Eukaryotic translation initiation factor 5 (eIF5), a monomeric protein of about 49 kDa in mammals and 46 kDa in the yeast Saccharomyces cerevisiae, in conjunction with GTP and other initiation factors plays an essential role in initiation of protein synthesis in eukaryotic cells. Following formation of the 40S initiation complex (40S . eIF3 . mRNA . Met-tRNAf . eIF2 . GTP) at the AUG codon of an mRNA, eIF5 interacts with the 40S initiation complex to promote the hydrolysis of bound GTP. Hydrolysis of GTP causes the release of bound initiation factors from the 40S subunit, an event that is essential for the subsequent joining of the 60S ribosomal subunit to the 40S complex to form the functional 80S initiation complex. Detailed characterization of the eIF5-promoted GTP hydrolysis reaction shows that eIF5 functions as a GTPase-activating protein (GAP) in translation initiation. First, eIF5 promotes hydrolysis of GTP only when the nucleotide is bound to eIF2 in the 40S initiation complex. eIF5, by itself, does not hydrolyze either free GTP or GTP bound to the Met-tRNAf . eIF2 . GTP ternary complex in the absence of 40S ribosomal subunits. Second, as with typical GAPs, eIF5 forms a complex with eIF2, the GTP-binding protein. This interaction, which occurs between the lysine-rich N-terminal region of the beta subunit of eIF2 and the glutamic acid-rich C-terminal region of eIF5, is essential for eIF5 function both in vitro and in vivo in yeast cells. Finally, like typical GAPs, eIF5 also contains an arginine-finger motif consisting of an invariant arginine residue at its N-terminus that is also essential for its function. This invariant arginine residue is presumably involved in the stabilization of the transition state of the GTP hydrolysis reaction catalyzed by initiation factor eIF2.