Phosphoenolpyruvate carboxylase kinase is a novel protein kinase regulated at the level of expression

Phosphorylation of phosphoenolpyruvate carboxylase plays a key role in the control of plant metabolism. Phosphoenolpyruvate carboxylase kinase is a Ca(2+)-independent enzyme that is activated by a process involving protein synthesis in response to a range of signals in different plant tissues. The component whose synthesis is required for activation has not previously been identified, nor has the kinase been characterised at a molecular level. We report the cloning of phosphoenolpyruvate carboxylase kinase from the Crassulacean Acid Metabolism plant Kalanchoe fedtschenkoi and the C(3) plant Arabidopsis thaliana. Surprisingly, phosphoenolpyruvate carboxylase kinase is a member of the Ca(2+)/calmodulin-regulated group of protein kinases. However, it lacks the auto-inhibitory region and EF hands of plant Ca(2+)-dependent protein kinases, explaining its Ca(2+)-independence. Its sequence is novel in that it comprises only a protein kinase catalytic domain with no regulatory regions; it appears to be the smallest known protein kinase. In K. fedtschenkoi, the abundance of phosphoenolpyruvate carboxylase kinase transcripts increases during leaf development. The transcript level in mature leaves is very low during the photoperiod, reaches a peak in the middle of the dark period and correlates with kinase activity. It exhibits a circadian oscillation in constant conditions. Protein kinases are typically regulated by second messengers, phosphorylation or protein/protein interactions. Phosphoenolpyruvate carboxylase kinase is an exception to this general rule, being controlled only at the level of expression. In K. fedtschenkoi, its expression is controlled both developmentally and by a circadian oscillator.