IF2

IF-2, or initiation factor 2, is a bacterial translation initiation factor encoded by the infB gene. It is a large GTP-binding protein that exists in several isoforms produced by alternative translation initiation, often described as long and short forms. IF-2 is essential for the initiation of protein synthesis in most bacteria and in the organelles of some eukaryotes.

During initiation, IF-2 forms a complex with GTP and the initiator tRNA, formylmethionyl-tRNAfMet, and delivers it to the P site of the 30S ribosomal subunit together with the initiation factors IF-1 and IF-3 and the mRNA. This facilitates correct start codon recognition and the assembly of the 30S initiation complex. Upon correct positioning, GTP hydrolysis by IF-2 triggers conformational changes that promote release of IF-3 and IF-2 and enable joining of the 50S subunit to form the 70S initiation complex. GTP hydrolysis and the subsequent release of IF-2 are key steps enabling the transition from initiation to elongation.

Structurally, IF-2 contains an N-terminal GTPase domain and a C-terminal region that binds tRNA and interacts with the ribosome. The C-terminal domains help position the initiator tRNA in the P site, and IF-2 preferentially binds fMet-tRNAfMet. In bacteria, IF-2 works in concert with IF-1 and IF-3; a functionally analogous factor in eukaryotes is eIF5B, which promotes subunit joining after start-site selection.