Modular assembly of mitochondrial complex III from a dynamic matrix domain pool

Cryo-EM shows Qcr1 and Qcr2 preassemble into matrix domain modules that exchange dynamically, and the Qcr1 C-terminal tail links modules to cytochrome b and cardiolipin in yeast complex III assembly.

• Why it matters: Complex III assembly requires joining a large matrix domain to a membrane-embedded core, but how Qcr1/Qcr2 are built, organized, and connected during dimer formation was unclear.
• What they did: The authors used cryo-EM on a cardiolipin-deficient yeast strain and recombinant proteins to resolve single, double, and triple matrix domains, then tested interface mutations and Qcr1 tail deletion; they also solved an assembly intermediate structure.
• The result: The module pool interconverts assembly species and exchanges subunits; interface mutations reshape this pool, Qcr1 tail deletion impairs respiratory growth, and a parkinsonism-linked tyrosine substitution exposes a conserved cysteine to oxidation.

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