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Earlier, we isolated eukaryotic initiation factor 2 (eIF-2)-stimulating protein (SP) as a homogeneous complex with eIF-2 (eIF-2-SP) and showed that, in the presence of Mg2+, eIF-2-SP promotes formation of a ternary complex with GTP and eukaryotic initiator methionyl tRNA (Met-tRNA,.) blocked by a-subunit, but not by 13-subunit, phosphorylation of eIF-2. The eIF-2 and GDP exchanges are compatible with the reaction eIF-2-GDP + SP F EIF-2-SP + GDP reminiscent of the exchange between the Tu and Ts components of prokaryotic elongation factor 1 (EF-Tu and EF-Ts, respectively) EF-Tu-GDP + EF-Ts = EF-Tu-EF-Ts + GDP. Due to the high affinity of GDP (4100 times greater than that of GTP) for eIF-2, 40S (eIF-2-GTP-Met-tRNA4-40S) to 80S (Met-tRNA,4-mRNA-80S) initiation complex conversion, which is accompanied by GTP hydrolysis, probably releases eIF-2 as eIF-2-GDP. Our results suggest that, in the presence of Mg2+, GDP binding restricts the availability of eIF-2 for chain initiation and that SP relieves this restriction in a catalytic fashion, provided that the a subunit of eIF-2 is not phosphorylated.Polypeptide chain initiation in eukaryotes begins with the formation ofa ternary complex between eukaryotic initiation factor 2 (eIF-2), GTP, and eukaryotic initiation methionyl tRNAj (Met-tRNA,) (eIF-2-GTP-Met-tRNAi), which binds to a 40S ribosome, giving rise to a 40S initiation complex (for review, see ref.

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