Structure of the [beta] subunit of translation initiation factor 2 from the Archaeon Methanococcus jannaschii by NMR : a representative of the eIF2[beta]/eIF5 family of proteins

eIF2b is one of the three subunits of eukaryotic initiation factor 2 (eIF2), a protein which recruits the initiator Met-tRNA and GTP to the 40S ribosomal subunit as part of the 43S pre-initiation complex. The three dimensional structure of aIF2b from Methanococcus jannaschii, an archaeal homologue of eIF2, was determined using multidimensional NMR methods. The aIF2b was found to consist of two independent structural domains. The N-terminal domain contains a four-stranded antiparallel b sheet and two a helices on one side of the sheet. The folding topology was found to be similar to that of the DNA-binding domain of a yeast heat shock transcription factor and a domain within the ribosomal protein S4, although there is no significant homology at the level of primary sequence. The C-terminal domain of aIF2b contains a 'zinc ribbon' motif of three antiparallel b strands, with four conserved cysteines arranged as two CXXC units separated by 17 residues. The role of zinc in the C-terminal domain was investigated with a synthetic peptide that corresponds to the last 50 amino acid residues (residues 94-143) of aIF2b. The N- and Cterminal domains of aIF2b are connected by a helical linker, and were found to be independent of each other. Conserved residues on the surface of each domain that are likely candidates for direct interaction with other components of the translational apparatus were identified. The significant primary sequence homology between archaeal aIF2b and the eukaryotic initiation factors eIF2b and eIF5 allows structural features to be predicted for these latter two proteins from this first structural model of aIF2b.