The atypical subunit composition of oxidative phosphorylation complexes is associated with original extra structural domains in Euglena gracilis

In mitochondrial oxidative phosphorylation, electron transfer from NADH or succinate to oxygen by a series of large protein complexes in the inner mitochondrial membrane (complexes I - IV) is coupled to the generation of an electrochemical proton gradient, the energy of which is utilized by F1FO ATP synthase (complex V) to generate ATP. In Euglena gracilis, a non-parasitic secondary green alga related to trypanosomes, these respiratory complexes totalize more than 40 Euglenozoa-specific subunits along with about 50 classical subunits described in other eukaryotes [1]. Here mitochondrial complexes I, III, IV and V were purified by a two-step chromatographic procedure and their subunit compositions were resolved by a three-dimensional gel electrophoresis (BN/SDS/SDS). The purified complexes were also studied by electron microscopy followed by single-particle analysis. Even if the overall structures of the four complexes are similar to the structure of canonical enzymes (e.g. from mammals), an extra domain located at the tip of the peripheral arm of complex I and a "helmet-like" domain on the top of the cytochrome c binding region in complex IV were observed. In the case of complex V, new features include a large membrane-spanning region joining the monomers, an external peripheral stalk and a structure that goes through the membrane and reaches the inter membrane space below the c-ring, the latter having not been reported for any mitochondrial F-ATPase [2, 3].