Protein synthesis in rabbit reticulocytes XXII: A heat stable dialyzable factor (EIF-1^*) modulates Met-tRNA_f binding to EIF-1

The peptide chain initiation factor EIF-1 forms a ternary complex, Met-tRNA_f·EIF-1·GTP in the absence of Mg⁺⁺ and the preformed complex is stable to Mg⁺⁺. However, with homogeneous preparations of EIF-1, addition of Mg⁺⁺ during the initial formation of the ternary complex strongly inhibits the complex formation. A heat stable dialyzable factor (EIF-1^*) which mostly remains associated with the high molecular weight protein complex, EIF-2 (TDF) during purification of the peptide chain initiation factors, has been purified using a phenol extraction procedure. EIF-1^* restores the Met-tRNA_f binding activity of EIF-1 in the presence of 1 mM Mg⁺⁺; in the presence of EIF-1^*, Met-tRNA_f binding by EIF-1 shows a sharp Mg⁺⁺ optimum around 1 mM. EIF-1^* is heat stable, alkali stable, dialyzable and pronase sensitive. The same EIF-1^* preparation also strongly inhibits Met-tRNA_f binding to EIF-1 in the absence of Mg⁺⁺ and stimulates protein synthesis in a mRNA-dependent rabbit reticulocyte lysate system.