The mitochondrial members of the highly conserved AAA family, Yta10p and Yta12p, constitute a membrane-embedded complex of about 850 kDa. As an ATP dependent metallopeptidase (AAA protease), the YTA10–12 complex mediates the degradation of nonassembled inner membrane proteins. In contrast to nucleotide-dependent complex formation and substrate binding, proteolysis of bound polypeptides depends on the hydrolysis of ATP and the metallopeptidase activity of both subunits. Independent of its proteolytic function, the chaperone-like activity of the YTA10–12 complex is required for assembly of the membrane-associated ATP synthase. We propose that proteolytic and chaperone-like activities in the YTA10–12 complex mediate assembly and degradation processes of membrane protein complexes and thereby exert key functions in the maintenance of membrane integrity.