At least eleven thylakoid proteins become phosphorylated under reducing conditions, and redox titration has identified a common midpoint potential of E m = +38 ± 4 mV, n = 0.95 ± 0.06. In the presence of the phosphatase inhibitor NaF (10 mM), the redox dependency of phosphorylation is found to be essentially unchanged: E m = +50 ± 3 mV, n = 1.02 ± 0.04. Thylakoid membranes were phosphorylated in the light and then incubated at various redox potentials for 15 min in the dark; no redox dependency was observed in the dephosphorylation of any of the 17 bands then distinguishable by autoradiography and phosphorimaging. The phosphoprotein phosphatase reactions can be divided arbitrarily into four kinetic classes: the fastest, class I, includes LHC II; the moderate class II includes D1 and D2; the slow class III includes CP43 and the 9 kDa phosphoprotein; finally, a 19.5 kDa protein exhibited no loss of32P at all. In separate experiments we measured thylakoid protein dephosphorylation initiated by changing the redox potential from −140 to +200 mV, in the presence or absence of fluoride. In this case the results are consistent with at least two kinetically distinguishable classes of phosphoprotein phosphatase reactions. We conclude that thylakoid protein phosphatase reactions are kinetically heterogeneous and redox-independent. It follows that the redox dependency of thylakoid protein phosphorylation is a property of thylakoid protein kinase reactions. Our observed E m and n values are consistent with a primary site of kinase redox control at the level of PQ/PQ.− of the Qi (Qn) site of the cytochrome b 6/ƒ complex.
