Coa1 is an inner mitochondrial membrane protein that associates with Shy1 and is required for cytochrome oxidase complex IV assembly. It contains a conserved hydrophobic segment (amino acids 74-92) with the potential to form a membrane-spanning helix. The N-terminus of Coa1 is rich in positively charged amino acids and could form an amphipathic alpha helix, characteristic of a mitochondrial presequence. A cleavage site for the mitochondrial processing peptidase is predicted adjacent to the presequence. Upon in vitro import into mitochondria, Coa1 is processed to a mature form, indicating that it possesses a cleavable presequence. The eukaryotic cytochrome oxidase complex consists of 12-13 subunits, with three mitochondrial encoded subunits, Cox1-Cox3, forming the core enzyme. Translation of the Cox1 transcript requires the two promoters, Pet309 and Mss51, and the latter has an additional role in translational elongation. Coa1 is necessary for linking the activity of Mss51 to Cox1 insertion into the assembly complex.