Figure 9.

Model of the interaction between subunits γ and α^chi in α₃^chiβ₃γ during rotation. A, the unique mycobacterial γ-loop (red), which is in the vicinity to the M. phlei c-ring (brown, PDB code 4V1G) (15), was inserted in the Gsγ (yellow). The arrow indicates the interaction between the ³⁹⁶AQFGSDLDK⁴⁰⁴ peptide (green) and the extra C-terminal domain of α^chi (light green) with the α-helical peptide ⁹⁰AYNSNVLRLVYQT¹⁰² of Gsγ. B, the proximity of the C-terminal helix of Mtϵ with the extended C-terminal stretch of α^chi. Subunit ϵ (blue) inside the α₃^chiβ₃γ complex was modeled based on the recently determined solution structure of Mtϵ (PDB code 5WY7), which was then superimposed onto the δ subunit of the bovine F₁-ATPase. At 80° rotation of the central stalk, the compact C-terminal helix of Mtϵ might interact with the extended C-terminal domain of α^chi on the way to form an extended conformation. C, subunits α^chi, β with nucleotide occupancy, and γ with rotational position are shown in green, orange, and yellow, respectively, and labeled according to the native crystal structure (60). The extended C terminus of α^chi is shown in dark green. Catalytic events are based on the model by Watanabe et al. (63). The state on the left shows the catalytic dwell. During phase 1 (see Fig. 5C) P_i release from β_E triggers a 40° rotation of subunit γ, which leads the enzyme to adopt the ATP-waiting dwell. During phase 2, ATP binding and ADP release, accompanied by a change in the conformational states of the α^chi and β subunits, causes subunit γ to further rotate by 80°. At a rotational position of 90° subunit γ is in close proximity to the extended C terminus of α^chi.