Partial Purification and Characterization of Phosphoenolpyruvate Carboxylase Protein-Serine Kinase from Illuminated Maize Leaves

Following in situ renaturation and assay of protein kinase activity after denaturing electrophoresis of relatively impure samples of maize phosphoenolpyruvate carboxylase (PEPC) kinase, a ∼30-kDa polypeptide was implicated as the best candidate for the PEPC kinase catalytic subunit. This kinase′s apparent native molecular weight was estimated at 28,000 by gel filtration on a calibrated Superose 12 column (HR 10/30), suggesting that the isolated PEPC kinase is monomeric. This protein-serine kinase was partially purified about 4000-fold from illuminated maize leaves by ammonium sulfate precipitation and sequential chromatography on Ultrogel AcA 54, hydroxylapatite, blue dextran-agarose, and an analytical AcA 54 column. Analysis by denaturing electrophoresis revealed that a 30-kDa polypeptide copurified with PEPC kinase activity during the final step. This highly purified kinase had an apparent Km (PEPC subunit) of 2.5 μM and a Km (total ATP) of 40 μM at pH 8.0, its pH optimum. Upon in vitro phosphorylation of dark-form (dephospho) C4 PEPC at Ser-15 (maize PEPC) or Ser-8 (sorghum), the malate sensitivity of the target enzyme decreased significantly. The maize PEPC kinase activity was markedly inhibited by L-malate, a negative allosteric effector of its protein substrate, in a concentration- and pH-dependent manner. Comparative phosphorylation studies with the catalytic subunit of mammalian cAMP-dependent protein kinase and casein revealed that a significant part of the malate inhibition of PEPC kinase activity in vitro was due to this effector′s interaction with PEPC. The activity of both the highly purified PEPC kinase and a less pure sample prepared rapidly in the presence of various protease inhibitors wasinsensitive to Ca2+ chelation or addition. It is concluded that the ∼30-kDa maize PEPC kinase is a low abundance, Ca2+-independent protein-serine kinase that activates its target enzyme by the exclusive phosphorylation of the regulatory serine residue near the N terminus and the resulting decrease in feedback inhibition by L-malate.