Regulatory seryl-phosphorylation of C4 phosphoenolpyruvate carboxylase by a soluble protein kinase from maize leaves

A reconstituted system composed of purified phosphoenolpyruvate carboxylase (PEP-Case) and a soluble protein kinase (PK) from green maize leaves was developed to critically assess the effects of in vitro protein phosphorylation on the catalytic and regulatory (malate sensitivity) properties of the target enzyme. The PK was partially purified from light-adapted leaf tissue by ammonium sulfate fractionation (0-60% saturation fraction) of a crude extract and blue dextran-agarose affinity chromatography. The resulting preparation was free of PEPCase. This partially purified protein kinase activated PEPCase from dark-adapted green maize leaves in an ATP-, Mg2+-, time-, and temperature-dependent fashion. Concomitant with these changes in PEPCase activity was a marked decrease in the target enzyme's sensitivity to feedback inhibition by L-malate. The PK-mediated incorporation of 32P from [gamma-32P]ATP into the protein substrate was directly correlated with these changes in PEPCase activity and malate sensitivity. The maximal molar 32P-incorporation value was about 0.25 per 100-kDa PEPCase subunit (i.e., 1 per holoenzyme). Phosphoamino acid analysis of the 32P-labeled target enzyme by two-dimensional thin-layer electrophoresis revealed the exclusive presence of phosphoserine. These in vitro results, together with our recent studies on the light-induced changes in phosphorylation status of green maize leaf PEPCase in vivo (J. A. Jiao and R. Chollet (1988) Arch. Biochem. Biophys. 261, 409-417), collectively provide the first unequivocal evidence that the seryl-phosphorylation of the dark-form enzyme by a soluble protein kinase is responsible for the changes in catalytic activity and malate sensitivity of C4 PEPCase observed in vivo during dark/light transitions of the parent leaf tissue.