Cytochrome b is an amphipathic mobile membrane protein that is predominantly located at the endoplasmic reticulum surface. It is an essential component of a number of membrane-bound redox systems. In animal and fungal cells cytochrome b is thought to be an electron donor for sterol modifying enzymes and fatty acid desaturases. Disruption of the Saccharomyces cytochrome b gene, however, yielded cells that had no nutritional requirement for either sterols or unsaturated fatty acids. Expression of sterol and fatty acid-modifying genes was increased in the cytochrome b-disrupted cells, however, suggesting that cytochrome b may play some nonessential role in these functions. Unsaturated fatty acids in yeast are formed by Ole1p, an oxygen-dependent -9 fatty acid desaturase that is an intrinsic endoplasmic reticulum membrane protein. Although the yeast -9 fatty acid desaturase does not appear to require cytochrome b, introduction of the rat liver stearoyl-CoA desaturase gene into an ole1-disrupted, cytochrome b-disrupted yeast strain revealed that this enzyme specifically requires cytochrome b to function. Comparison of the coding sequences of the yeast and rat desaturase genes showed that the yeast protein contains a 113-amino acid carboxyl-terminal extension not found in the rat enzyme. That extension has regions of strong homology to cytochrome b, particularly in the heme binding and electron transfer motifs. Truncation or disruption of the desaturase cytochrome b-like domain in cells that contain the wild type diffusible b produced unsaturated fatty acid auxotrophy, suggesting that the cytochrome b-like domain of Ole1p plays an essential role in the desaturase reaction.

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