To power up the cell, mitochondria establish a proton gradient across their inner membranes that is transformed into chemical energy in the form of ATP. But how the individual respiratory complexes of mitochondria work has been under debate.
A study published this week in Science helps reconcile those two models, presenting evidence that respiratory complexes in the mitochondria of can both form supercomplexes and remain separate, depending on the available fuel sources and the type of cell (1).
“The [solid- and fluid-state] models are extreme situations in which these machines can assemble to perform their roles in slightly different ways,” said study author José Antonio Enríquez, senior investigator at the National Center for Investigation of Cardiovascular Diseases in Madrid, Spain. “We think that our previous conception that the system works identically mechanistically was conceptually wrong.”
In 2008, Enríquez showed the presence of various supercomplexes—different combinations of the respiratory complexes CI, CII, CIII, and CIV—in mouse liver mitochondria using blue-native gel electrophoresis, a qualitative method for determining whether molecules physically associate (2). The finding added to mounting evidence that interactions between parts of the respiratory chain are much more complicated than once thought.
In the new study, the scientists characterized the physical interactions and electron flow between complexes by depleting the levels of individual complexes in mouse fibroblasts.
Screening for proteins that might be associated with supercomplexes, but not free respiratory complexes, the group found one candidate: cytochrome c oxidase subunit VIIa polypeptide 2-like (Cox7a2l) protein. After uncovering a mutation in this protein in mouse cell lines that were missing the CIII and CIV supercomplex, the team dubbed the protein SCAFI, or supercomplex assembly factor I.
“SCAFI represents the first protein whose role is just to allow the interactions of these two machines, the CIII and the CIV,” Enríquez said. The group is now looking for more proteins that might regulate assembly.
Interestingly, cells from standard C57BL/6J mice have a truncated form of SCAFI, no assembly between CIII and CIV, and a higher respiration rate. The presence of SCAFI changes the flow of electrons through the respiratory chain and seems to segment the components of the electron transport chain into distinct populations, according to the authors. The downstream effects are still unknown, however.
From the new study, “the reason why the supercomplexes are formed, at least in animal mitochondria, is now clear,” said Egbert Boekema, professor of electron microscopy at the University of Groningen in The Netherlands, who was not involved with the study. That is, these pools of supercomplexes and free complexes seem to ensure the efficient breakdown of all available substrates.
The new study relied on blue-native gel electrophoresis, which involves breaking apart cells using detergent and therefore can’t approximate what happens in situ. But the eventual plan is to investigate the process of assembly in intact cells, using superresolution imaging and other methods.
Determining the mechanisms of supercomplex assembly will be crucial for understanding mitochondrial diseases, according to Enríquez. “We think that the basic knowledge that we can get from the mouse is going to be immediately applicable to human, although we are using human cells to corroborate that,” he said.
1. Lapuente-Brun, E., R. Moreno-Loshuertos, R. Acín-Pérez, A. Latorre-Pellicer, C. Colás, E. Balsa, E. Perales-Clemente, P. M. Quirós, E. Calvo, M. A. Rodríguez-Hernández, P. Navas, R. Cruz, Ã. Carracedo, C. López-Otín, A. Pérez-Martos, P. Fernández-Silva, E. Fernández-Vizarra, and J. A. Enríquez. 2013. Supercomplex assembly determines electron flux in the mitochondrial electron transport chain. Science 340(6140):1567-1570.
2. Acín-Pérez, R., P. Fernández-Silva, M. L. L. Peleato, A. Pérez-Martos, and J. A. A. Enriquez. 2008. Respiratory active mitochondrial supercomplexes. Molecular cell 32(4):529-539.