Site-directed mutagenesis of the avian retrovirus nucleocapsid protein, pp 12, at Serine 40, the primary site of phosphorylation in vivo.

The major nucleocapsid protein of avian retroviruses, pp 12, 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 pp 12, 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 p 12 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 p 12 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.