Viral precursor protein P3 and its processed products perform discrete and essential functions in the poliovirus RNA replication complex.

The differential use of protein precursors and their products is a key strategy used during poliovirus replication. To characterize the role of protein precursors during replication, we examined the complementation profiles of mutants that inhibited 3D polymerase or 3C-RNA binding activity. We showed that 3D entered the replication complex in the form of its precursor, P3 (or 3CD), and was cleaved to release active 3D polymerase. Furthermore, our results showed that P3 is the preferred precursor that binds to the 5'CL. Using reciprocal complementation assays, we showed that one molecule of P3 binds the 5'CL and that a second molecule of P3 provides 3D. In addition, we showed that a second molecule of P3 served as the VPg provider. These results support a model in which P3 binds to the 5'CL and recruits additional molecules of P3, which are cleaved to release either 3D or VPg to initiate RNA replication.