Dephosphorylation of central photosynthetic proteins regulates their turnover in plant thylakoid membranes, A membrane protein phosphatase from spinach thylakoids was purified 13000-fold using detergent-engaged FPLC, The purified enzyme exhibited characteristics typical of eukaryotic Ser/Thr phosphatases of the PP2A family in that it was inhibited by okadaic acid (IC50 = 0.4 nM) and tautomycin (IC50 = 25 nM), irreversibly bound to microcystin-agarose, and recognized by a polyclonal antibody raised against a recombinant catalytic subunit of human PP2A. Furthermore, the anti-PP2A antibody inhibited protein dephosphorylation in isolated thylakoids. The phosphatase copurified with TLP40, a cyclophilin-like peptidyl-prolyl isomerase located in the thylakoid lumen. TLP40 could be released from the phosphatase immobilized on microcystin-agarose by high-salt treatment.: Binding of cyclosporin A (CsA) to TLP40 led to thylakoid phosphatase activation,while cyclophilin substrates, prolyl-containing oligopeptides, inhibited protein dephosphorylation. This dephosphorylation could be modulated by CsA or oligopeptides only after the thylakoids had been ruptured to expose the lumenal membrane surface where the TLP40 is located. Regulation of the PP2A-like phosphatase at the outer thylakoid surface is likely to operate via reversible binding of TLP40 to the inner membrane surface. This is a first example of transmembrane regulation in which the activity of phosphatase is altered-by the binding of a cyclophilin to-a site other than the active one. We propose that signaling from TLP40 to the protein phosphatase coordinates dephosphorylation and protein folding, two processes required for protein turnover during the repair of photoinhibited photosystem II reaction centers.