Contribution of the two dsRBM motifs to the double-stranded RNA binding and protein interactions of PACT

PACT is a stress-modulated activator of protein kinase PKR (protein kinase, RNA activated), which is involved in antiviral innate immune responses and stress-induced apoptosis. Stress-induced phosphorylation of PACT is essential for PACT's increased association with PKR leading to PKR activation, phosphorylation of translation initiation factor eIF2α, inhibition of protein synthesis, and apoptosis. PACT-induced PKR activation is negatively regulated by TRBP (transactivation response element RNA-binding protein), which dissociates from PACT after PACT phosphorylation in response to stress signals. The conserved double-stranded RNA binding motifs (dsRBMs) in PKR, PACT, and TRBP mediate protein-protein interactions, and the stress-dependent phosphorylation of PACT changes the relative strengths of PKR-PACT, PACT-TRBP, and PACT-PACT interactions to bring about a timely and transient PKR activation. This regulates the general kinetics as well as level of eIF2α phosphorylation, thereby influencing the cellular response to stress either as recovery and survival or elimination by apoptosis. In the present study, we evaluated the effect of specific mutations within PACT's two evolutionarily conserved dsRBMs on dsRNA-binding, and protein-protein interactions between PKR, PACT, and TRBP. Our data show that the two motifs contribute to varying extents in dsRNA binding, and protein interactions. These findings indicate that although the dsRBM motifs have high sequence conservation, their functional contribution in the context of the whole proteins needs to be determined by mutational analysis. Furthermore, using a PACT mutant that is deficient in PACT-PACT interaction but competent for PACT-PKR interaction, we demonstrate that PACT-PACT interaction is essential for efficient PKR activation.