Sialyltransferases of the mammalian ST8Sia family catalyze oligo- and polysialylation of surface-localized glycoproteins and glycolipids through transfer of sialic acids from CMP-sialic acid to the nonreducing ends of sialic acid acceptors. The crystal structure of human ST8SiaIII at 1.85-Å resolution presented here is, to our knowledge, the first solved structure of a polysialyltransferase from any species, and it reveals a cluster of polysialyltransferase-specific structural motifs that collectively provide an extended electropositive surface groove for binding of oligo-polysialic acid chain products. The ternary complex of ST8SiaIII with a donor sugar analog and a sulfated glycan acceptor identified with a sialyltransferase glycan array provides insight into the residues involved in substrate binding, specificity and sialyl transfer.
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