Substrate specificity and kinetics of thylakoid phosphoprotein phosphatase reactions

A synthetic 15-amino-acid phosphopeptide analogue of an N-terminal phosphorylated segment of LHC II was found to inhibit dephosphorylation not only of phospho-LHC II but of all other thylakoid phosphoproteins resolved by phosphorimaging. The results suggest that structural features required for recognition of the phosphoprotein phosphatase are common to different thylakoid phosphoproteins as well as to the phosphopeptide itself: at least one thylakoid phosphoprotein phosphatase exhibits a broad substrate specificity. Dephosphorylation reaction rates of all 13 thylakoid phosphoproteins were determined, and the dephosphorylation half-times were found to range from 7 min to more than 180 min. Most of the phosphoprotein dephosphorylation reactions were partially inhibited by NaF, and were insensitive to antimycin A and okadaic acid. Nevertheless, both antimycin A and NaF stimulated the phosphorylation of LHC II and 9 kDa protein. Possible reasons for differences in sensitivity to these inhibitors are discussed.