In silico analysis suggests that PH0702 and PH0208 encode for methylthioribose-1-phosphate isomerase and ribose-1,5-bisphosphate isomerase, respectively, rather than aIF2Bβ and aIF2Bδ.

The overall process of protein biosynthesis across all domains of life is similar; however, detailed insights reveal a range of differences in the proteins involved. For decades, the process of protein translation in archaea has been considered to be closer to eukaryotes than to bacteria. In archaea, however, several homologues of eukaryotic proteins involved in translation initiation have not yet been identified; one of them being the initiation factor eIF2B consisting of five subunits (α, β, γ, δ and ε). Three open reading frames (PH0440, PH0702 and PH0208) in Pyrococcus horikoshii have been proposed to encode for the α-, β- and δ-subunits of aIF2B, respectively. The crystal structure of PH0440 shows similarity toward the α-subunit of eIF2B. However, the capability of PH0702 and PH0208 to function as the β- and δ-subunits of eIF2B, respectively, remains uncertain. In this study, we have taken up the task of annotating PH0702 and PH0208 using bioinformatics methods. The phylogenetic analysis of protein sequences belonging to IF2B-like family along with PH0702 and PH0208 revealed that PH0702 belonged to methylthioribose-1-phosphate isomerase (MTNA) group of proteins, whereas, PH0208 was found to be clustered in the group of ribose-1,5-bisphosphate isomerase (R15PI) proteins. A careful analysis of protein sequences and structures available for eIF2B, MTNA and R15PI confirms that PH0702 and PH0208 contain residues essential for the enzymatic activity of MTNA and R15PI, respectively. Additionally, the protein PH0208 comprises of the residues required for the dimer formation which is essential for the biological activity of R15PI. This prompted us to examine all eIF2B-like proteins from archaea and to annotate their function. The results reveal that majority of these proteins are homologues of the α-subunit of eIF2B, even though they lack the residues essential for their functional activity. A better understanding of the mechanism of GTP exchange during translation initiation in archaea is henceforth required.