Phosphorylation of ribosomal proteins by the vaccinia virus B1R protein kinase

Two proteins of the 40S ribosomal subunit were shown to be phosphorylated in vitro by a vaccinia virus-encoded serine/threonine protein kinase. These were identified by two-dimensional gel electrophoresis as ribosomal proteins Sa and S2 and were shown by phosphoamino acid analysis to both be phosphorylated on serine and threonine residues. The three phosphorylated forms of S2 produced by the B1R protein kinase in vitro matched the phosphorylated forms of S2 previously observed in cells infected with vaccinia virus. These data strongly suggest that this enzyme is responsible for the phosphorylation of S2 and Sa which occurs early during vaccinia virus infection.