Abstract
Chloroplast NAD(P)H dehydrogenase (NDH) is a homolog of complex I in mitochondria and bacteria. NDH mediates the backflow of electrons from stroma reductants to plastoquinone in cyclic electron transport around photosystem I (CET). Recently, the biochemical analysis indicated that NDH forms a supercomplex with PSI in thylakoid membranes. However, in contrast to the elucidation of fine structure of complex I, the entire subunit composition and electron input site of NDH are still unclear. Here we report the identification and characterization of a novel protein required for NDH activity. Its homologs were also found in cyanobacteria but not in Chlamydomonas, which lacks NDH. The protein localizes to thylakoid membranes but has no transmembrane and any known motifs. The Arabidopsis knockout mutant did not show a transient increase in Fo of Chl fluorescence after turning off of actinic light, indicating the impairment of NDH activity in vivo. This protein co-migrated with NDH-PSI supercomplex on BN-PAGE. However, the lack of the protein did not affect the stabilities of the supercomplex and other NDH subunits.
