Illumination increases the phosphorylation state of maize leaf phospho enolpyruvate car☐ylase by causing an increase in the activity of a protein kinase

Illumination of maize leaves increases the phosphorylation state of phospho enol pyruvate carboxylase and reduces the sensitivity of the enzyme to feedback inhibition by malate. Red, white and blue light were each found to be equally potent, and the effect of light was blocked by 3(3,4-dichlorophenyl)-1,1-dimethylurea. A phospho enol pyruvate carboxylase kinase was partially purified from illuminated maize leaves by a three-step procedure. Phosphorylation of phospho enol pyruvate carboxylase by this protein kinase reached 0.7-0.8 molecules/subunit and correlated with a 3- to 4-fold increase in K(i) for malate. The protein kinase was inhibited by L-malate, but was insensitive to a number of other potential regulators. Freshly prepared and desalted extracts of darkened maize leaves contained very little kinase activity, but the activity appeared when leaves were illuminated for 30-60 min before extraction. The catalytic subunit of protein phosphatase 2A from rabbit skeletal muscle, but not that of protein phosphatase 1, could dephosphorylate phospho enol pyruvate carboxylase. The protein phosphatases 1 and 2A activities of maize leaves were not affected by illumination. It is suggested that the major means by which light stimulates the phosphorylation of phospho enol pyruvate carboxylase is by an increase in the activity of the protein kinase.