferase Pvg1. Collectively, these results provide insight into β-galactosylation catalyzed by Pvg3 and the supporting role of Pvg5 in PvGalβ biosynthesis.