Multiple roles for the C‐terminal domain of eIF5 in translation initiation complex assembly and GTPase activation

eIF5 stimulates the GTPase activity of eIF2 bound to Met‐tRNAiMet, and its C‐terminal domain (eIF5‐CTD) bridges interaction between eIF2 and eIF3/eIF1 in a multifactor complex containing Met‐tRNAiMet. The tif5‐7A mutation in eIF5‐CTD, which destabilizes the multifactor complex in vivo, reduced the binding of Met‐tRNAiMet and mRNA to 40S subunits in vitro. Interestingly, eIF5‐CTD bound simultaneously to the eIF4G subunit of the cap‐binding complex and the NIP1 subunit of eIF3. These interactions may enhance association of eIF4G with eIF3 to promote mRNA binding to the ribosome. In vivo, tif5‐7A eliminated eIF5 as a stable component of the pre‐initiation complex and led to accumulation of 48S complexes containing eIF2; thus, conversion of 48S to 80S complexes is the rate‐limiting defect in this mutant. We propose that eIF5‐CTD stimulates binding of Met‐tRNAiMet and mRNA to 40S subunits through interactions with eIF2, eIF3 and eIF4G; however, its most important function is to anchor eIF5 to other components of the 48S complex in a manner required to couple GTP hydrolysis to AUG recognition during the scanning phase of initiation.

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