DEPHOSPHORYLATION OF THE THYLAKOID MEMBRANE LIGHT-HARVESTING COMPLEX-II BY A STROMAL PROTEIN PHOSPHATASE

Light-harvesting complex-II (LHC-II) phosphatase activity has generall y been examined in the intact thylakoid membrane. A recent report of p eptide-phosphatase activity associated with the chloroplast stromal fr action (Hammer, M.F. et al. (1995) Photosynth Res 44: 107-115) has led to the question of whether this activity is capable of dephosphorylat ing membrane-bound LHC-II. To this end, heat-treated thylakoid membran es were examined as a potential LHC-II phosphatase substrate. Followin g incubation of the thylakoid membrane at 60 degrees C for 15 min, the endogenous protein phosphatase and kinase activities were almost elim inated. Heat-inactivated phosphomembranes exhibited minimal dephosphor ylation of the light harvesting complex-II. Peptide-phosphatase activi ties isolated from the thylakoid and stromal fraction were able to dep hosphorylate LHC-II in heat-inactivated phosphomembranes. The stromal phosphatase showed highest activity against LHC-II at pH 9. Dephosphor ylation of the LHC-II by the stromal enzyme was not inhibited by molyb date, vanadate or tungstate ions, but was partially inhibited by EDTA and a synthetic phosphopeptide mimicking the LHC-II phosphorylation si te. Thus, the previously identified stromal phosphatase does appear ca pable of dephosphorylating authentic LHC-II in vivo.