Effects of phosphorylation of avian retrovirus nucleocapsid protein pp12 on binding of viral RNA

The major nucleocapsid protein of avian retroviruses, pp12, preferentially binds to the single-stranded regions of 60 S viral RNA with an apparent binding constant (K(app)) of 1.2 x 10¹¹ M^-1. If the phosphate associated with serine residues of pp12 is hydrolyzed by either alkali treatment or with partially purified phosphoprotein phosphatase activities isolated from virions, the K(app) for binding to 60 S RNA decreases 100-fold. The high affinity binding of pp12 to viral RNA can be restored by phosphorylation of the protein with a protein kinase, protease-activated kinase I. The same serine residues phosphorylated in vivo are phosphorylated by protease kinase I in vitro. These residues have been identified as serine residues 40 and either 76 or 77. The protein purified from virions is phosphorylated primarily at serine residue 40 (>90%). This suggests that phosphoserine residue 40 is responsible for modulating the binding of the protein to RNA. Thus, the phosphorylation state of pp12 can be reversibly altered in vitro, resulting in the interconversion of the protein between a state of high and low affinity for single-stranded viral RNA.