Structural Switch of the γ Subunit in an Archaeal aIF2αγ Heterodimer

SummaryEukaryotic and archaeal initiation factors 2 (e/aIF2) are heterotrimeric proteins (αβγ) supplying the small subunit of the ribosome with methionylated initiator tRNA. This study reports the crystallographic structure of an aIF2αγ heterodimer from Sulfolobus solfataricus bound to Gpp(NH)p-Mg2+. aIF2γ is in a closed conformation with the G domain packed on domains II and III. The C-terminal domain of aIF2α interacts with domain II of aIF2γ. Conformations of the two switch regions involved in GTP binding are similar to those encountered in an EF1A:GTP:Phe-tRNAPhe complex. Comparison with the EF1A structure suggests that only the γ subunit of the aIF2αγ heterodimer contacts tRNA. Because the α subunit markedly reinforces the affinity of tRNA for the γ subunit, a contribution of the α subunit to the switch movements observed in the γ structure is considered.