Factors from wheat germ that enhance the activity of eukaryotic initiation factor eIF-2. Isolation and characterization of Co-eIF-2 alpha.

Two factors have been isolated from wheat germ that enhance the ability of initiation factor 2(eIF-2) to form a ternary complex with GTP and Met-tRNAf. One of these factors, Co-eIF-2 beta, is a monomeric protein with a molecular weight of approximately 83,000 (Lax, S. R., Osterhout, J.J., and Ravel, J.M. (1982) J. Biol. Chem. 257, 8233-8237). The purification and properties of Co-eIF-2 alpha are described in this report. The most highly purified preparations of Co-eIF-2 alpha contain two polypeptides with molecular weights of 21,000 and 19,000. Both Co-eIF-2 alpha and Co-eIF-2 beta are heat-stable factors that stimulate ternary complex formation in the presence and absence of Mg2+ and overcome the inhibitory effect of aurintricarboxylic acid. Co-eIF-2 alpha differs from Co-eIF-2 beta in that Co-eIF-2 beta stimulates the formation of a binary complex between eIF-2 and GDP and Co-eIF-2 alpha does not. Also. The stimulatory effects of Co-eIF-2 alpha and Co-eIF-2 beta on the ternary complex formation are close to additive, strongly suggesting that the two factors function independently. Neither Co-eIF-2 alpha nor Co-eIF-2 beta enhances the rate of exchange between GDP bound to eIF-2 and free GDP, indicating that neither factor functions as a guanine nucleotide exchange factor.