The mechanism of translation initiation on T ype 1 picornavirus IRES s

Picornavirus T ype 1 IRES s comprise five principal domains ( dII – dVI ). Whereas dV binds eIF 4 G , a conserved AUG in dVI was suggested to stimulate attachment of 43 S ribosomal preinitiation complexes, which then scan to the initiation codon. Initiation on T ype 1 IRES s also requires IRES trans‐acting factors ( ITAF s), and several candidates have been proposed. Here, we report the in vitro reconstitution of initiation on three T ype 1 IRES s: poliovirus ( PV ), enterovirus 71 ( EV 71), and bovine enterovirus ( BEV ). All of them require eIF 2, eIF 3, eIF 4A, eIF 4G, eIF 4B, eIF 1A, and a single ITAF , poly( C ) binding protein 2 ( PCBP 2). In each instance, initiation starts with binding of eIF 4 G / eIF 4 A . Subsequent recruitment of 43 S complexes strictly requires direct interaction of their eIF 3 constituent with eIF 4G. The following events can differ between IRES s, depending on the stability of dVI . If it is unstructured ( BEV ), all ribosomes scan through dVI to the initiation codon, requiring eIF 1 to bypass its AUG. If it is structured (PV, EV71), most initiation events occur without inspection of dVI , implying that its AUG does not determine ribosomal attachment.