A light-dependent dicyclohexylcarbodiimide-sensitive Ca-ATPase activity in chloroplasts which is not coupled to proton translocation.

The early observation of light-dependent Ca-ATPase activity in chloroplast thylakoids [Avron, M. (1962) J. Biol. Chem. 237, 2011-2017] has been reinvestigated. It is demonstrated that in contrast to light-triggered Mg-ATP activity, Ca-ATPase activity is strictly dependent on delta microH+, the transthylakoid membrane electrochemical potential gradient, since (a) there is an absolute requirement for continuous illumination; (b) electron-transport mediators that catalyze proton uptake, like phenazinemethosulphate, methylviologen of ferricyanide, are essential and (c) uncouplers inhibit the activity. The Ca-ATPase activity is essentially unaffected by dithiols, but is inhibited by CF0-CF1 inhibitors including tentoxin, dicyclohexylcarbodiimide and antisera to CF1. Addition of Ca-ATP to thylakoids does not induce delta pH or delta psi (the electrical potential gradient) formation either in the light or following preillumination with dithiols, demonstrating that it is not coupled to proton translocation. It is also demonstrated that Ca-ATP or Ca-ADP does not induce a proton leak through CF0-CF1. It is concluded that the Ca-ATPase activity in chloroplast thylakoid reflects a partial reaction of ATP synthesis catalyzed by CF0-CF1, which is internally uncoupled from proton translocation but is dependent on energization by a transmembrane delta microH+.