Novel nuclear-encoded subunits of the chloroplast NAD(P)H dehydrogenase complex.

The NAD(P)H dehydrogenase (NDH) complex functions in photosystem I cyclic electron transfer in higher plant chloroplasts and is crucial for plant responses to environmental stress. Chloroplast NDH complex is a close relative to cyanobacterial NDH-1L complex, and all fifteen subunits so far identified in NDH-1L have homologs in the chloroplast NDH complex. Here we report on the identification of two nuclear-encoded proteins NDH48 and NDH45 in higher plant chloroplasts and show their intimate association with the NDH complex. These two membrane proteins are shown to interact with each other and with the NDH complex enriched in stroma thylakoids. Moreover, the deficiency of either the NDH45 protein or the NDH48 protein in respective mutant plants leads to severe defects in both the accumulation and the function of the NDH complex. The NDH48 and NDH45 proteins are not components of the hydrophilic connecting domain of the NDH complex but are strongly attached to the hydrophobic membrane domain. We conclude that NDH48 and NDH45 are novel nuclear-encoded subunits of the chloroplast NDH complex and crucial both for the stable structure and function of the NDH complex.