MiP2005: Session 5 - Young Investigator Presentation
Mitochondrial Physiology Network 10.9: 63-64 (2005) - download pdf
The early stage of Complex I assembly is linked with Complex III and IV supercomplex.
Christophe Rocher*, W Fan, E Ruiz-Pesini, DC Wallace
Center Molecular Mitochondrial Medicine Genetics, Univ. California, Irvine, CA 92697-3940; *Present address: U 688 - INSERM, Physiopathologie Mitochondriale, Univ. Victor Segalen Bordeaux 2, Bordeaux, France. - email@example.com
Mitochondrial respiratory chain complex I is the largest complex of the oxidative phosphorylation (OXPHOS) system with about 46 subunits. This complex has a L-shaped structure, with an arm buried in the mitochondrial inner membrane and the other arm protruding in the mitochondrial matrix. According to a recently proposed model of a modular complex I assembly, these two parts are preassembled independently and then combined to form a full complex .
In order to study this model, we used a mouse cell line with a frameshift mutation in the ND6 mitochondrial complex I subunit gene and a consequent lack of ND6 synthesis. Results obtained by Western blot after PAGE and BN-PAGE showed a lack of full complex I assembly, confirmed by absence of (1) complex I enzyme activity and (2) O2 consumption on pyruvate-malate. In addition, we found some assembly intermediates for the matrix arm but not for the membrane arm, which confirmed the assembly sequence proposed earlier .
Moreover, we found a high molecular weight complex I assembly intermediate including the 17kD subunit. Further investigations revealed a specific interaction between this subunit and supercomplex III and IV. This result was obtained only in our cell model but not in control cells, which may underline a putative early step for complex I assembly, involving complex III and IV. Indeed, the 17 kD subunit that is the first protein implied in the membrane arm assembly pathway  may need to be associated to these complexes in order to be associated with the ND6 subunit. This is stressed by the fact that in our ND6 deficient cell line, we found also an increase of complex III and complex IV activity and protein amount, reinforcing the hypothesis of a link of these two complexes in complex I assembly.
As already shown , OXPHOS supercomplexes are significant not only for a biochemical goal like substrate channeling but also for assembly and stabilization of protein complexes. Indeed, they suggested that a lack of complex III assembly had an effect on complex I assembly. Putting these results together, we propose that the association of the complex I 17 kD subunit and supercomplex III+IV may be the first step of complex I assembly.
1. Ugalde C, Vogel R, Huijbens R, Van Den Heuvel B, Smeitink J, Nijtmans L (2004) Human mitochondrial complex I assembles through the combination of evolutionary conserved modules: a framework to interpret complex I deficiencies. Hum. Mol. Genet. 13: 2461-2472.
2. Acin-Perez R, Bayona-Bafaluy MP, Fernandez-Silva P, Moreno-Loshuertos R, Perez-Martos A, Bruno C, Moraes CT, Enriquez JA (2004) Respiratory complex III is required to maintain complex I in mammalian mitochondria. Mol. Cell. 13: 805-815.