Figure 8.

Model for concerted PilT motions. (A) The quasi 2-fold symmetric C2 crystal structure has two peripheral wide-open subunits (B, E; blue), two central “active” subunits (C, F; orange) and two central “resting” subunits (A, D; green). Four CTD:CTD interfaces have engaged arginine fingers (double lines). The remaining two have disengaged fingers (zig-zag). Subunit F is clamped around bound nucleotide. (B) When ATP (red) binds in the E cleft, the two domains close around the ligand (short black arrows), causing the β5/β6 arginines to approach the ATP. Because of the extensive CTD_D:NTD_E interface, the motion of NTD_E forces the swiveling of CTD_D (in particular the C-terminal helices) toward the periphery of the hexamer (long grey arrow). Consequently, the D arginine fingers engage in the E active site (double lines). On the other side of CTD_D, the CTD_C:CTD_D interface likewise rearranges, disengaging the C arginine fingers from the D active site (zig-zag). (C) Subunit D is now poised as the most peripheral, wide open subunit and ready to bind nucleotide; E is clamped around nucleotide and contributing to an engaged CTD:CTD interface on either side.