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Abstract
Co-eIF-2A binds to preformed ternary complex, Met-tRNA(,f)(.)eIF-2(.)GTP and forms a stable quarternary complex. The quarternary complex is significantly more resistant to aurin tricarboxylic acid (3 x 10('-5) M) than the ternary complex. Several laboratories have reported that mRNAs can bind to eIF-2 and can cause extensive breakdown of the ternary complex. We now report that addition of homogeneous Co-eIF-2A preparation completely reverses mRNA inhibition of ternary complex formation; in the presence of Co-eIF-2A, the quarternary complex formed is completely resistant to mRNAs. This observation suggests that under the physiological conditions, the initiation complex exists mostly as the quarternary complex and is stable in the presence of physiological mRNA concentrations.
Co-eIF-2B is a high molecular weight protein complex. Partially purified Co-eIF-2B promotes dissociation of the Met-tRNA(,f)(.)eIF-2(.)GTP complex in the presence of divalent cations, Mg('2+), Mn('2+), Ca('2+) and Sr('2+). This dissociation is not dependent upon GTP hydrolysis. Co-eIF-2B also promotes binding of Met-tRNA(,f) to 40S ribosomes to form Met-tRNA(,f)(.)40S initiation complex.
