The second stalk of Escherichia coli ATP synthase.

Two stalks link the F(1) and F(0) sectors of ATP synthase. The central stalk contains the gamma and epsilon subunits and is thought to function in rotational catalysis as a rotor driving conformational changes in the catalytic alpha(3)beta(3) complex. The two b subunits and the delta subunit associate to form b(2)delta, a second, peripheral stalk extending from the membrane up the side of alpha(3)beta(3) and binding to the N-terminal regions of the alpha subunits, which are approx. 125 A from the membrane. This second stalk is essential for binding F(1) to F(0) and is believed to function as a stator during rotational catalysis. In vitro, b(2)delta is a highly extended complex held together by weak interactions. Recent work has identified the domains of b which are essential for dimerization and for interaction with delta. Disulphide cross-linking studies imply that the second stalk is a permanent structure which remains associated with one alpha subunit or alphabeta pair. However, the weak interactions between the polypeptides in b(2)delta pose a challenge for the proposed stator function.