Molecular and genetic evidence for a virus-encoded glycosyltransferase involved in protein glycosylation

The major capsid protein, Vp54, of chlorella virus PBCV-1 is a glycoprotein that contains either one glycan of similar to 30 sugar residues or two sim ilar glycans of similar to 15 residues. Previous analysis of PBCV-1 antigen ic mutants that contained altered Vp54 glycans led to the conclusion that u nlike other glycoprotein-containing viruses, most, if not all, of the enzym es involved in the synthesis of the Vp54 glycan are probably encoded by PBC V-1 (I.-N. Wang et al., 1993, Proc. Natl. Acad Sci. USA 90, 3840-3844). In this report we used molecular and genetic approaches to begin to identify t hese virus genes. Comparing the deduced amino acid sequences of the putativ e 375 PBCV-1 protein-encoding genes to databases identified seven potential glycosyltransferases. One gene, designated a64r, encodes a 638-amino-acid protein that has four motifs conserved in "Fringe type" glycosyltransferase s. Analysis of 13 PBCV-1 antigenic mutants revealed mutations in a64r that correlated with a specific antigenic variation. Dual-infection experiments with different antigenic mutants indicated that viruses that contained wild -type a64r could complement and recombine with viruses that contained mutan t a64r to form wild-type virus. Therefore, we conclude that a64r encodes a glycosyltransferase involved in synthesizing the Vp54 glycan. This is the f irst report of a virus-encoded glycosyltransferase involved in protein glyc osylation. (C) 2001 Academic Press.