SUMMARY: Redox titrations were carried out on the protein kinase reactions of isolated 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 Em = +38 ± 4 mV, n = 0.95 ± 0.06. Two other proteins, including one at 63 kDa, were phosphorylated only under oxidizing conditions, Em = +33 ± 11 mV, n = 0.67 ± 0.09. These midpoint potentials and n values suggest that either cytochrome b6 or else a semiquinone associated with the cytochrome b6f complex may be the controlling redox sensor. The "reverse" redox dependence of phosphorylation of the 63 kDa and 46 kDa bands suggests that more than one redox-controlled protein kinase (or phosphatase) functions in the thylakoid membrane. We also suggest that a protein kinase (or phosphatase) is itself regulated by phosphorylation in a redox-controlled reaction.

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