3CD Cleavage of the Poliovirus P1 Precursor: A Model for Complex Proteinase/Substrate Interactions

Abstract

Eukaryotic viruses have evolved numerous strategies of replication that allow the propagation of their nucleic acid genomes in a potentially hostile host environment. Such strategies often include the limitation of genomic RNA or DNA sequences to only those essential, the production of multiple gene products from a single viral mRNA species, the use of host-cellular replication and translation machinery, and the selective inhibition of host-cell gene expression. Storage of viral genetic information in the form of ribonucleic acid is an additional replication strategy employed by a subgroup of eukaryotic viruses (RNA viruses). Most RNA viruses (excluding orthomyxoviruses and retroviruses) have eliminated the need for a nuclear phase in their viral life cycles, thus facilitating viral gene expression. An instructive example of a well-studied RNA virus which incorporates all of the above mentioned strategies to orchestrate a productive and efficient replication cycle is poliovirus, a member of the family Picornaviridae. Poliovirus is a small, positive-sense RNA virus that encodes at least 14 distinct viral gene products within a monocistronic genome of 7.5 kb (Rueckert, 1985; Lawson and Semler, 1990). This is achieved by translation of the viral mRNA into a single polyprotein that is subsequently cleaved into mature structural and nonstructural viral proteins by viral proteinases 2A and 3C. Proteinase 2A specifically recognizes a tyrosine-glycine (Y-G) amino acid pair and completes the initial cleavage on the polyprotein at the P1/P2 junction (Toyoda et al., 1986). The remainder of the important cleavages within the poliovirus P2 and P3 regions of the polyprotein is mediated by proteinase 3C, which exclusively recognizes glutamine-glycine (Q-G) amino acid pairs (Semler et al., 1981; Emini et al., 1982; Hanecak et al., 1982; Larsen et al., 1982). Cleavage of the P1 precursor requires polymerase sequences (3D) as well as 3C sequences in the form of proteinase 3CD (Jore et al. 1988; Ypma-Wong et al., 1988a). The poliovirus genomic RNA is also characterized by a large untranslated region (∼740 nt) located at the 5′ end (5′ UTR). The 5′ UTR facilitates the translation of viral mRNA by host-cell translation machinery via a novel cap-independent mechanism of translation initiation (Pelletier et al., 1988; Trono et al., 1988). The use of a cap-independent translation initiation mechanism allows for the selective inhibition of host-cell cap-dependent translation of eukaryotic mRNAs, an event mediated by proteinase 2A (Etchison et al., 1982; Sun and Baltimore, 1989).