EIF-2B and the exchange of guanine nucleotides bound to eIF-2

Abstract

1. 1. Available data for the formation of the ternary complex eIF-2·GTP·methionyl-tRNA i involved in eukaryotic initiation and of the inhibition of ternary complex formation by GDP have been examined with a view to determining the mechanism by which eIF-2B facilitates nucleotide exchange. 2. 2. Two mechanisms have been considered—first a displacement reaction in which eIF-2B displaces GDP and GTP in a manner analogous to a “ping-pong” enzyme mechanism, anand secondly the possibility that binding of eIF-2B to eIF-2·nucleotide complexes enhances the rate of nucleotide exchange without itself inducing nucleotide displacement. 3. 3. Comparison has been made between the properties of eIF-2 and eIF-2B and of the bacterial elongation factors Tu and Ts. 4. 4. It seems most probable that, as previously suggested by others for Ts, eIF-2B effectively catalyses an exchange reaction through a “ping-pong” type mechanism. 5. 5. Possible explanations of data suggesting otherwise are put forward. Both eIF-2 and bacterial Tu are complex allosteric proteins subject to a variety of influences which in the case of eIF-2 include phosphorylation of the α subunit. 6. 6. This phosphorylation appears to change the equilibria in the reaction mechanism such that the transferred entity (eIF-2) becomes firmly bound to the catalyst (eIF-2B). 7. 7. Minimum rate constants for the formation of eIF-2·eIF-2B from eIF-2·GTP and reverse reactions are derived. 8. 8. These values suggest that the initiation factors are likely to have to operate in a restricted environment if rates of protein synthesis seen in vivo are to be sustained.