Phosphorylation of simian virus 40 proteins in a cell-free system

We have shown previously that all the structural proteins of simian virus 40 (SV40) are phosphoproteins. Virus phosphorylated in vivo could be further phosphorylated with exogenous cellular protein kinases in a cell-free system containing gamma-(32)P-ATP as phosphate donor. In intact infectious virus only polypeptides 1 and 2 (mol wt 49,000 and 40,800, respectively) were further phosphorylated in vitro. However, when infectious SV40 was partially disrupted, treated with nucleases, and then phosphorylated in vitro, all five structural polypeptides accepted additional phosphate groups. Similarly, all polypeptides of intact empty capsids, derived from infected cells, were further phosphorylated in vitro. Phosphorylation of empty capsids and infectious SV40 in vitro was enhanced from 4- to 11-fold after prior treatment of virus with alkali. The phosphate group was linked only to serine residues of the viral polypeptides phosphorylated both in vitro and in vivo.