Mitochondrial contact site and cristae organizing system promotes modular assembly of cytochrome c oxidase

Mitochondrial cytochrome c oxidase, the complex IV of the respiratory chain, is assembled in a modular fashion from mitochondrial- as well as nuclear-encoded subunits, guided by numerous assembly factors. This intricate process is further complicated by the characteristic architecture of the inner mitochondrial membrane with its functional asymmetry. The mitochondrial contact site and cristae organizing system (MICOS) maintains the stability of crista junctions that connect the cristae, the site of mitochondrial respiration, with the inner boundary membrane where newly imported respiratory subunits first arrive. Here, we report that MICOS facilitates specific assembly steps of complex IV and associates with intermediates of the Cox1 and Cox3 modules. Moreover, MICOS recruits a variety of assembly factors even in the absence of ongoing complex IV biogenesis. Our results establish MICOS as an important agent in efficient respiratory chain assembly that directly supports and coordinates complex IV biogenesis within the complex inner membrane architecture.