Polypeptide chain initiation in eukaryotes: reversibility of the ternary complex-forming reaction.

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Abstract
In the last step of polypeptide chain initiation in eukaryotes, the interaction of the 40S preinitiation complex eIF-2.cntdot.GTP.cntdot.Met-tRNA.cntdot.40S [the complex between the 40S ribosomal subunit and the ternary complex containing equimolar amounts of eukaryotic initiation factor 2 (eIF-2), GTP, and eukaryotic initiator methionyl tRNA (Met-tRNAi)] with a 60S ribosomal subunit in the presence of mRNA, cap binding protein (with capped messengers), ATP, and the initiation factors eIF-3, eIF-4a, -4b, -4c and eIF-5, results in the formation of an 80S initiation complex (Met-tRNA.cntdot.80S.cntdot.mRNA) with concomitant hydrolysis of GTP and liberation of eIF-2 for recycling in subsequent initiation events. However, at physiological Mg2+ concentrations, GDP is known to have .apprxeq. 100-fold greater affinity than GTP for eIF-2 and eIF-2 is believed to be released in the form of an eIF-2-GDP complex. Previously, it was shown that initiation factor SP (for eIF-2-stimulating protein) promotes the exchange of eIF-2-bound GDP for GTP and catalyzes ternary complex formation in the presence of Met-tRNAi. Binding of GDP by eIF-2 is indeed so tight that homogeneous preparations of [rabbit reticulocyte] eIF-2 contain upward of 0.5 mol of GDP/mol of eIF-2. In the presence of Mg2+ and catalytic amounts of SP, ternary complex formation conforms to the overall reversible reaction eIF-2.cntdot.GDP + GTP + Met-tRNA .dblarw. eIF-2.cntdot.GTP.cntdot.Met-tRNAi + GDP.

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