The phosphorylation of Sorghum leaf phosphoenolpyruvate carboxylase is a Ca++-calmodulin dependent process

Regulation of the in vitro phosphorylation process of the photosynthetic form (G form) of Sorghum leaf Phosphoenolpyruvate carboxylase (PEPC: EC 4.1.1.31) was studied. Results established that: 1) PEPC was efficiently phosphorylated on seryl residues in crude leaf extract 2) Pyruvate, orthophosphate dikinase (EC 2.7.9.1.) which has been supposed to interfere with the process, was found not to be significantly phosphorylated in our experimental conditions 3) KF, as well as both Ca++ and Mg++ ions increased the radioactive signal detected 4) addition of EDTA or EGTA nullified it and Ca++ alone was found to reverse the inhibitory effect exerted by both chelators 5) addition of anti-Calmodulin antibodies to the medium also abolished the PEPC phosphorylation. Present data demonstrated that the post-translational modification of the C4-plant photosynthetic PEPC is a Ca++/Calmodulin dependent process.