Key result
Three noncontiguous p57-binding sites were located within the poliovirus 5' nontranslated region, and binding was not solely dependent on a polyuridine tract.
The study identifies specific binding sites for the p57 polypeptide (PTB) within the poliovirus 5' nontranslated region, contributing to the understanding of viral translation initiation.
PTB-p57 equivalence in viral extracts leaves open species-specific roles; prospective functional studies required.
Initiation of translation of poliovirus RNA by ribosomal entry into an internal segment of the 742-nucleotide (nt)-long 5' nontranslated region involves trans-acting factors, including p57, a 57-kDa polypeptide which has been identified as the pyrimidine tract-binding protein (PTB). A UV cross-linking assay was used to compare the RNA-binding properties of the p57 present in various mammalian cytoplasmic extracts with those of purified murine p57 and recombinant human PTB. Three noncontiguous p57-binding sites were located within the poliovirus 5' nontranslated region, between nt 70 and 288, and 443 and 539 (domain V), and 630 and 730. With the same assay, a novel 34-kDa polypeptide was identified that bound nt 1 to 629 specifically. A single A-->G substitution of nt 480 which attenuates poliovirus did not alter UV cross-linking of p57 to domain V. Although UV cross-linking of p57 to the internal ribosome entry site was specifically reduced by competition with poly(U) but not by competition with poly(C), poly(G), and poly(A) homoribopolymers, the presence of a polyuridine tract was not a sufficient determinant for binding of RNA to the p57 present in cytoplasmic extracts, nor was the polypyrimidine tract downstream of domain V necessary for binding to this site.
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Hellen et al. (1994) studied this question. p57 (pyrimidine tract-binding protein) was evaluated on RNA-binding properties and binding site locations. Three noncontiguous p57-binding sites were located within the poliovirus 5' nontranslated region, and binding was not solely dependent on a polyuridine tract.
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