CcdA is a thylakoid membrane protein required for the transfer of reducing equivalents from stroma to thylakoid lumen in the higher plant chloroplast.

In order to transfer reducing equivalents into the thylakoid lumen, a specific thylakoid membrane transfer system is suggested that mediates the disulfide bond reduction of proteins in the thylakoid lumen of higher plant chloroplasts. In this system, although stromal thioredoxin can supply the reducing equivalents to a thioredoxin-like protein HCF164 in the thylakoid lumen, a mediator protein for electron transfer in the thylakoid membranes is proposed to be required to link the two suborganellar compartments. CcdA is a candidate protein as a component for this transfer system since CcdA- and HCF164-deficient mutants in Arabidopsis thaliana show the same phenotype. We now show that CcdA is localized in the thylakoid membrane and that its redox state, as well as that of HCF164, is modulated in thylakoids by stromal m-type thioredoxin. Our results strongly suggest that CcdA may act as a mediator in thylakoid membranes by transferring reducing equivalents from the stromal to the lumenal side of the thylakoid membrane in chloroplasts.