Isolation, Characterisation and Crystallisation of a Water‐Soluble Fragment of the Rieske Iron‐Sulfur Protein of Bovine Heart Mitochondrial bc1 Complex

A water‐soluble fragment of the bc1 complex from bovine heart mitochondria was isolated containing the intact Rieske [2Fe‐2S] cluster. The fragment consists of the last 129 amino acid residues of the Rieske iron‐sulfur protein and has a molecular mass of 14592 Da including two iron atoms. The absorption, visible CD, and EPR spectra of the fragment are indistinguishable from those of the membrane‐bound iron‐sulfur protein. The redox potential as determined by EPR‐monitored redox titration was +306 mV. The far‐ultraviolet CD spectrum is indicative of a protein with little regular secondary structure, while significant α‐helix content was detected in the membrane anchor of the complete iron‐sulfur protein. The fragment could be crystallized using poly(ethylene glycol) 6000 as precipitant. Needle‐shaped single crystals have been grown by the hanging‐drop vapor diffusion technique. These crystals belong to the space group P21 and diffract well beyond 0.2 nm resolution. Phase determination using the multiple‐wavelength anomalous‐scattering technique is underway.