A cell cycle-dependent protein serves as a template-specific translation initiation factor

Cap-independent translation initiation on picornavirus mRNAs is mediated by an internal ribosomal entry site (IRES) in the 5' untranslated region (5' UTR) and requires both eukaryotic initiation factors (eIFs) and IRES-specif ic cellular trans-acting factors (ITAFs). We show here that the requirement s for trans-acting factors differ between related picornavirus IRESs and ca n account for cell type-specific differences in IRES function. The neurovir ulence of Theiler's murine encephalomyelitis virus (TMEV; GDVII strain) was completely attenuated by substituting its IRES by that of foot-and-mouth d isease virus (FMDV). Reconstitution of initiation using fully fractionated translation components indicated that 48S complex formation on both IRESs r equires eIF2, eIF3, eIF4A, eIF4B, eIF4F, and the pyrimidine tract-binding p rotein (PTB) but that the FMDV IRES additionally requires ITAF(45), also kn own as murine proliferation-associated protein (Mpp1), a proliferation-depe ndent protein that is not expressed in murine brain cells. ITAF(45) did not influence assembly of 48S complexes on the TMEV IRES. Specific binding sit es for ITAF(45), PTB, and a complex of the eIF4G and eIF4A subunits of eIF4 F were mapped onto the FMDV IRES, and the cooperative function of PTB and I TAF(45) in promoting stable binding of eIF4G/4A to the IRES was characteriz ed by chemical and enzymatic footprinting. Our data indicate that PTB and I TAF(45) act as RNA chaperones that control the functional state of a partic ular IRES and that their cell-specific distribution may constitute a basis for cell-specific translational control of certain mRNAs.