In-cell architecture of the mitochondrial respiratory chain

Mitochondria produce energy through oxidative phosphorylation, carried out by five membrane-bound complexes collectively known as the respiratory chain. These complexes work in concert to transfer electrons and pump protons, leading to ATP regeneration. The precise organization of these complexes in native cells is debated, notably their assembly into higher-order supercomplexes called respirasomes. Here, we use in situ cryo-electron tomography to visualize the native structures and organization of several major mitochondrial complexes inside Chlamydomonas reinhardtii cells. ATP synthases and respiratory complexes are segregated into curved and flat crista membrane domains, respectively. Respiratory complexes I, III, and IV assemble into a single type of respirasome, from which we determined a native 5 Angstrom-resolution structure showing the binding of electron carrier cytochrome c. Combined with single-particle cryo-electron microscopy reconstruction at 2.4 Angstrom resolution, we assemble a detailed model of how the respiratory complexes interact with each other inside native mitochondria.