P52rIPK regulates the molecular cochaperone P58IPK to mediate control of the RNA-dependent protein kinase in response to cytoplasmic stress.

The 52 kDa protein referred to as P52 r I P K was first identified as a regulator of P58 I P K , a cellular inhibitor of the RNA-dependent protein kinase (PKR). P52 r I P K and P58 I P K each possess structural domains implicated in stress signaling, including the charged domain of P52 r I P K and the tetratricopeptide repeat (TPR) and DnaJ domains of P58 I P K . The P52 r I P K charged domain exhibits homology to the charged domains of Hsp90, including the Hsp90 geldanamycin-binding domain. Here we present an in-depth analysis of P52 r I P K function and expression, which first revealed that the 114 amino acid charged domain was necessary and sufficient for interaction with P58 I P K . This domain bound specifically to P58 I P K TPR domain 7, the domain adjacent to the TPR motif required for P58 I P K interaction with PKR, thus providing a mechanism for P52 r I P K inhibition of P58 I P K function. Both the charged domain of P52 r I P K and the TPR 7 domain of P58 I P K were required for P52 r I P K to mediate downstream control of PKR activity, eIF2α phosphorylation, and cell growth. Furthermore, we found that P52 r I P K and P58 I P K formed a stable intracellular complex during the acute response to cytoplasmic stress induced by a variety of stimuli. We propose a model in which the P52 r I P K charged domain functions as a TPR-specific signaling motif to directly regulate P58I I P K within a larger cytoplasmic stress signaling cascade culminating in the control of PKR activity and cellular mRNA translation.