The role of the membrane-spanning and extra-membranous regions of the iron-sulfur protein in its assembly into the cytochrome bc1 complex of yeast mitochondria.

The assembly of six deletion mutants of the Rieske iron-sulfur protein into the cytochrome bc1 complex was investigated by immunoprecipitation from detergent-solubilized mitochondria with specific antisera against either the iron-sulfur protein or the intact cytochrome bc1 complex. After import, the mutant proteins lacking residues 41-55 or 66-78, located at the membrane-spanning region of the protein, and residues 182-196 located at the C-terminus of the protein, were assembled in vitro into the bc1 complex approximately 50% as effectively as the wild type iron-sulfur protein suggesting that these regions of the iron-sulfur protein may not be critical for the assembly. By contrast, only trace amounts of the mutant proteins lacking residues 80-95, 122-135, 138-153 located in the extra-membranous region of the iron-sulfur protein were assembled into the bc1 complex. After import in vitro into mitochondria isolated from a cytochrome b-deficient yeast strain, the mutants lacking residues 41-55 and 182-196 were assembled as efficiently as the wild type; however, the mutants lacking residues 55-66 and 66-78 were assembled less efficiently in the absence of cytochrome b suggesting that the hydrophobic membrane-spanning region, residues 55-78, of the iron-sulfur protein, may interact with cytochrome b during the assembly of the bc1 complex.