InterPro domain: IPR042529

Initiation factor 2 binds to Met-tRNA, GTP and the small ribosomal subunit. The eukaryotic translation initiation factor EIF-2B is a complex made up of five different subunits, alpha, beta, gamma, delta and epsilon, and catalyses the exchange of EIF-2-bound GDP for GTP. This family includes initiation factor 2B alpha, beta and delta subunits from eukaryotes; related proteins from archaebacteria and IF-2 from prokaryotes and also contains a subfamily of proteins in eukaryotes, archaeae (e.g. Pyrococcus furiosus), or eubacteria such as Bacillus subtilis and Thermotoga maritima. Many of these proteins were initially annotated as putative translation initiation factors despite the fact that there is no evidence for the requirement of an IF2 recycling factor in prokaryotic translation initiation. Recently, one of these proteins from B. subtilis has been functionally characterised as a 5-methylthioribose-1-phosphate isomerase (MTNA) [ 1 ]. This enzyme participates in the methionine salvage pathway catalysing the isomerisation of 5-methylthioribose-1-phosphate to 5-methylthioribulose-1-phosphate [ 2 ]. The methionine salvage pathway leads to the synthesis of methionine from methylthioadenosine, the end product of the spermidine and spermine anabolism in many species.

This superfamily represents the C-terminal domain of the eukaryotic translation initiation factor (eIF-2B) and are found in initiation factor 2B alpha, beta and delta subunits from eukaryotes; related proteins from archaebacteria and IF-2 from prokaryotes in addition to a subfamily of proteins in eukaryotes, archaea, or eubacteria.

1. A functional link between RuBisCO-like protein of Bacillus and photosynthetic RuBisCO. Science 302, 286-90
2. Crystal structure of yeast Ypr118w, a methylthioribose-1-phosphate isomerase related to regulatory eIF2B subunits. J. Biol. Chem. 279, 37087-94