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Reversible seryl-phosphorylation contributes to the light/dark regulation of C(4)-leaf phosphoenolpyruvate carboxylase (PEPC) activity in vivo. The specific regulatory residue that, upon in vitro phosphorylation by a maize-leaf protein-serine kinase(s), leads to an increase in catalytic activity and a decrease in malate-sensitivity of the target enzyme has been recently identified as Ser-15 in (32)P-phosphorylated/activated dark-form maize PEPC (J-A Jiao, R Chollet [1990] Arch Biochem Biophys 283: 300-305). In order to ascertain whether this N-terminal seryl residue is, indeed, the in vivo regulatory phosphorylation site, [(32)P]phosphopeptides were isolated and purified from in vivo(32)P-labeled maize and sorghum leaf PEPC and subjected to automated Edman degradation analysis. The results show that purified light-form maize PEPC contains 14-fold more (32)P-radioactivity than the corresponding dark-form enzyme on an equal protein basis and, more notably, only a single N-terminal serine residue (Ser-15 in maize PEPC and its structural homolog, Ser-8, in the sorghum enzyme) was found to be (32)P-phosphorylated in the light or dark. These in vivo observations, combined with the results from our previous in vitro phosphorylation studies (J-A Jiao, R Chollet [1989] Arch Biochem Biophys 269: 526-535; [1990] Arch Biochem Biophys 283: 300-305), demonstrate that an N-terminal seryl residue in C(4) PEPC is, indeed, the regulatory site that undergoes light/dark changes in phosphorylation-status and, thus, plays a major, if not cardinal role in the light-induced changes in catalytic and regulatory properties of this cytoplasmic C(4)-photosynthesis enzyme in vivo.

In Vivo Regulatory Phosphorylation Site in C4-Leaf Phosphoenolpyruvate Carboxylase from Maize and Sorghum

In Vivo Regulatory Phosphorylation Site in C₄-Leaf Phosphoenolpyruvate Carboxylase from Maize and Sorghum