A pair of salt bridges mediating coupling between the weak-to-strong state transition and ATPase activation. (A and B) Model of the MTBD–MT interaction in the weak and strong binding states, respectively. In the weak binding state (A), MTBD exhibited diffusional motion constrained in the electronegative potential valley of MT, with its electropositive surface orienting toward MT (Fig. 5 E). In the strong binding state (B), navigated by the flipped ion pair of E3390 and R403, MTBD found its binding site on MT (Fig. 5 C). The molecular structures of the MTBD and α- and β-tubulin at the MTBD–MT interface in the respective states are shown in the insets.