ABSTRACT
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 (â6)-α-d-Gal-(1â4)-α-Neu5Ac-(2â)n in NmW and (â6)-α-d-Glc-(1â4)-α-Neu5Ac-(2â)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).
Subject(s)
Bacterial Capsules/chemistry , Bacterial Proteins/chemistry , Hexosyltransferases/chemistry , Neisseria meningitidis/chemistry , Sialyltransferases/chemistry , Amino Acid Sequence , Bacterial Capsules/enzymology , Bacterial Proteins/classification , Bacterial Proteins/genetics , Bacterial Proteins/metabolism , Gene Expression , Hexosyltransferases/classification , Hexosyltransferases/genetics , Hexosyltransferases/metabolism , Molecular Sequence Data , Neisseria meningitidis/enzymology , Phylogeny , Polysaccharides, Bacterial/chemistry , Protein Folding , Protein Structure, Tertiary , Sequence Alignment , Sequence Homology, Amino Acid , Sialyltransferases/classification , Sialyltransferases/genetics , Sialyltransferases/metabolismABSTRACT
Neisseria meningitidis (Nm) is a leading cause of bacterial meningitis and sepsis. Crucial virulence determinants of pathogenic Nm strains are the polysaccharide capsules that support invasion by hindering complement attack. In NmW-135 and NmY the capsules are built from the repeating units (â 6)-α-D-Gal-(1 â 4)-α-Neu5Ac-(2 â)n and (â 6)-α-D-Glc-(1 â 4)-α-Neu5Ac-(2 â)n, respectively. These unusual heteropolymers represent unique examples of a conjugation between sialic acid and hexosyl-sugars in a polymer chain. Moreover, despite the various catalytic strategies needed for sialic acid and hexose transfer, single enzymes (SiaDW-135/Y) have been identified to form these heteropolymers. Here we used SiaDW-135 as a model system to delineate structure-function relationships. In size exclusion chromatography active SiaDW-135 migrated as a monomer. Fold recognition programs suggested two separate glycosyltransferase domains, both containing a GT-B-fold. Based on conserved motifs predicted folds could be classified as a hexosyl- and sialyltransferase. To analyze enzyme properties and interplay of the two identified glycosyltransferase domains, saturation transfer difference NMR and mutational studies were carried out. Simultaneous and independent binding of UDP-Gal and CMP-Sia was seen in the absence of an acceptor as well as when the catalytic cycle was allowed to proceed. Enzyme variants with only one functionality were generated by site-directed mutagenesis and shown to complement each other in trans when combined in an in vitro test system. Together the data strongly suggests that SiaDW-135 has evolved by fusion of two independent ancestral genes encoding sialyl- and galactosyltransferase activity.
