Phosphorylation of pea chloroplast acetyl‐CoA carboxylase

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Abstract
We have examined whether chloroplast acetyl-CoA carboxylase is a phosphoprotein. Pea (Pisum sativum) chloroplasts were incubated in the presence of [γ-33P]-ATP and radiolabeled proteins were examined after immunoprecipitation with antibodies against all four known subunits of heteromeric chloroplast acetyl-CoA carboxylase. The β-subunit of the carboxyltransferase was found to be labeled by33P. Phosphoamino acid analysis of the immunoprecipitated β-subunit of the carboxyltransferase indicates that it is phosphorylated on serine residues. Incorporation of33P into carboxyltransferase β-subunit decreased in chloroplasts transferred to dark conditions after labeling in the light. Dephosphorylation of pea chloroplast extracts by an alkaline phosphatase-agarose conjugate reduced in vitro acetyl-CoA carboxylase activity by 67%. Furthermore, while acetyl-CoA carboxylase activity and its carboxyltransferase half-reaction were reduced in dephosphorylated extracts, the biotin carboxylase half-reaction was not inhibited. The evidence presented here points to the carboxyltransferase β-subunit of chloroplast acetyl-CoA carboxylase as a candidate for regulation by protein phosphorylation/dephosphorylation.