PURIFICATION OF A PROTEIN PHOSPHATASE FROM CHLOROPLAST STROMA CAPABLEOF DEPHOSPHORYLATING THE LIGHT-HARVESTING COMPLEX-II

A protein phosphatase was purified from the stroma of Pea (Pisum sativ um L.) chloroplasts that is capable of dephosphorylating synthetic pho sphopeptides. Following chromatographic purification of greater than 4 00-fold, two-dimensional electrophoresis indicated that the stromal pr otein phosphatase is a 29-kD protein. A similar molecular size was det ermined for the protein-phosphatase activity using gel-permeation chro matography, indicating that the stromal protein phosphatase is probabl y a monomer. The purified enzyme was able to dephosphorylate synthetic phosphopeptides, which mimic the thylakoid light-harvesting complex-I I (LHC-II) N terminus, as well as LHC-II in thylakoid membranes, but d id not dephosphorylate the major 64-kD phosphoprotein in the stroma. T he stromal protein phosphatase did not discriminate between dephosphor ylation of phosphothreonine and phosphoserine residues in synthetic pe ptide substrates, providing further evidence that this enzyme is disti nct from the protein phosphatase localized in thylakoid membranes. The exact physiological role of the stromal protein phosphatase has yet t o be determined, but it may function in the dephosphorylation of LHC-I I.