Protein phosphorylation in pea root plastids

Protein phosphorylation has been investigated in nonphotosynthetic plastids of pea roots. Intact and lysed preparations of plastids were incubated wit h [gamma-P-32]ATP and three stromal proteins of sizes 41, 58 and 62 kDa wer e phosphorylated on a serine residue, No other proteins were significantly labelled under the conditions used, The 62 kDa protein is probably phosphog lucomutase and represents a phosphoenzyme catalytic intermediate. The prote in kinase(s) and phosphatase(s) acting on the other proteins were not sensi tive to exogenous calcium but were sensitive to magnesium, The protein phos phatase which acts on the 41 kDa protein is possibly of type 2C, whereas th at acting on the 58 kDa phosphoprotein did not fall into any class defined by mammalian systems, Metabolism of exogenous glucose 6-phosphate by the ox idative pentose phosphate pathway in intact plastids abolished the phosphor ylation of the 58 kDa protein. Dihydroxyacetone phosphate, phosphoenolpyruv ate and 3-phosphoglycerate also inhibited phosphorylation of the 58 kDa pro tein and had a time-dependent effect on the phosphorylation of the 41 kDa p rotein. The significance of these results in relation to a possible role fo r protein phosphorylation in these plastids is considered.