A protein inhibitor of phosphoenolpyruvate carboxylase kinase from Kalanchoë fedtschenkoi

Abstract

The nocturnal fixation of CO2 in CAM plants is catalysed by phosphoenolpyruvate carboxylase. This enzyme is regulated by reversible phosphorylation; it occurs in a phosphorylated, malate-insensitive form at night and a less active, malate-sensitive dephosphorylated form during the day. The main factor controlling the phosphorylation state and activity of phosphoenolpyruvate carboxylase is the activity of phosphoenolpyruvate carboxylase kinase. This protein kinase is regulated at the level of expression; its activity is high in the middle of the night and low but finite during the day. The activity of phosphoenolpyruvate carboxylase kinase in leaf extracts of the CAM plant Kalanchoë fedtschenkoi increases markedly on dilution. This is caused by the presence of a protein that inhibits the kinase. The inhibitor protein has been separated from the kinase and purified partially. It inhibits the kinase reversibly, presumably by a direct interaction; it is neither a protease nor a protein phosphatase. The amounts of kinase and inhibitor in leaves have been estimated following separation by hydrophobic chromatography. The amount of inhibitor in K. fedtschenkoi leaves is sufficient to inhibit the basal level of kinase activity present during the day and the early stages of the night. The inhibitor has also been detected in C4 and C3 plants. The function of the phosphoenolpyruvate carboxylase kinase inhibitor may be to inhibit the basal level of kinase present in conditions under which rapid flux through phosphoenolpyruvate carboxylase is not required.