POLIOVIRUS RNA-SYNTHESIS UTILIZES AN RNP COMPLEX FORMED AROUND THE 5'-END OF VIRAL-RNA

The structure of a ribonucleoprotein complex formed at the 5'-end of p oliovirus RNA was investigated. This complex involves the first 90 nuc leotides of poliovirus genome which fold into a cloverleaf-like struct ure and interact with both uncleaved 3CD, the viral protease-polymeras e precursor, and a 36 kDa ribosome-associated cellular protein. The ce llular protein is required for complex formation and interacts with un paired bases in one stem-loop of the cloverleaf RNA. Amino acids withi n the 3C protease which are important for RNA binding were identified by site-directed mutagenesis and the crystal structure of a related pr otease was used to model the RNA binding domain within the viral 3CD p rotein. The physiologic importance of the ribonucleo-protein complex i s suggested by the finding that mutations that disrupt complex formati on abolish RNA replication but do not affect RNA translation or stabil ity. Based on these structural and functional findings we propose a mo del for the initiation of poliovirus RNA synthesis where an initiation complex consisting of 3CD, a cellular protein, and the 5'-end of the positive strand RNA catalyzes in trans the initiation of synthesis of new positive stranded RNA.