Role of positively charged transmembrane segments in the insertion and assembly of mitochondrial inner-membrane proteins

The biogenesis of membrane oligomeric complexes is an intricate process tha t requires the insertion and assembly of transmembrane (TM) domains into th e lipid bilayer. The Oxa1p family plays a key role in this process in organ elles and bacteria. Hell et al. (2001, EMBO J., 20, 1281-1288) recently hav e proposed that Oxa1p could act as part of a general membrane insertion mac hinery for mitochondrial respiratory complex subunits. We have previously s hown that mutations in the TMI domain of Cyt1p can partially compensate for the absence of Oxa1p. Here, we demonstrate that a single amino acid substi tution in the TMI domain of Qcr9p can bypass Oxa1p in yeast. Qcr9p and Cyt1 p are two subunits of the respiratory complex bc1 and their relative roles in the assembly of other respiratory complexes have been investigated. The mutations we have isolated in Cyt1p or Qcr9p introduce positively charged a mino acids, and we show that the mutant TM domain of Cyt1p mediates the res toration of complex assembly. We propose that the positive charges introduc ed in Cyt1p and Qcr9p TM domains promote interactions with negatively charg ed TM domains of other respiratory complex subunits, allowing the coinserti on of both domains into the membrane, in the absence of Oxa1p. This model a rgues in favor of a role of Oxa1p in the insertion and the lateral exit of less hydrophobic TM domains from the translocation site into the lipid bila yer.