The rotation of the gamma-subunit has been included in the binding-change mechanism of ATP synthesis/hydrolysis by the proton ATP synthase (FOF1), The Escherichia coli ATP synthase was engineered for rotation studies such that its ATP hydrolysis and synthesis activity is similar to that of wild type. A fluorescently labeled actin filament connected to the gamma-subunit of the F-1 sector rotated on addition of ATP, This progress enabled us to analyze the gamma M23K (the gamma-subunit Met-23 replaced by Lys) mutant, which is defective in energy coupling between catalysis and proton translocation. We found that the F-1 sector produced essentially the same frictional torque, regardless of the mutation. These results suggest that the gamma M23K mutant is defective in the transformation of the mechanical work into proton translocation or vice versa.