Conformation of the gamma subunit at the gamma-epsilon-c interface in the complete Escherichia coli F(1)-ATPase complex by site-directed spin labeling

Structure-function relationships of the gamma-epsilon-c subunit interface of F(O)F(1) ATP synthase, a region of subunit interactions important in coupling between catalysis and transport, were investigated by site-directed spin labeling and electron paramagnetic resonance (EPR) spectroscopy. The EPR line widths and collision accessibilities of 18 spin-labeled, unique cysteine F(1) mutants from gammaLeu198 to gammaLeu215 indicate an alternating pattern in the mobility and accessibility parameters for positions gamma201-209, which is reminiscent of a beta-strand. Labels at positions gamma204 and gamma210 show tertiary contact upon F(1) binding to F(O) and gammaD210C has reduced coupling efficiency. gammaE208C could not be spin labeled, but the uncoupling effects of gammaE208K are suppressed by second-site mutations in the polar loop of subunit c Ketchum, C. J. and Nakamoto, R. K. (1998) J. Biol. Chem. 273, 22292-22297]. The restricted mobility and accessibility of spin labels in the odd-numbered positions between gamma201 and gamma207 plus the 2-4-fold higher values in k(cat) for ATP hydrolysis of these same mutant F(1) indicate that the interactions of these residues with the epsilon subunit mediate its inhibitory activity. Disrupted interactions with epsilon subunit also cause reduced coupling efficiency. We propose a model for the gamma-epsilon-c interface of Escherichia coli F(O)F(1) ATP synthase in which side chains from the odd-numbered residues of the gammaLys201-gammaTyr207 beta-strand directly and functionally interact with the epsilon subunit, while the even-numbered, acidic residues gammaAsp204, gammaGlu208, and gammaAsp210 interact with the F(O) sector, probably with subunit c. gamma Subunit interactions with both subunits in this region are important for coupling efficiency.