Coi1 is a novel assembly factor of the yeast complexes III-IV supercomplex

Abstract

Mitochondria are important organelles that are required for generation of ATP, and are also involved in regulation and monitoring various cellular processes. They are divided into several sub-compartments, among them is the mitochondrial inner membrane (IM), which harbors the highly structured multi-subunit respiratory chain complexes. Most of the subunits of these complexes are nuclear-encoded and imported from the cytosol into mitochondria. A small group of the subunits of the respiratory complexes are synthesized within the organelle and are then directed to their proper destination. Components of the respiratory chain complexes from both origins are assembled into the mitochondrial IM with the help of various assembly factors. The integration of the core subunits of cytochrome bc1 complex (complex III) and cytochrome c oxidase complex (complex IV) into the IM is well studied. However, the assembly and organization of their supercomplexes are still not completely understood. This work characterizes a previously uncharacterized yeast protein (ORF YDR381C-A) that we named cytochrome c oxidase interacting protein 1 (Coi1). The protein was found to be a new regulatory factor that is involved in the assembly of complex IV and supercomplexes. The results demonstrated that Coi1 is a small mitochondrial IM protein with a single transmembrane segment toward its N-terminus whereas the C-terminus faces the intermembrane space (IMS). Deletion of COI1 leads to a severe growth phenotype on non-fermentable carbon sources, a decrease in membrane potential, hampered respiration, and reduced enzymatic activity of complex IV. In addition, the steady-state levels of subunits of complexes III and IV as well as of the assembled complexes and supercomplexes are reduced. Furthermore, Coi1 was found to physically interact with subunits of both complexes, although it appears to be a stoichiometric subunit neither of complex III nor of complex IV. Of note in absence of Coi1, heme A in complex IV was completely diminished. Collectively, this work identifies a novel protein that plays a role in the assembly of the mitochondrial respiratory chain supercomplexes.