THE PLASTID NDH GENES CODE FOR AN NADH-SPECIFIC DEHYDROGENASE - ISOLATION OF A COMPLEX-I ANALOG FROM PEA THYLAKOID MEMBRANES

The plastid genomes of several plants contain ndh genes-homologues of genes encoding subunits of the proton-pumping NADH:ubiquinone oxidored uctase, or complex I, involved in respiration in mitochondria and euba cteria, From sequence similarities with these genes, the ndh gene prod ucts have been suggested to form a large protein complex (Ndh complex) ; however, the structure and function of this complex remains to be es tablished, Herein we report the isolation of the Ndh complex from the chloroplasts of the higher plant Pisum sativum, The purification proce dure involved selective solubilization of the thylakoid membrane with dodecyl maltoside, followed by two anion-exchange chromatography steps and one size-exclusion chromatography step, The isolated Ndh complex has an apparent total molecular mass of approximately 550 kDa and acco rding to SDS/PAGE consists of at least 16 subunits including NdhA, Ndh I, NdhJ, NdhK, and NdhH, which were identified by N-terminal sequencin g and immunoblotting. The Ndh complex showed an NADH-and deamino-NADH- specific dehydrogenase activity, characteristic of complex I, when eit her ferricyanide or the quinones menadione and duroquinone were used a s electron accepters, This study describes the isolation of the chloro plast analogue of the respiratory complex I and provides direct eviden ce for the function of the plastid Ndh complex as an NADH:plastoquinon e oxidoreductase. Our results are compatible with a dual role for the Ndh complex in the chlororespiratory and cyclic photophosphorylation p athways.