Posttranslational regulation of phosphoeno/pyruvate carboxylase in C4 and crassulacean acid metabolism plants

Abstract

Control of C4 photosynthesis and Crassulacean acid metabolism (CAM) is, in part, mediated by the diel regulation of phosphoeno/pyruvate carboxylase (PEPC) activity. The nature of this regulation of PEPC in the leaf cell cytoplasm of C4 and CAM plants is both metabolite-related and posttranslational. Specifically, the regulatory properties of the enzyme vary in accord with the physiological activity of C4 photosynthesis and CAM: PEPC is less sensitive to feedback Inhibition by L-malate under light (C4 plants) or at night (CAM plants) than in darkness (C4) or during the day (CAM). While the view that a light-induced change in the aggregation state of the holoenzyme is a general mechanism for the diel regulation of PEPC activity in CAM plants is currently in dispute, there is no supportive in vivo evidence for such a tetramer/dimer interconversion in C4 plants. In contrast, a wealth of in vitro and in vivo data has accumulated in support of the view that the reversible phosphorylation of a specific, N-terminal regulatory serine residue in PEPC (e.g. Ser-15 or Ser-8 in the maize or sorghum enzymes, respectively) plays a key, if not cardinal, role in the posttranslational regulation of the carboxylase by light/ dark or day/night transitions in both C4 and CAM plants, respectively.