Regulation of phosphoenolpyruvate carboxylase: an example of signal transduction via protein phosphorylation in higher plants

There is now good evidence that the malate sensitivity of PEPc is regulated by phosphorylation/dephosphorylation in the leaf tissue of C4 and CAM plants. This statement is based on the assessment of the phosphorylation state of PEPc in [32P]-labeled intact tissue by immunoprecipitation and the correlation between phosphorylation state and malate sensitivity that has been observed during incubation of purified PEPc in vitro with protein kinases or protein phosphatases. The phosphorylation of PEPc in the CAM plant B. fedtschenkoi is controlled by an endogenous rhythm whereas that PEPc in the C4 plant maize is triggered directly by light. In neither case has the mechanism of signal transduction been identified. It is hoped that further work on the protein kinases and protein phosphatases involved will reveal the nature of the signalling systems. Preliminary work suggests that plant protein phosphatases are very similar to their mammalian counterparts. It is also noteworthy that higher plant genes very similar to the genes encoding the cyclic nucleotide-dependent protein kinases and the protein kinase C family have recently been identified (31). It is interesting to speculate that the protein kinases and phosphatases involved in signal transduction systems in plants may prove to be closely related to well-studied mammalian enzymes