INTERACTIONS BETWEEN THE F1 AND F-0 PARTS IN THE ESCHERICHIA-COLI ATP-SYNTHASE - ASSOCIATIONS INVOLVING THE LOOP REGION OF C-SUBUNITS

The N-ethylmaleimide reactivity of c subunits in Escherichia coli F1F0 ATP synthase (ECF1F0) isolated from five mutants, each with a cystein e at a different position in the polar loop region (positions 39, 40, 42, 43, and 44), has been investigated. The maleimide was found to rea ct with Cys placed at positions 42, 43, and 44 but not at 39 or 40, Al l copies of the c subunit reacted similarly when the Cys was at positi on 43 or 44, In contrast, the Cys in the mutant cQ42C reacted as two c lasses, with 60% reacting relatively rapidly and 40% reacting at a rat e 40-fold slower. After removing F-1, all copies of the c subunit in t his mutant reacted equally fast. Therefore, the slow class in the cQ42 C mutant represents c subunits shielded by, and probably involved dire ctly in, the interaction of the F-0 with gamma and epsilon subunits of the F-1 part, Based on the estimated stoichiometry of c subunits in t he ECF1F0 complex, 4 or 5 c subunits are involved in this F-1 interact ion. N-Ethylmaleimide modification of all of the c subunits reduced AT Pase activity by only 30% in ECF1F0 from mutant cQ42C. Modification of the more rapidly reacting class had little effect on ATP hydrolysis-d riven proton translocation, and did not alter the DCCD inhibition of A TPase activity. However, as those c subunits involved in the F-1 inter action became modified, DCCD inhibition was progressively lost, as was coupling between ATP hydrolysis and proton translocation.