Identification of linkage-specific sequence motifs in sialyltransferases.

Eukaryotic sialyltransferases (SiaTs) comprise a superfamily of enzymes catalyzing the transfer of sialic acid (Sia) from a common donor substrate to various acceptor substrates in different linkages. These enzymes have been classified as ST3Gal, ST6Gal, ST6GalNAc, and ST8Sia families based on linkage- and acceptor monosaccharide-specificities and sequence similarities. It was recognized early on that SiaTs contain certain well-conserved motifs, and these were denoted as L (large)-, S (small)-, and VS (very small)-motifs; recently, a fourth motif, denoted as motif III, was identified. These four motifs are common to all the SiaTs, irrespective of the linkage- and acceptor saccharide-specificities. In this study, the sequences of the various families have been analyzed, and sequence motifs that are unique to the various families have been identified. These unique motifs are expected to contribute to the characteristic linkage- and acceptor saccharide-specificities of the family members. One of the linkage specific motifs is contiguous to L-motif. Members of ST3Gal and ST8Sia families share significant sequence similarities; in contrast, the ST6Gal family is distinct from the ST6GalNAc family. The latter consists of two subfamilies, one comprising ST6GalNAc I and ST6GalNAc II, and the other comprising ST6GalNAc III, ST6GalNAc IV, ST6GalNAc V, and ST6GalNAc VI. Each of these subfamilies has characteristic sequence motifs not present in the other subfamily.