Multimerization of poly(rC) binding protein 2 is required for translation initiation mediated by a viral IRES

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Abstract
The cellular protein, poly(rC) binding protein 2 (PCBP2), is known to function in picornavirus cap-independent translation. We have further examined the RNA binding properties and protein–protein interactions of PCBP2 necessary for translation. We have studied its putative multimerization properties utilizing the yeast two-hybrid assay and in vitro biochemical methods, including glutathione S-transferase (GST) pull-down assays and gel filtration. Through genetic analysis, the multimerization domain has been localized to the second K-homologous (KH) RNA binding domain of the protein between amino acids 125 and 158. To examine the function of multimerization in poliovirus translation, we utilized the truncated protein, ΔKH1-PCBP2, which is capable of multimer formation, but does not bind poliovirus stem–loop IV RNA (an interaction required for translation). Utilizing RNA binding and in vitro translation assays, this protein was shown to act as a dominant negative, suggesting that PCBP2 multimerization functions in poliovirus translation and RNA binding. Additionally, PCBP2 containing a deletion in the multimerization domain (ΔKH2-PCBP2) was not able to bind poliovirus stem–loop IV RNA and could not rescue translation in extracts that were depleted of endogenous PCBP2. Results from these experiments suggest that the multimerization of PCBP2 is required for efficient RNA binding and cap-independent translation of poliovirus RNA. By examining the functional interactions of the cellular protein PCBP2, we have discovered a novel determinant in the mechanism of picornavirus cap-independent translation.