Chloroplast ATP synthase contains one single copy of subunit δ that is indispensable for photophosphorylation

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Abstract
FoF1 ATP synthases synthesize ATP in their F1 portion at the expense of free energy supplied by proton flow which enters the enzyme through their channel portion F0. The smaller subunits of F1, especially subunit .delta., may act as energy transducers between these rather distant functional units. We have previously shown that chloroplast .delta., when added to thylakoids partially depleted of the coupling factor CF1, can reconstitute photophosphorylation by inhibiting proton leakage through exposed coupling factor CF0. In view of controversies in the literature, we reinvestigated two further aspects related to subunit .delta., namely (a) its stoichiometry in CF0CF1 and (b) whether or not .delta. is required for photophosphorylation. By rocket immunoelectrophoresis of thylakoid membranes and calibration against purified .delta., we confirmed a stoichiometry of one .delta. per CF0CF1. In CF1-depleted thylakoids photophosphorylation could be reconstituted not only by adding CF1 and subunit .delta. but, surprisingly, also by CF1(-.delta.). We found that the latter was attributable to a contamination of CF1(-.delta.) preparations with integral CF1. To lesser extent CF1(-.delta.) acted by complementary rebinding to CF0 channels that were closed because they contained .delta. [CF0(+.delta.)]. This added catalytic capacity to proton-tight thylakoid vesicles. The ability of subunit .delta. to control proton flow through CF0 and the absolute requirement for .delta. in restoration of photophosphorylation suggest an essential role of this small subunit at the interface between the large portions of ATP synthase: .delta. may be part of the coupling site between electrochemical, conformational and chemical events in this enzyme.
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