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Eukaryotic translation initiation factor 2 (eIF2) is composed of three subunits, eIF2 alpha, eIF2 beta (EIF2S2), and eIF2 gamma, which are present in equal molar amounts. eIF2 beta plays a central role in the maintenance of what is generally considered a rate-limiting step in mRNA translation. In the early steps of protein synthesis, eIF2 beta binds GTP and Met-tRNA and transfers Met-tRNA to the 40S ribosomal subunit. At the end of the initiation process, GTP bound to eIF2 beta is hydrolyzed to GDP and the eIF2/GDP complex is released from the ribosome. The exchange of GDP bound to eIF2 beta for GTP is a prerequisite to binding Met-tRNA and is mediated by eIF2 beta, which recycles the eIF2 complex for another round of initiation.