Dissection of hexosyl- and sialyltransferase domains in the bifunctional capsule polymerases from Neisseria meningitidis W and Y defines a new sialyltransferase family.

Crucial virulence determinants of disease causing Neisseria meningitidis species are their extracellular polysaccharide capsules. In the serogroups W and Y, these are heteropolymers of the repeating units (→6)-α-d-Gal-(1→4)-α-Neu5Ac-(2→)n in NmW and (→6)-α-d-Glc-(1→4)-α-Neu5Ac-(2→)n in NmY. The capsule polymerases, SiaDW and SiaDY, which synthesize these highly unusual polymers, are composed of two predicted GT-B fold domains separated by a large stretch of amino acids (aa 399-762). We recently showed that residues critical to the hexosyl- and sialyltransferase activity are found in the predicted N-terminal (aa 1-398) and C-terminal (aa 763-1037) GT-B fold domains, respectively. Here we use a mutational approach and synthetic fluorescent substrates to define the boundaries of the hexosyl- and sialyltransferase domains. Our results reveal that the active sialyltransferase domain extends well beyond the predicted C-terminal GT-B domain and defines a new glycosyltransferase family, GT97, in CAZy (Carbohydrate-Active enZYmes Database).