Structure of the β Subunit of Translation Initiation Factor 2 from the Archaeon Methanococcus jannaschii: A Representative of the eIF2β/eIF5 Family of Proteins,

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Abstract
The β subunit of archaeal translation initiation factor 2 (aIF2β) is a representative of a family of proteins whose members include the β subunit of eukaryotic translation initiation factor 2 (eIF2β) and the N-terminal domain within translation initiation factor 5 (eIF5); no members of this family of proteins have been structurally characterized up to this time. In the work presented here, aIF2β from Methanococcus jannaschii was expressed in Escherichia coli, purified, and analyzed using multidimensional NMR methods. The aIF2β was found to contain two independent structural domains. The N-terminal domain contains a four-stranded antiparallel β sheet and two α helices, and is structurally similar to the DNA-binding domain of a yeast heat shock transcription factor and a domain within ribosomal protein S4. This structural similarity was an unanticipated result, since no significant homology was detected at the level of primary sequence. The C-terminal domain of aIF2β contains a zinc-binding motif of three antiparallel β strands, with four conserved cysteines arranged as two CXXC units separated by 17 residues. 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 aIF2β and the eukaryotic eIF2β and eIF5, when combined with the structural results in the work presented here, permitted structural features to be predicted for these latter two eukaryotic proteins.