Coordinated movement of eukaryotic translation initiation factors 1, 1A, and 5 within the 43S ribosomal preinitiation complex (PIC) mediates the response to start codon recognition.

Abstract

Eukaryotic translation initiation factor (eIF) 5 acts as a GTPase activating protein for eIF2 that carries the first tRNA onto the ribosome. Recent data have shown that eIF5 antagonizes eIF1, enhancing its release from the PIC in response to start codon recognition. eIF1 release promotes the conversion of the PIC from open conformation that scan the mRNA to closed one arrested on the start codon. Recent data indicate that an interaction occurs between eIF1A and eIF5 upon start codon recognition by PIC. We observed fluorescence energy transfer (FRET) between fluorescently labeled derivatives of eIF1A and eIF5 within the PIC that is strongly enhanced upon start codon recognition. Our data indicate that the C‐terminal tail (CTT) of eIF1A moves closer to the N‐terminal region of eIF5 upon start codon recognition. This conformational change is involved in stabilizing the closed state of PIC. The kinetics of this FRET change mirror the kinetics of eIF1 release from the PIC on start codon recognition, suggesting two events are coupled. Mutations in eIF1A CTT that decrease the fidelity of start codon recognition in vivo uncouple eIF1 release from eIF1A CTT movement towards eIF5 NTD as well as from release of inorganic phosphate from eIF2. Our results suggest a model in which eIF1 release from PIC promotes movement of the eIF1A CTT towards eIF5 NTD that in turn promotes Pi release from eIF2, converting it to its inactive, GDP‐bound state.