The ribosome-associated Mba1 escorts Cox2 from insertion machinery to maturing assembly intermediates.

The three conserved core subunits of the cytochrome c oxidase are mitochondrial-encoded in close to all eukaryotes. The Cox2 subunit spans the inner membrane twice, exposing N- and C-terminus into the intermembrane space. For this the N-terminus is exported cotranslationally by Oxa1 and subsequently undergoes proteolytic maturation in Saccharomyces cerevisiae Little is known about the translocation of the C-terminus but Cox18 has been identified as a critical protein in this process. Here we find that the scaffold protein Cox20, which promotes processing of Cox2, is in complex with the ribosome-receptor Mba1 and translating mitochondrial ribosomes in a Cox2-dependent manner. The Mba1-Cox20 complex accumulates when export of the C-terminus of Cox2 is blocked by loss of the Cox18 protein. While Cox20 engages with Cox18, Mba1 is no longer present at this stage. Our analyses indicate that Cox20 associates with nascent Cox2 and Mba1 to promote Cox2 maturation cotranslationally. We suggest that Mba1 stabilizes the Cox20-ribosome complex and supports handover of Cox2 to the Cox18 tail export machinery.