The cytochrome bc1 complex (also known as ubiquinol:cytochrome c oxidoreductase, ubiquinol:ferricytochrome c oxidoreductase, and respiratory complex III) is a highly conserved enzyme of the mitochondrial respiratory chain (reviewed in 4). In S. cerevisiae it consists of three catalytic subunits, Cobp, Rip1p, and Cyt1p, plus seven additional subunits: Cor1p, Qcr2p, Qcr6p, Qcr7p, Qcr8p, Qcr9p, and Qcr10p (4, 5). The crystal structure of the complex shows that two functional units, each containing these ten subunits, associate with each other in the mitochondrial inner membrane (6). Assembly of a functional complex requires two proteins, Cbp3p and Cbp4p, that are not components of the complex but may associate with it during assembly (7). It also requires Bcs1p, an AAA-family ATPase that interacts with a precursor of the complex to mediate incorporation of the Rip1p and Qcr10p subunits (8). The mechanism of energy transfer by the complex, known as the protonmotive Q cycle, has been studied in detail (reviewed in 4). The net result of the Q cycle is the stepwise transfer of an electron through the complex from ubiquinol to cytochrome c (Cyc1p), coupled with the translocation of a proton across the mitochondrial inner membrane (4). The function of the cytochrome bc1 complex is essential to the energy-generating process of oxidative phosphorylation, which is carried out by the enzyme complexes of the mitochondrial respiratory chain.

Qcr6p is required for activity of the cytochrome bc1 complex at high temperature: the null mutant displays normal respiratory growth at 30 degrees C, but is unable to respire and lacks complex III activity at 37 degrees (1). At the nonpermissive temperature, the qcr6 null mutant also accumulates unprocessed cytochrome c1 (Cyt1p) (1). The response to varying ionic strength of complex III activity in the qcr6 null mutant suggests that the relatively acidic Qcr6p may have a role in association of the basic cytochrome c (Cyc1p) with the complex (9). Qcr6p has a 25-amino acid presequence that is cleaved upon mitochondrial import but, in contrast to most other mitochondrial presequences, is not required for import (10). Qcr6p is conserved in other eukaryotes and is homologous to the so-called "hinge" protein subunit of bovine complex III, which is thought to mediate interaction between cytochromes c1 and c (2).

Last updated: 2007-07-26