The C-terminal domain of the epsilon subunit of the chloroplast ATP synthase is not required for ATP synthesis.

The epsilon subunit of the ATP synthases from chloroplasts and Escherichia coli regulates the activity of the enzyme and is required for ATP synthesis. The epsilon subunit is not required for the binding of the catalytic portion of the chloroplast ATP synthase (CF1) to the membrane-embedded part (CFo). Thylakoid membranes reconstituted with CF1 lacking its epsilon subunit (CF1-epsilon) have high ATPase activity and no ATP synthesis activity, at least in part because the membranes are very leaky to protons. Either native or recombinant epsilon subunit inhibits ATPase activity and restores low proton permeability and ATP synthesis. In this paper we show that recombinant epsilon subunit from which 45 amino acids were deleted from the C-terminus is as active as full-length epsilon subunit in restoring ATP synthesis to membranes containing CF1-epsilon. However, the truncated form of the epsilon subunit was significantly less effective as an inhibitor of the ATPase activity of CF1-epsilon, both in solution and bound to thylakoid membranes. Thus, the C-terminus of the epsilon subunit is more involved in regulation of activity, by inhibiting ATP hydrolysis, than in ATP synthesis.