Phosphoenolpyruvate carboxylase kinase is a novel protein kinase regulatedat 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 synt hesis in response to a range of signals in different plant tissues. The com ponent whose synthesis is required for activation has not previously been i dentified, nor has the kinase been characterised at a molecular level. We r eport the cloning of phosphoenolpyruvate carboxylase kinase from the Crassu lacean Acid Metabolism plant Kalanchoe fedtschenkoi and the C-3 plant Arabi dopsis thaliana. Surprisingly, phosphoenolpyruvate carboxylase kinase is a member of the Ca2+/calmodulin-regulated group of protein kinases. However, it lacks the auto-inhibitory region and EF hands of plant Ca2+-dependent pr otein kinases, explaining its Ca2+-independence. Its sequence is novel in t hat it comprises only a protein kinase catalytic domain with no regulatory regions; it appears to be the smallest known protein kinase. In K. fedtsche nkoi, the abundance of phosphoenolpyruvate carboxylase kinase transcripts i ncreases during leaf development. The transcript level in mature leaves is very low during the photoperiod, reaches a peak in the middle of the dark p eriod and correlates with kinase activity. It exhibits a circadian oscillat ion in constant conditions. Protein kinases are typically regulated by seco nd messengers, phosphorylation or protein/protein interactions. Phosphoenol pyruvate carboxylase kinase is an exception to this general rule, being con trolled only at the level of expression. In K. fedtschenkoi, its expression is controlled both developmentally and by a circadian oscillator.