Phosphorylation of pea chloroplast acetyl-CoA carboxylase

We have examined whether chloroplast acetyl-CoA carboxylase is a phosphoprotein. Pea (Pisum sativum) chloroplasts were incubated in the presence of [gamma-33P]-ATP and radiolabeled proteins were examined after immunoprecipitation with antibodies against all four known subunits of heteromeric chloroplast acetyl-CoA carboxylase. The beta-subunit of the carboxyltransferase was found to be labeled by 33P. Phosphoamino acid analysis of the immunoprecipitated beta-subunit of the carboxyltransferase indicates that it is phosphorylated on serine residues. Incorporation of 33P into carboxyltransferase beta-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 beta-subunit of chloroplast acetyl-CoA carboxylase as a candidate for regulation by protein phosphorylation/dephosphorylation.