New initiation factor activity required for globin mRNA translation.

A reconstituted reticulocyte translation system originally designed to be deficient in eukaryotic initiation factor 4B (eIF-4B) was used to identify a new activity required for maximal synthesis of rabbit globin. This new activity purifies as a stable, high molecular weight complex by a variety of chromatographic procedures and is termed eIF-4F. The purified globin stimulatory activity also restores translation of capped mRNAs in extracts of poliovirus-infected HeLa cells. Like restoring activity that was obtained as a protein complex by different procedures (Tahara, S. M., Morgan, M. A. and Shatkin, A. J. (1981) J. Biol. Chem. 256, 791-794), eIF-4F includes the 24,000-dalton cap binding protein and major polypeptides of Mr approximately 200,000 and approximately 46,000. The latter component comigrates with eIF-4A by two-dimensional gel electrophoresis and, like eIF-4A, chemically cross-links to the 5'-end of capped mRNA by an ATP-dependent, m7GDP-sensitive reaction. Unlike eIF-4F, cap binding protein of Mr approximately 24,000 isolated by affinity chromatography on m7GDP-Sepharose does not stimulate globin synthesis in the reconstituted system.