Phosphoenolpyruvate PyrP carboxylase (PyrPC) and PyrPC kinase were copurified from darkadapted leaves of the common ice plant Mesembryanthemum crystallinum L . with crassulacean-acid metabolism (CAM). Purification by (NH&S04 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 of255 pmol P,. mol-' PyrPC . s-'. 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 pM 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 -3051 that Serll is the phosphorylation site of the CAM PyrPC of M . crystallinum. At pH 7.0, the K,,, for ATP of the protein kinase was 25 pM; phosphorylation of PyrPC was maximal after 30 min at pH 7.0. The kinase showed also activity with histone 111-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 C, and constitutive CAM plants.