Isolation and characterization of factors from wheat germ that exhibit eukaryotic initiation factor 4B activity and overcome 7-methylguanosine 5'-triphosphate inhibition of polypeptide synthesis.

Three highly purified preparations (preparations I, II-1, and II-2) have been obtained from wheat germ and shown to support in vitro polypeptide synthesis directed by capped or uncapped mRNAs in a eukaryotic initiation factor 4B (eIF-4B)-deficient system. The three preparations differ, however, in polypeptide composition and in the ability to overcome the inhibitory effect of 7-methylguanosine 5'-triphosphate (m7GTP) on in vitro polypeptide synthesis. Preparation I contains two polypeptides (Mr = 80,000 and 28,000), which are present in a 1:1 molar ratio and are associated in a high molecular weight complex. Preparation II-1 contains two major polypeptides (Mr = 220,000 and 26,000) and preparation II-2 also contains two major polypeptides (Mr = 110,000 and 26,000). Preparations II-1 and II-2 are high molecular weight complexes; neither contains detectable amounts of a Mr 80,000 or a Mr 50,000 component. Preparations II-1 and II-2 both overcome m7GTP inhibition, whereas preparation I does not. These findings raise several questions with regard to the identity of eIF-4B and its relationship to cap recognition factors.