SUBSTRATE-SPECIFICITY AND KINETICS OF THYLAKOID PHOSPHOPROTEIN PHOSPHATASE REACTIONS

A synthetic 15-amino-acid phosphopeptide analogue of an N-terminal pho sphorylated segment of LHC II was found to inhibit dephosphorylation n ot only of phospho-LHC II but of all other thylakoid phosphoproteins r esolved by phosphorimaging. The results suggest that structural featur es required for recognition of the phosphoprotein phosphatase are comm on to different thylakoid phosphoproteins as well as to the phosphopep tide itself: at least one thylakoid phosphoprotein phosphatase exhibit s a broad substrate specificity. Dephosphorylation reaction rates of a ll 13 thylakoid phosphoproteins were determined, and the dephosphoryla tion half-times were found to range from 7 min to more than 180 min. M ost of the phosphoprotein dephosphorylation reactions were partially i nhibited by NaF, and were insensitive to antimycin A and okadaic acid. Nevertheless, both antimycin A and NaF stimulated the phosphorylation of LHC II and the 9 kDa protein. Possible reasons for differences in sensitivity to these inhibitors are discussed.