Large T antigen (T Ag), the major regulatory protein produced by the primate polyomaviruses, is a multifunctional phosphoprotein expressed early in the viral life cycle. T Ag performs many functions essential to viral DNA replication, and studies with SV40 T Ag indicate that the regulation of these functions is modulated, in part, by the phosphorylation status of this oncoprotein. In this study, we demonstrate that JC virus (JCV) T Ag obtained from lytically infected and transformed cells is phosphorylated at serine and threonine residues. Analysis of JCV T Ag via two-dimensional tryptic peptide mapping generates 14 phosphopeptides. Additional mapping studies of intact, hybrid, mutant, and truncated forms of JCV T Ag have aided the localization of phosphorylation sites to the N- or C-terminal region of the protein; both serine and threonine residues are modified at each terminus. The data indicate that, unlike the corresponding regulatory phosphorylation site Ser677 in SV40, Thr664 is not phosphorylated in JCV T Ag. The phosphorylation sites utilized for JCV T Ag, and the regulatory role of these sites, are predicted to contribute to the unique biology of this human virus.
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