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Glycobiology, 2001, Vol. 11, No. 11 997-1008
© 2001 Oxford University Press

The polysialyltransferase ST8Sia II/STX: posttranslational processing and role of autopolysialylation in the polysialylation of neural cell adhesion molecule

Brett E. Close, Jennifer M. Wilkinson, Tracy J. Bohrer, Clement P. Goodwin, Lucy J. Broom and Karen J. Colley1

Department of Biochemistry and Molecular Biology, University of Illinois College of Medicine, 1819 West Polk Street M/C 536, Chicago, IL 60612, USA

The presence of {alpha}2,8-linked polysialic acid on the neural cell adhesion molecule (NCAM) is known to modulate cell interactions during development and oncogenesis. Two enzymes, the {alpha}2,8-polysialyltransferases ST8Sia IV/PST and ST8Sia II/STX are responsible for the polysialylation of NCAM. We previously reported that both ST8Sia IV/PST and ST8Sia II/STX enzymes are themselves modified by {alpha}2,8-linked polysialic acid chains, a process called autopolysialylation. In the case of ST8Sia IV/PST, autopolysialylation is not required for enzymatic activity. However, whether the autopolysialylation of ST8Sia II/STX is required for its ability to polysialylate NCAM is unknown. To understand how autopolysialylation impacts ST8Sia II/STX enzymatic activity, we employed a mutagenesis approach. We found that ST8Sia II/STX is modified by six Asn-linked oligosaccharides and that polysialic acid is distributed among the oligosaccharides modifying Asn 89, 219, and 234. Coexpression of a nonautopolysialylated ST8Sia II/STX mutant with NCAM demonstrated that autopolysialylation is not required for ST8Sia II/STX polysialyltransferase activity. In addition, catalytically active, nonautopolysialylated ST8Sia II/STX does not polysialylate any endogenous COS-1 cell proteins, highlighting the protein specificity of polysialylation. Furthermore, immunoblot analysis of NCAM polysialylation by autopolysialylated and nonautopolysialylated ST8Sia II/STX suggests that the NCAM is polysialylated to a higher degree by autopolysialylated ST8Sia II/STX. Therefore, we conclude that autopolysialylation of ST8Sia II/STX, like that of ST8Sia IV/PST, is not required for, but does enhance, NCAM polysialylation.

1 To whom correspondence should be addressed


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