Characterization of GTP-dependent Met-tRNAf binding protein.

Purified GTP-dependent Met-tRNAf binding protein (EIF2) prepared from the 0.5 M KCl eluate of reticulocyte polyribosomes was successfully resolved into its three-component subunits by isoelectric focusing in the presence of urea. The 37,000-dalton subunit focused at a pH of 5.8 and was resolved into two spots; the 48,000-dalton subunit focused at a pH of 6.6 and was resolved into three spots; and the 52,000-dalton subunit exhibited an isoelectric point of 8.9 and migrated as a single spot. When isolated 37,000- and 48,000-dalton subunit was found to possess Met-tRNAf and mRNA binding activities while the 37,000-dalton subunit was found to possess GDP binding activity. Phosphorylation of EIF2 by protein kinases present in reticulocyte lysates was demonstrated using [gamma-32P]GTP or [gamma-32P]ATP as the phosphate donor. The 37,000-dalton subunit was preferentially phosphorylated when [gamma-32P]ATP was used as substrate; the 48,000-dalton subunit was preferentially phosphorylated when the [gamma-52P]GTP was used as phosphate donor, although some phosphorylation of the 37,000-dalton subunit was also observed. The 37,000-dalton subunit of ribosome-associated EIF2 was present predominantly in a dephosphorylated form following purification.