Cryo-EM reveals the central steps of mitochondrial complex III assembly and the cooperative assembly of supercomplex CIII2CIV

Recent advances in cryo-electron microscopy (cryo-EM) have provided new insights into the assembly process of mitochondrial complex III, a crucial component of the respiratory chain in eukaryotes. Previously, biochemical studies suggested a sequential and parallel mechanism for subunit addition in complex III, but cryo-EM studies have now challenged these assumptions, revealing a more complex and cooperative assembly process.

The cryo-EM investigation observed that subunit incorporation and folding can be uncoupled, as demonstrated by the folding of cytochrome c1. This finding deviates from the earlier understanding of parallel subunit addition in the two protomers after dimerization. Moreover, the study found that after the two protomers of complex III assemble, the assembly process does not proceed in parallel for both protomers, specifically highlighting the folding of the intermembrane space domain.

In addition to challenging the previously accepted assembly mechanism, the cryo-EM structures also shed light on the formation of the supercomplex CIII2CIV in non-vertebrates. This supercomplex is intricately intertwined with the assembly of complex III, revealing a new understanding of how these complexes form. The work suggests a generalizable model for the maturation of complex III and proposes a cooperative assembly model for supercomplex formation, both of which can potentially apply to other protein complexes.

Furthermore, the study emphasizes the critical role of cryo-EM in unraveling the complex assembly mechanisms that are not easily predictable by computational models like AlphaFold. The findings from this research not only reshape our knowledge of complex III assembly but also pave the way for further exploration of the maturation processes of other protein complexes.