Site-directed mutagenesis of the avian retrovirus nucleocapsid protein, pp12, at serine 40, the primary site of phosphorylation in vivo

The major nucleocapsid protein of avian retroviruses, pp12, binds to single-stranded viral RNA with high affinity. Phosphorylation at Ser-40 is necessary for this binding. In order to examine the role of phosphorylation of serine 40 in the biological function of pp12, we have introduced a series of amino acid substitutions at this position in the Rous sarcoma virus (Pr-C) protein. Substitution of threonine, alanine, or three other amino acids for Ser-40 had very little or no detectable effect on viral replication, nor did the control substitution of glycine for Ser-43, a nonphosphorylated residue. In vivo and in vitro, the Ala-40 and probably the Thr-40 substituted p12 proteins are phosphorylated at alternative sites which are phosphorylated to a minor extent in vivo in the wild type protein. A study of the RNA binding properties of Ala-40 substituted p12 has indicated that the protein has been stabilized in a high affinity RNA binding state which is independent of phosphorylation. The viability of the Ala-40 mutant virus indicates that this high binding affinity may be required for biological activity.