Structural and dynamical features of protein P7 from bacteriophage phi12: Insights into a functional role in the cystoviral polymerase complex

Cystoviruses are a class of enveloped double-stranded RNA viruses that use a multi-protein polymerase complex (PX) to replicate and transcribe the viral genome. The cystoviral PX, that is amenable to in vitro and in vivo manipulation, comprises a unique model system for similar polymerase machinery. Containing three segmented double stranded RNA genome, the cystoviral PX is a simplified model for the polymerase machinery in more complex RNA viruses like reoviruses sharing structural and functional similarities at the level of the constituent proteins. Though the structures of the RNA dependent RNA polymerase (RdRp) and ATPase components of the cystoviral PX are known and their functional behaviors understood to a large extent, no atomic-resolution structural information is available for the major capsid protein P1 that defines the overall structure and symmetry of the viral capsid, and the essential protein P7. Towards obtaining a complete structural and functional understanding of the cystoviral PX, we have obtained the structure of P7 from the cystovirus o12 at a resolution of 1.8 A. The N-terminal core region (1-129) of P7 forms a novel homodimeric alphabeta-fold with structural similarities with BRCT domains implicated in multiple protein-protein interactions in DNA repair proteins. Our results combined with the known role of P7 in stabilizing the nucleation complex during capsid assembly hints towards its participation in key protein-protein interactions within the cystoviral PX. Additionally, we have found through solution NMR studies that the C-terminal tail of P7 (130-169) that is essential for virus viability, though highly disordered, contains a nascent helix. We demonstrate through NMR-based titrations, that P7 is capable of interacting with RNA. We find that both the N-terminal core and the dynamic C-terminus tail of P7 play a role in RNA recognition leading to a significant reduction of the degree of disorder in the C-terminal tail. Given the additional role of P7 in maintaining transcriptional fidelity, our data suggest a central biological role for P7/RNA interactions.