SUPPRESSOR MUTATIONS IN F1 SUBUNIT-EPSILON RECOUPLE ATP-DRIVEN H-MUTANT OF ESCHERICHIA-COLI F1F0 ATP SYNTHASE( TRANSLOCATION IN UNCOUPLED Q42E SUBUNIT C)

The Q42E mutation in the polar loop of subunit c of the Escherichia co li F1F0 ATP synthase leads to an uncoupling of H+ translocation throug h F-0 and ATP synthesis/hydrolysis in F-1. We have isolated four secon d-site suppressor mutants in which the coupling defect is corrected. S ubstitutions for Glu(31) in F-1 subunit epsilon were found in each sup pressor mutant, where the substitutitions were E31G, E31V, and E31K (t he last being found twice). The different substitutions vary in effect iveness in restoring wild type growth properties in the order epsilon E31G > epsilon E31V > epsilon E31K. Biochemical properties of epsilon E31G/cQ42E and epsilon E31K/cQ42E membranes were compared. In epsilon E31G/cQ42E mutant membranes, ATP-driven H+ translocation by F1F0 and t he binding and coupling of F-1 to F-0 showed a striking pH dependence. Near normal function was observed at pH 7.0, but function was lost at pH 7.8. The function of epsilon E31K/cQ42E membranes was much less af fected by changes in pH. Relative to epsilon E31G/cQ42E membranes, the ATP-driven H+ transport function of epsilon E31K/cQ42E membranes was approximately the same at pH 7.5, greater at pH 7.8, and less at pH 7. 0. The differences between mutants could be explained if cGlu(42) ioni zed at pH 7.8 with loss of function in epsilon E31G/cQ42E membrane and a similar ionization were compensated for by the positively charged L ys in the epsilon E31K/cQ42E membrane.