The epsilon subunit of Escherichia coli coupling factor 1 is required for its binding to the cytoplasmic membrane.

The coupling factor, F1-ATPase of Escherichia coli (ECF1) contains five different subunits, alpha, beta, gamma, delta, and epsilon. Properties of delta-deficient ECF1 have previously been described. F1-ATPase containing only the alpha, beta, and gamma subunits was prepared from E. coli by passage of delta-deficient ECF1 through an affinity column containing immobilized antibodies to the epsilon subunit. The delta, epsilon-deficient enzyme has normal ATPase activity but cannot bind to ECF1-depleted membrane vesicles. Both the delta and epsilon subunits are required for the binding of delta, epsilon-deficient ECF1 to membranes and the restoration of oxidative phosphorylation. Either delta or epsilon will bind to the deficient enzyme to form a four-subunit complex. Neither four-subunit enzyme binds to depleted membranes. The epsilon subunit, does, however, slightly improve the binding affinity between delta and delta-deficient enzyme suggesting a possible interaction between the two subunits. Neither subunit binds to trypsin-treated ECF1, which contains only the alpha and beta subunits. A role for gamma in the binding of epsilon to F1 is suggested. epsilon does not bind to ECF1-depleted membranes. Therefore, the in vitro reconstitution of depleted membranes requires an initial complex formation between epsilon and the rest of ECF1 prior to membrane attachment. Reconstitution experiments indicate that only one epsilon is required per functional ECF1 molecule.