Lack of Types 1 and 2A Protein Serine(P)/Threonine(P) Phosphatase Activities in Chloroplasts

Protein phosphatase activity in crude leaf extracts and in purified intact chloroplasts of wheat (Triticum aestivum) and pea (Pisum sativum) was analyzed using exogenously supplied phosphoproteins or endogenous thylakoid proteins. Leaf extracts contain readily detectable amounts of protein phosphatase activity measured with either phosphohistone or phosphorylase a, substrates of mammalian protein phosphatases. No significant chloroplast protein phosphatase activity was detected using these exogenous phosphoproteins. The dephosphorylation of endogenous thylakoid light-harvesting chlorophyll a/b binding proteins in situ was inhibited by fluoride, but not by microcystin-LR or okadaic acid, diagnostic inhibitors of mammalian types 1 and 2A protein phosphatases. Additionally, no evidence for a pea chloroplast alkaline phosphatase activity was found using β-glycerolphosphate or 4-methylum-belliferyl phosphate as substrates. From these results, we conclude that phosphohistone and phosphorylase a are not useful substrates for chloroplast thylakoid protein phosphatase activity and that the chloroplast enzymes may not fit into one of the canonical classifications currently used for protein phosphatases.