Specific binding of coupling factor 1 lacking the delta and epsilon subunits to thylakoids.

An improved procedure for the preparation of chloroplast coupling factor 1 (CF1) lacking the delta subunit is described. In addition, CF1 deficient in the epsilon subunit was isolated by a new method and CF1 lacking both of the smaller subunits was prepared. The ability of the subunit-deficient forms and of CF1, either heated or incubated with dithiothreitol to activate its ATPase activity, to bind to thylakoids from which CF1 had been removed was studied. All CF1 preparations bound in a cation-dependent manner to similar extents. CF1 lacking the delta subunit required higher cation concentrations for maximal binding. All preparations competed similarly with control CF1 for binding sites on the depleted membranes. The alpha subunit of all forms of CF1 in solution was rapidly cleaved by trypsin. After reconstitution, however, the alpha subunit of CF1, as well as of the subunit-deficient and the activated forms, was resistant to attack by trypsin. Moreover, treatment of the membranes with either trypsin or N,N'-dicyclohexylcarbodiimide inhibited the binding of all CF1 forms. These results suggest that the binding of the subunit-deficient and activated forms of CF1 is specific. CF1 lacking the epsilon subunit restored neither proton uptake nor ATP synthesis to the depleted membranes. In contrast to our previous results, CF1 lacking the delta subunit was partially effective. Previously, we used a suboptimal Mg2+ concentration for binding the delta-deficient enzyme which we show here was partially deficient in the epsilon subunit. These results show that the delta and epsilon subunits are not required for binding CF1 to the membranes and that the delta subunit is not an absolute requirement for ATP synthesis.