REDOX TITRATION OF MULTIPLE PROTEIN PHOSPHORYLATIONS IN PEA CHLOROPLAST THYLAKOIDS

Redox titrations were carried out on the protein kinase reactions of i solated pea chloroplast thylakoid membranes. Of the 13 phosphoproteins observed by autoradiography, all were found to titrate with the same midpoint potential of around +40 mV. Eleven proteins, including LHCII, D1, D2, CP43, a 9 kDa and a 55 kDa protein, were phosphorylated under reducing conditions, with a midpoint potential of E(m) = +38 +/- 4 mV , n = 0.95 +/- 0.06. Two other proteins, including one at 63 kDa, were phosphorylated only under oxidizing conditions, E(m) = +33 +/- 11 mV, n = 0.67 +/- 0.09. These midpoint potentials and n values suggest tha t either cytochrome b6 or else a semiquinone associated with the cytoc hrome b6f complex may be the controlling redox sensor. The 'reverse' r edox dependence of phosphorylation of the 63 kDa and 46 kDa bands sugg ests that more than one redox-controlled protein kinase (or phosphatas e) functions in the thylakoid membrane. We also suggest that a protein kinase (or phosphatase) is itself regulated by phosphorylation in a r edox-controlled reaction.