Protein synthesis in brine shrimp embryos and rabbit reticulocytes. The effect of Mg2+ on binary (eukaryotic initiation factor 2 X GDP) and ternary (eukaryotic initiation factor 2 X GTP X met-tRNAf) complex formation.

We have prepared eukaryotic initiation factor 2 (eIF-2) from rabbit reticulocytes and Artemia embryos and studied the effect of Mg2+ on binary (eIF-2 X GDP) and ternary (eIF-2 X GTP X Met-tRNAf) complex formation. Under conditions where Mg2+ inhibits Met-tRNAf binding to reticulocyte eIF-2, ternary complex formation with Artemia eIF-2 is not inhibited. Similarly, the formation of eIF-2 X GDP with Artemia eIF-2 is stimulated by Mg2+, whereas the corresponding reticulocyte binary complex is strongly inhibited. In the presence of 1 mM Mg2+, the isolated Artemia eIF-2 X GDP complex is stable in the absence of any added nucleotide, but readily exchanges bound GDP for free GTP. However, the reticulocyte eIF-2 X GDP complex is significantly more stable in the presence of GTP, and nucleotide exchange is dependent upon the addition of a factor isolated from either the postribosomal supernatant or the high salt wash of rabbit reticulocyte ribosomes. This factor also stimulates Met-tRNAf binding to both Artemia and reticulocyte eIF-2.