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This family includes initiation factor 2B alpha, beta and delta subunits from eukaryotes, initiation factor 2B subunits 1 and 2 from archaebacteria and some proteins of unknown function from prokaryotes. Initiation factor 2 binds to Met-tRNA, GTP and the small ribosomal subunit. Members of this family have also been characterised as 5-methylthioribose- 1-phosphate isomerases, an enzyme of the methionine salvage pathway. The crystal structure of Ypr118w, a non-essential, low-copy number gene product from Saccharomyces cerevisiae, reveals a dimeric protein with two domains and a putative active site cleft .
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) [PUBMED:14551435]. This enzyme participates in the methionine salvage pathway catalysing the isomerisation of 5-methylthioribose-1-phosphate to 5-methylthioribulose-1-phosphate [PUBMED:15215245]. The methionine salvage pathway leads to the synthesis of methionine from methylthioadenosine, the end product of the spermidine and spermine anabolism in many species.
Kyrpides NC, Woese CR; , Proc Natl Acad Sci U S A 1998;95:3726-3730.: Archaeal translation initiation revisited: the initiation factor 2 and eukaryotic initiation factor 2B alpha-beta-delta subunit families. PUBMED:9520434 EPMC:9520434 .
Bumann M, Djafarzadeh S, Oberholzer AE, Bigler P, Altmann M, Trachsel H, Baumann U; , J Biol Chem 2004; [Epub ahead of print]: Crystal structure of yeast Ypr118w, a methylthioribose-1-phosphate isomerase related to regulatory eIF2B subunits. PUBMED:15215245 EPMC:15215245.