X-ray crystal structure of rabbit N-acetylglucosaminyltransferase I:: catalytic mechanism and a new protein superfamily

N-acetylglucosaminyltransferase I (GnT I) serves as the gateway from oligomannose to hybrid and complex N-glycans and plays a critical role in mammalian development and possibly all metazoans, We have determined the X-ray crystal structure of the catalytic fragment of GnT I in the absence and presence of bound UDP-GlcNAc/Mn2+ at 1.5 and 1.8 Angstrom resolution, respectively. The structures identify residues critical for substrate binding and catalysis and provide evidence for similarity, at the mechanistic level, to the deglycosylation step of retaining beta-glycosidases. The structuring of a 13 residue loop, resulting from UDP-GlcNAc/Mn2+ binding, provides an explanation for the ordered sequential 'Bi Bi' kinetics shown by GnT I, Analysis reveals a domain shared with Bacillus subtilis glycosyltransferase SpsA, bovine beta-1,4-galactosyltransferase 1 and Escherichia coli N-acetylglucosamine-1-phosphate uridyltransferase. The low sequence identity, conserved fold and related functional features shown by this domain define a superfamily whose members probably share a common ancestor. Sequence analysis and protein threading show that the domain is represented in proteins from several glycosyltransferase families.