The N-terminus of the Qcr7 Protein of the Cytochrome bc1 Complex in S. cerevisiae May Be Involved in Facilitating Stability of the Subcomplex with the Qcr8 Protein and Cytochrome b

Subunit 7 or the Qcr7 protein of the cytochrome bc1 complex in yeast is a nuclear-encoded 14-kDa protein and is essential for formation of a functional bc1 complex and respiration. It was shown previously that the N-terminal region of the Qcr7 protein might play little role in import but may be essential for correct assembly of the bc1 complex. To examine the role of the N-terminus in assembly of the bc1 complex, various N-terminus deletion and point mutants of the QCR7 gene were expressed in yeast strains where the endogenous QCR7 gene has been inactivated. Deletion of the first 8–15 residues after methionine at the N-terminus of the Qcr7 protein was studied and it was shown that except for the Qcr7p-Δ7 mutant, the strains overexpressing deletion mutants (Qcr7p-Δ8 to Qcr7p-Δ14) displayed decreased steady-state levels of iron–sulfur protein (ISP) and 14-kDa (Qcr7) and 11-kDa (Qcr8) subunits, as well as a respiratory defect. It was shown that introducing mutations at the N-terminus of the QCR7 gene in the Δ7 context had more drastic effects on respiration and assembly than in the full-length context, suggesting that the first seven residues at the N-terminus have a role in increasing the stability of the holocomplex. This led to a respiratory-deficient phenotype and drastic decrease in the steady-state levels of Qcr7 (14-kDa) and Qcr8 (11-kDa) proteins. The mutants Qcr7p-Δ7 (R10I), Qcr7p-Δ7 (G12V), Qcr7p-Δ7 (D13G), Qcr7p-Δ7 (D13V), and Qcr7p-Δ7 (D13Y) showed decreases in the steady-state levels of ISP and 14- and 11-kDa subunits, as well as defects in respiration. These results are interpreted in the light of the X-ray crystal structure of the yeast bc1 complex.