Abstract
In C4 plants, PEPC, which is expressed in the cytosol of mesophyll cell, plays a key role in CO2 fixation. PEPC is activated by phosphorylation of the conserved Ser residue near N-terminus in response to light. Recently, we first cloned a specific protein kinase (PEPC-PK) from Flaveria trinervia, a C4 plant. Furthermore, we showed that recombinant PEPC-PK was redox-regulated. Here, we report two experimental results relating to the mechanism of redox-regulation. (1) Inactivated PEPC-PK by oxidation could be recovered with cytosolic thioredoxin system (1 μM Trx, 100 μM NADPH, 50 nM thioredoxin reductase). Trx-h, a cytosol-type Trx, activated PEPC-PK more effectively than chloroplast-type Trx-f and -m. (2) Among six Cys residues, a pairwise replacement of Cys53 and Cys250 to Ala abolished the sensitivity to oxidants and reductants. These results suggested that PEPC-PK activity was redox-regulated via thioredoxin through an intramolecular thiol/disulfide formation between Cys53 and Cys250.
