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(rIPK) was first identified as a regulator of P58(IPK), a cellular inhibitor of the RNA-dependent protein kinase (PKR). P52(rIPK) and P58(IPK) each possess structural domains implicated in stress signaling, including the charged domain of P52(rIPK) and the tetratricopeptide repeat (TPR) and DnaJ domains of P58(IPK). The P52(rIPK) 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(rIPK) function and expression, which first revealed that the 114 amino acid charged domain was necessary and sufficient for interaction with P58(IPK). This domain bound specifically to P58(IPK) TPR domain 7, the domain adjacent to the TPR motif required for P58(IPK) interaction with PKR, thus providing a mechanism for P52(rIPK) inhibition of P58(IPK) function. Both the charged domain of P52(rIPK) and the TPR 7 domain of P58(IPK) were required for P52(rIPK) to mediate downstream control of PKR activity, eIF2alpha phosphorylation, and cell growth. Furthermore, we found that P52(rIPK) and P58(IPK) 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(rIPK) charged domain functions as a TPR-specific signaling motif to directly regulate P58(IPK) within a larger cytoplasmic stress signaling cascade culminating in the control of PKR activity and cellular mRNA translation.