In-cell architecture of the mitochondrial respiratory chain.

Mitochondria regenerate adenosine triphosphate (ATP) through oxidative phosphorylation. This process is carried out by five membrane-bound complexes collectively known as the respiratory chain, working in concert to transfer electrons and pump protons. The precise organization of these complexes in native cells is debated. We used in situ cryo–electron tomography to visualize the native structures and organization of several major mitochondrial complexes in Chlamydomonas reinhardtii cells. ATP synthases and respiratory complexes segregate into curved and flat crista membrane domains, respectively. Respiratory complexes I, III, and IV assemble into a respirasome supercomplex, from which we determined a native 5-angstrom (Å) resolution structure showing binding of electron carrier cytochrome c. Combined with single-particle cryo–electron microscopy at 2.4-Å resolution, we model how the respiratory complexes organize inside native mitochondria. Editor's summary: The mitochondrial respiratory chain consists of membrane-integral complexes that build up the proton gradient across the membranes of the cristae, thereby powering ATP synthase. Waltz et al. used cryo–electron tomography to resolve native structures of these complexes directly within the green alga Chlamydomonas reinhardtii. Their findings reveal how respiratory complexes I, III, and IV assemble into a respirasome supercomplex, which is restricted to flat membrane regions apart from rows of ATP synthase at the curved tips of cristae. The work also captures fine structural details of how respirasomes are held together and how electron-carrier proteins bind these assemblies, offering insights into mitochondrial respiration inside native cells. —Stella M. Hurtley [ABSTRACT FROM AUTHOR]