Gamma subunit of initiation factor 2 (eIF2 gamma)eIF2 is a heterotrimeric translation initiation factor that consists of alpha, beta, and gamma subunits. The GTP-bound gamma subunit also binds initiator methionyl-tRNA and delivers it to the 40S ribosomal subunit. Following hydrolysis of GTP to GDP, eIF2:GDP is released from the ribosome. The gamma subunit has no intrinsic GTPase activity, but is stimulated by the GTPase activating protein (GAP) eIF5, and GDP/GTP exchange is stimulated by the guanine nucleotide exchange factor (GEF) eIF2B. eIF2B is a heteropentamer, and the epsilon chain binds eIF2. Both eIF5 and eIF2B-epsilon are known to bind strongly to eIF2-beta, but have also been shown to bind directly to eIF2-gamma. It is possible that eIF2-beta serves simply as a high-affinity docking site for eIF5 and eIF2B-epsilon, or that eIF2-beta serves a regulatory role. eIF2-gamma is found only in eukaryotes and archaea. It is closely related to SelB, the selenocysteine-specific elongation factor from eubacteria. The translational factor components of the ternary complex, IF2 in eubacteria and eIF2 in eukaryotes are not the same protein (despite their unfortunately similar names). Both factors are GTPases; however, eubacterial IF-2 is a single polypeptide, while eIF2 is heterotrimeric. eIF2-gamma is a member of the same family as eubacterial IF2, but the two proteins are only distantly related. This family includes translation initiation, elongation, and release factors.