Structure - Function relationships of the intact aIF2α subunit from the archaeon Pyrococcus abyssi

Eukaryotic and archaeal initiation factor 2 (e- and aIF2, respectively) are heterotrimeric proteins (αβγ) supplying the small subunit of the ribosome with methionylated initiator tRNA. The γ subunit forms the core of the heterotrimer. It resembles elongation factor EF1-A and ensures interaction with Met-tRNA_iMet. In the presence of the α subunit, which is composed of three domains, the γ subunit expresses full tRNA binding capacity. This study reports the crystallographic structure of the intact aIF2α subunit from the archaeon Pyrococcus abyssi and that of a derived C-terminal fragment containing domains 2 and 3. The obtained structures are compared with those of N-terminal domains 1 and 2 of yeast and human eIF2α and with the recently determined NMR structure of human eIF2α. We show that the three-domain organization in the α subunit is conserved in archaea and eukarya. Domains 1 and 2 form a rigid body linked to a mobile third domain. Sequence comparisons establish that the most conserved regions in the aIF2α polypeptide lie at opposite sides of the protein, within domain 1 and domain 3, respectively. These two domains are known to exhibit RNA binding capacities. We propose that domain 3, which is known to glue the α subunit onto the γ subunit, participates in Met-tRNA _i^Met binding while domain 1 recognizes either rRNA or mRNA on the ribosome. Thereby, the observed structural mobility within the e- and aIF2α molecules would be an integral part of the biological function of this subunit in the heterotrimeric e- and aIF2αβγ factors.