Zea mays C₄-type PEP carboxylase (PEPC) : Mechanisms of Malate-desensitization by Phosphorylation and Recognition of PEPC by PEPC kinase (PEPCk)

Abstract

PEPC catalyzes the primary CO₂-fixation reaction in C₄ photosynthesis. Zea mays PEPC (ZmPEPC) is feedback-inhibited by L-malate (MA) in an allosteric manner. Plant enzyme is also regulated by phosphorylation of Ser near the N-terminus by a specific protein kinase (PEPCk), which reduces the sensitivity to MA.
We constructed two mutant PEPCs, ΔN_1-34ZmPEPC, plant specific N-terminus region truncated mutant of ZmPEPC, and +N_1-34EcPEPC, N-terminus region of ZmPEPC attached to E.coli PEPC (EcPEPC). ΔN_1-34ZmPEPC showed reduced sensitivity to MA. Although almost the same amount of ³²P as wild-type ZmPEPC was incorporated into +N_1-34EcPEPC, its sensitivity to MA was not affected by phosphorylation. These results suggest that not only N-terminus but also the other plant-specific sites may be involved in the desensitization of ZmPEPC, followed by phosphorylation. The phosphorylation of wild-type ZmPEPC was inhibited either by ΔN_1-34ZmPEPC or by wild-type EcPEPC. It suggests that PEPCk recognizes some sites of ZmPEPC, besides N-terminus.