The P58 cellular inhibitor complexes with the interferon-induced, double-stranded RNA-dependent protein kinase, PKR, to regulate its autophosphorylation and activity.

The 58-kDa protein, referred to as P58, is a cellular inhibitor of the interferon-induced, double-stranded RNA-activated protein kinase, PKR. The P58 protein inhibits both the autophosphorylation of PKR and the phosphorylation of the PKR natural substrate, the alpha subunit of eukaryotic initiation factor eIF-2. Sequence analysis revealed that P58 is a member of the tetratricopeptide family of proteins. Utilizing experimental approaches, which included coprecipitation or coselection of native and recombinant wild-type and mutant proteins, we found that P58 can efficiently complex with the PKR protein kinase. Attempts to map the P58 interactive sites revealed a correlation between the ability of P58 to inhibit PKR in vitro and bind to PKR. The DnaJ sequences, present at the carboxyl terminus of P58, were dispensable for binding in vitro, while sequences containing the eIF-2 alpha similarity region were essential for efficient complex formation. Furthermore, not all tetratricopeptide motifs were necessary for PKR-P58 interactions. Initial experiments to map the binding domains present in PKR showed that P58 complexed with PKR molecules that lacked the first RNA binding domain but did not bind to a PKR mutant containing only the amino terminus. These data, taken together, demonstrate that P58 inhibits PKR through a direct interaction, which is likely independent of the binding of double-stranded RNA to the protein kinase.