Regulatory protein phosphorylation of phosphoenolpyruvate carboxylase in the facultative crassulacean-acid-metabolism plant Mesembryanthemum crystallinum L.
Phosphoenolpyruvate PyrP carboxylase (PyrPC) and PyrPC kinase were copurified from dark-adapted leaves of the common ice plant Mesembryanthemum crystallinum L. with crassulacean-acid metabolism (CAM). Purification by (NH4)2SO4 fractionation, chromatography on Fractogel-DEAE and hydroxylapatite resulted in a PyrPC preparation with a specific activity of 23-25 U/mg protein and a protein kinase activity of 255 mumol Pi.mol-1 PyrPC.s-1. After in vitro phosphorylation, the most prominently phosphorylated polypeptide was identified as PyrPC by immunoblotting and sequencing. Phosphorylation of PyrPC in vitro by incubation with 400 microM MgATP decreased its sensitivity towards malate. When purified in the absence of the protease inhibitor chymostatin, PyrPC lost an N-terminal sequence of 128 amino acids. Although the carboxylation reaction was unaffected, the truncated PyrPC could neither be phosphorylated in vitro nor inhibited by malate. This result and data obtained by limited proteolysis concur with the hypothesis [Jiao, J.A. & Chollet, R. (1989) Arch. Biochem. Biophys. 283, 300-305] that Ser11 is the phosphorylation site of the CAM PyrPC of M. crystallinum. At pH 7.0, the Km for ATP of the protein kinase was 25 microM; phosphorylation of PyrPC was maximal after 30 min at pH 7.0. The kinase showed also activity with histone III-S but not with dephosphorylated casein. It was inhibited by malate. The results show, that reversible protein phosphorylation is an important factor in the regulation of PyrPC in the facultative CAM plant M. crystallinum, similar to C4 and constitutive CAM plants.