Fig. 1.

S. solfataricus aIF2αβγ. (A) Overall structure of aIF2α3βγ bound to GDP. aIF2γ is yellow, switch 1 is red (g-31 to g-49), and switch 2 is orange (g-93 to g-113). The aIF2α3 domain (a-174 to a-266) is cyan. The aIF2β-helix 1 (b-3 to b-17) is green. Residues b-23 to b-27 are not visible. The aIF2β central domain (b-28 to b-103) is dark green, and the aIF2β ZBD (b-104 to b-139) is light green. The same color coding is used in all four structures. (B) Putative structure of the full aIF2αβγ heterotrimer, as deduced from the available data. This structure results from superimposition of aIF2γ, as in the present aIF2α3βγ structure, to aIF2γ, as in the aIF2αγ:GDPNP-Mg²⁺ structure (20). The three domains of aIF2α are colored as follows: the N-terminal domain is dark blue (a-1 to a-84), the central domain is marine (a-85 to a-173), and the C-terminal domain is cyan (a-174 to a-266). (C) View of aIF2αγ:GDPNP-Mg²⁺ as obtained in ref. 20. Residues g-36 to g-43 in switch 1 are not visible. Switch 1 is ON, and the conserved g-T46 contacts the magnesium ion. Switch 2 is ON, and the NH group of g-G96 contacts the γ-phosphate. (D) Overall structure of Ss-aIF2α3β1γ bound to GDP–Mg²⁺. Residues g-35 to g-48 in switch 1 are not visible. Figs. 1–3 and SI Figs. 7–9 were drawn with PyMol (DeLano Scientific LLC).