New complexes containing the internal alternative NADH dehydrogenase (Ndi1) in mitochondria of Saccharomyces cerevisiae

Mitochondria of Saccharomyces cerevisiae lack the respiratory complex I, but contain three rotenone-insensitive NADH dehydrogenases distributed on both the external (Nde1 and Nde2) and internal (Ndi1) surfaces of the inner mitochondrial membrane. These enzymes catalyse the transfer of electrons from NADH to ubiquinone without the translocation of protons across the membrane. Due to the high resolution of the Blue Native PAGE (BN-PAGE) technique combined with digitonin solubilization, several bands with NADH dehydrogenase activity were observed on the gel. The use of specific S. cerevisiae single and double mutants of the external alternative elements (ΔNDE1, ΔNDE2, ΔNDE1/ΔNDE2) showed that the high and low molecular weight complexes contained the Ndi1. Some of the Ndi1 associations took place with complexes III and IV, suggesting the formation of respirasome-like structures. Complex II interacted with other proteins to form a high molecular weight supercomplex with a molecular mass around 600 kDa. We also found that the majority of the Ndi1 was in a dimeric form, which is in agreement with the recently reported three-dimensional structure of the protein.