Comparison of nonphosphorylated and phosphorylated species of polyomavirus major capsid protein VP1 and identification of the major phosphorylation region.

The major virion protein of polyomavirus, VP1, consists of about six isoelectric species designated A through F. The minor species D, E, and F are phosphorylated and are thought to serve as viral receptors. We first wanted to distinguish whether all VP1 species are derived by post-translational modification from a common amino acid sequence or whether one or more of the species contain a region(s) of altered amino acid sequence resulting from alternate mRNA processing. We compared the VP1 species by detailed peptide mapping with several combinations of specific protease and radioisotopic labels. This approach enabled us to examine more than 80% of the predicted VP1 sequence, including the amino-and carboxy-termini. We found no evidence of sequence differences among any of the VP1 species. The specific incorporation of 32Pi was found to be the same for all of the phosphorylated species. Comparison of the phosphorylation sites of in vivo 32Pi-labeled D, E, and F by peptide mapping showed them to be identical. Each phosphorylated species contained a single major phosphopeptide and several minor phosphopeptides. The major phosphoamino acid, identified by acid hydrolysis, was phosphothreonine, with phosphoserine also present. By using chemical cleavage methods, we localized the major phosphorylation region to a central portion of the VP1 sequence. We discuss some features of this region and relate this information to functional implications of phosphorylation.