The initiation of protein synthesis in eukaryotic cells is regulated by interactions between protein initiation factors and RNA molecules. The eukaryotic initiation complex is composed of three subunits, designated eIF2a, eIF2? and eIF2g (eukaryotic translation initiation factor 2 a, ? and g, respectively), all of which work in concert to form a ternary complex with GTP and tRNA in the early stages of protein synthesis. eIF2a, also known as EIF2S1 or EIF2, is a 315 amino acid subunit of the eukaryotic initiation complex that functions to bind tRNA to the 40S ribosomal subunit (in a GTP-dependent manner), thereby initiating translation. In addition, the phosphorylation state of eIF2a controls the rate of tRNA translation. When eIF2a is not phosphorylated, translation occurs at a normal rate. However, upon phosphorylation by one of several kinases, eIF2a is stabilized, thus preventing the GDP/GTP exchange reaction and slowing translation.