Heterogeneous Nuclear Ribonucleoprotein I (hnRNP-I/PTB) Selectively Binds the Conserved 3′ Terminus of Hepatitis C Viral RNA

Abstract

Hepatitis C virus (HCV) is a positive-strand RNA virus whose genome is replicated by a direct RNA-to-RNA mechanism. Initiation of negative-strand RNA synthesis is believed to proceed from the 3′ end of the genomic RNA. The high conservation of the 3′ terminus suggests that this region directs the assembly of proteins required for the initiation of RNA replication. We sought to determine whether host proteins bind specifically to this RNA structure. We observed specific binding of cellular proteins to labeled 3′-terminal RNA by mobility shift analysis. UV crosslinking revealed that the predominant 3′-terminal RNA-binding protein migrates as a single, 60-kDa species that can be precipitated by monoclonal antibodies directed against heterogeneous nuclear ribonucleoprotein I, also called polypyrimidine tract-binding protein (hnRNP-I/PTB), a protein previously shown to bind to the 5′ internal ribosome entry site (IRES) of the HCV genome. Purified hnRNP-I/PTB also bound selectively to the 3′ end of the HCV genome. hnRNP-I/PTB binding requires the upstream two stem-loop structures (SL2 and SL3) but not the most 3′-terminal stem-loop (SL1). Minor alteration of either the stem or loop sequences in SL2 or SL3 severely compromised hnRNP-I/PTB binding, suggesting extremely tight RNA structural requirements for interaction with this protein. hnRNP-I/PTB does not bind to either end of the antigenomic RNA strand and binds to the 5′ IRES element of the genome at least 10-fold less avidly than to the 3′ terminus. The strong, selective, and preferential binding of hnRNP-I/PTB to the 3′ end of the HCV genome suggests that it may be recruited to participate in viral replication, helping to direct initiation of negative-strand RNA synthesis, stabilize the viral genome, and/or regulate encapsidation of genomic RNA.