FIG. 6.

Model of molecular switching from translation to replication of poliovirus RNA. Proteins identified as interacting with the 5"NTR of poliovirus are depicted by ovals. Different proteins are shaded differently. For simplicity of the model, only one PTB binding is drawn, even though several PTB-binding sites are present in polioviral IRES. The events required for translation at the early stage of viral infection are depicted with thin lines. The effect of the interaction between PCBP and PTB on translation is hypothetical, even though the protein-protein interaction between these proteins was demonstrated experimentally. Changes after the production of 3CD^pro are depicted by thick lines. PTB is cleaved by 3CD^pro as indicated by the folded line on PTB. The C-terminal PTB may stay on the IRES element blocking translation from this mRNA. At the same time, the removal of the N-terminal end of PTB prevents its interaction with other proteins, such as PCBP, hnRNP K, hnRNP L, and PTB itself (48). The PCBP, previously bound to the IRES element, may be transferred to the CL element in the presence of 3CD^pro. With these changes, replication will commence.