Purification and characterization of two initiation factors required for maximal activity of a highly fractionated globin mRNA translation system.

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Abstract
Two additional initiation factors (IF-M4 and IF-M5) were purified [from rabbit reticulocytes] and characterized both physically and biologically. IF-M4 is active as a single polypeptide chain with a MW of 48,000. In contrast, IF-M5 is active as a complex with a MW of about 500,000 and consists of 7 major and several minor polypeptide components. Analysis of IF-M5 in 2 polyacrylamide gel electrophoresis systems indicated that 1 of the major polypeptide chains of IF-M5 was the 35,000 dalton subunit of IF-MP. This analysis also revealed that IF-M2A, IF-M3, and elongation factor 2 were present as minor components. Both IF-M4 and IF-M5 are required to achieve maximal activity in an assay system dependent on exogenous globin mRNA, but neither factor was observed to stimulate model reactions that utilize artificial templates [poly(U) or AUG].