Eukaryotic translation initiation factor 5 (elF5) acts as a classical GTPase-activator protein

GTP hydrolysis occurs at several specific stages during the initiation, elongation, and termination stages of mRNA translation. However, it is unclear how GTP hydrolysis occurs; it has previously been suggested to involve a GTPase active center in the ribosome, although proof for this is lacking. Alternatively, it could involve the translation factors themselves, e.g., be similar to the situation for small G in which the GTPase active site involves arginine residues contributed by a further protein termed a GTPase-activator protein (GAP). During translation initiation in eukaryotes, initiation factor elF5 is required for hydrolysis of GTP bound to elF2 (the protein which brings the initiator Met-tRNA, to the 40S subunit). Here we show that elF5 displays the hallmarks of a classical GAP (e.g., RasGAP). Firstly, its interaction with elF2 is enhanced by AlF₄^-. Secondly, elF5 possesses a conserved arginine (Arg15) which, like the "arginine fingers" of classical GAPs, is flanked by hydrophobic residues. Mutation of Arg15 to methionine abolishes the ability of elF5 either to stimulate GTP hydrolysis or to support mRNA translation in vitro. Mutation studies suggest that a second conserved arginine (Arg48) also contributes to the GTPase active site of the elF2.elF5 complex. Our data thus show that elF5 behaves as a classical GAP and that GTP hydrolysis during translation involves proteins extrinsic to the ribosome. Indeed, inspection of their sequences suggests that other translation factors may also act as GAPs.