FIGURE 1.

Schematic of ST8Sia IV/PST and PST mutants. Shown are full-length ST8SiaIV/PST and the truncated and mutant proteins used in this study. All truncated PST mutants lack the sialyl motifs (SML, SMS, motif III, SMVS) found in full-length PST, and hence are catalytically inactive. Soluble PST mutants begin at residue 21 and do not contain the cytosolic tail or transmembrane region (TM) (sPST and sPST140). Mutation of His³³¹ in PST to lysine (PST H331K) was previously shown to render a full-length PST protein catalytically inactive (57). The polybasic region (PBR) is found between residues 71 and 105 in PST. Mutation of Arg⁸² and Arg⁹³ in the PBR was previously shown to greatly reduce NCAM polysialylation (61). The PSTD is a stretch of 32 amino acids (residues 246–277 in PST) that is contiguous with the SMS and is conserved in the two polySTs (60). Previous work demonstrated that selected residues in the PSTD are required for NCAM polysialylation (60, 61).