An Src Homology 3 Domain-Like Fold Protein Forms a Ferredoxin Binding Site for the Chloroplast NADH Dehydrogenase-Like Complex in Arabidopsis

Some subunits of chloroplast NAD(P)H dehydrogenase (NDH) are related to those of the respiratory complex I, and NDH mediates photosystem I (PSI) cyclic electron flow. Despite extensive surveys, the electron donor and its binding subunits have not been identified. Here, we identified three novel components required for NDH activity. CRRJ and CRRL are J- and J-like proteins, respectively, and are components of NDH subcomplex A. CRR31 is an Src homology 3 domain-like fold protein, and its C-terminal region may form a tertiary structure similar to that of PsaE, a ferredoxin (Fd) binding subunit of PSI, although the sequences are not conserved between CRR31 and PsaE. Although CRR31 can accumulate in thylakoids independently of NDH, its accumulation requires CRRJ, and CRRL accumulation depends on CRRJ and NDH. CRR31 was essential for the efficient operation of Fd-dependent plastoquinone reduction in vitro. The phenotype of crr31 pgr5 suggested that CRR31 is required for NDH activity in vivo. We propose that NDH functions as a PGR5-PGRL1 complex-independent Fd: plastoquinone oxidoreductase in chloroplasts and rename it the NADH dehydrogenase-like complex.