NADH:ubiquinone oxidoreductase from bovine heart mitochondria: sequence of a novel 17.2-kDa subunit

The sequences of 41 subunits of complex I (NADH:ubiquinone oxidoreductase) from bovine heart mitochondria have been described previously. Seven of them are encoded in mitochondrial DNA, and the remainder are nuclear gene products that are imported into the organelle from the cytoplasm. By electrospray mass spectrometry experiments conducted on complex I and on two related subcomplexes, an additional protein has been identified with a mass not corresponding to any of the known subunits of the enzyme. This protein has also been found in samples of the enzyme fractionated on two dimensional polyacrylamide gels. Material from these gels has been digested with trypsin and peptide sequences have been determined, confirming that the protein did not correspond to any of the known subunits of complex I. The cDNA sequence of this protein, determined with the aid of the peptide sequences, demonstrates that it is a novel subunit of complex I, and that it is related to a 13-kDa human protein associated with differentiation.