Modulation of phosphoenolpyruvate carboxylase phosphorylation in leaves of amaranthus hypochondriacus, a NAD-ME Type of C₄ Plant

Abstract

PEP carboxylase (PEPC) in leaves of C4 plants is activated by phosphorylation of enzyme by a PEPC-protein kinase (PEPC-PK). We reevaluated the pattern of PEPC phosphorylation in leaf extracts of Amaranthus hypochondriacus. It was dependent on Ca²⁺, the optimum concentration of which for stimulation was 10 mM. The extent of stimulation was inhibited by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a Ca²⁺ chelator. The inhibition by BAPTA was relieved by the addition of Ca²⁺ but not by the addition of Mg²⁺. The stimulation by Ca²⁺ of PEPC phosphorylation was marginally enhanced by calmodulin (CaM), but not by diacylglycerol (DAG). Phosphorylation was strongly restricted by Ca²⁺ or Ca²⁺-CaM-dependent protein kinase inhibitors. Thus phosphorylation of PEPC is Ca²⁺-dependent in leaves of A. hypochondriacus and a calcium-dependent protein kinase (CDPK) may modulate PEPC-PK and subsequently the phosphorylation status of PEPC.