During
picornavirus infection, several cellular
proteins are cleaved by virus-encoded
proteinases. Such cleavage events are likely to be involved in the changing dynamics during the intracellular viral life cycle, from viral translation to host shutoff to RNA replication to virion assembly. For example, it has been proposed that there is an active switch from poliovirus translation to RNA replication mediated by changes in
RNA-binding protein affinities. This switch could be a mechanism for controlling template selection for translation and negative-strand
viral RNA synthesis, two processes that use the same positive-strand
RNA as a template but proceed in opposing directions. The cellular
protein poly(rC)-
binding protein (PCBP) was identified as a primary candidate for regulating such a mechanism. Among the four different
isoforms of PCBP in mammalian cells, PCBP2 is required for translation initiation on picornavirus genomes with type I
internal ribosome entry site elements and also for RNA replication. Through its three K-homologous (KH) domains, PCPB2 forms functional
protein-
protein and
RNA-
protein complexes with components of the viral translation and replication machinery. We have found that the
isoforms PCBP1 and -2 are cleaved during the mid-to-late phase of poliovirus
infection. On the basis of in vitro cleavage assays, we determined that this cleavage event was mediated by the viral
proteinases 3C/3CD. The primary cleavage occurs in the linker between the KH2 and KH3 domains, resulting in truncated PCBP2 lacking the KH3 domain. This cleaved
protein, termed PCBP2-DeltaKH3, is unable to function in translation but maintains its activity in
viral RNA replication. We propose that through the loss of the KH3 domain, and therefore loss of its ability to function in translation, PCBP2 can mediate the switch from viral translation to RNA replication.