Substitution of Escherichia coli F1Fo ATP synthase residues βD372 or γS12 with groups that are unable to form a hydrogen bond at this location decreased ATP synthase-dependent cell growth by 2 orders of magnitude, eliminated the ability of F1Fo to catalyze ATPase-dependent proton pumping in inverted E. coli membranes, caused a 15-20% decrease in the coupling efficiency of the membranes as measured by the extent of succinate-dependent acridine orange fluorescence quenching, but increased soluble F1-ATPase activity by about 10%. Substitution of γK9 to eliminate the ability to form a salt bridge with βD372 decreased soluble F1-ATPase activity and ATPase-driven proton pumping by 2-fold but had no effect on the proton gradient induced by addition of succinate. Mutations to eliminate the potential to form intersubunit hydrogen bonds and salt bridges between other less highly conserved residues on the γ subunit N-terminus and the β subunits had little effect on ATPase or ATP synthase activities. These results suggest that the βD372-γK9 salt bridge contributes significantly to the rate-limiting step in ATP hydrolysis of soluble F 1 while the βD372-γS12 hydrogen bond may serve as a component of an escapement mechanism for ATP synthesis in which αβγ intersubunit interactions provide a means to make substrate binding a prerequisite of proton gradient-driven γ subunit rotation.
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