Chloroplast Phosphoproteins. Evidence for a Thylakoid‐Bound Phosphoprotein Phosphatase

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Abstract
1 Isolated intact pea (Pisum sativum) chloroplasts incorporate [32P]orthophosphate into several thylakoid polypeptides in the light. Transfer of the labelled chloroplasts to darkness results in rapid dephosphorylation of the polypeptides. The most rapidly dephosphorylated phosphoproteins are the 26000‐Mr doublet derived from the light‐harvesting chlorophyll a/b binding complex. 2 Incubation of isolated 32P‐labelled thylakoids in buffer in the absence of stromal components also results in rapid protein dephosphorylation. Again, the most rapidly dephosphorylated phosphoproteins are the 26000‐Mr light‐harvesting doublet. Dephosphorylation of all thylakoid phosphoproteins is accelerated by addition of up to 10 mM MgCl2. 3 The enzyme responsible for dephosphorylation is a phosphatase rather than a phosphotransferase or the thylakoid protein kinase acting in reverse. The enzyme is specifically and totally inhibited by NaF and does not require phosphoryl group acceptors such as ADP. Unlike the protein kinase, the phosphatase is indifferent to light and the electron transport inhibitor 3(3,4‐dichlorophenyl)‐1,1‐dimethylurea. 4 The phosphorylated regions of the thylakoid phosphoproteins protrude from the outer surface of the membrane and are removed by trypsin treatment.