The Rieske protein from purple sulfur bacteria is an extrinsic protein.

The mode of membrane attachment of the Rieske iron-sulfur protein from cytochrome bc1 complex of Rhodospirillum rubrum has been studied using biochemical approaches. In contrast to cytochrome c1 the bacterial Rieske protein was extracted from chromatophores using chaotropic agents (NaSCN, urea, guanidine), an alkaline pH and relatively low concentration of Triton X-100. The results presented here lead to the conclusion, that the Rieske protein from chromatophores is extrinsic and that their association with the rest of the complex involves hydrophobic interactions.