Transmembrane helices mediate the formation of a stable ternary complex of cyt b5 reductase, cyt b5, and SCD1

Mammalian cytochrome b5 (cyt b5) and cytochrome b5 reductase (b5R) are electron carrier proteins required for many membrane-embedded oxidoreductases. Both cyt b5 and b5R have a cytosolic domain anchored to the membrane by a single transmembrane helix (TM). It is not clear if b5R, cyt b5 and their partner oxidoreductases assemble as binary or ternary complexes. Here we show that b5R and cyt b5 form a stable binary complex, and that b5R, cyt b5 and a membrane-embedded oxidoreductase, stearoyl-CoA desaturase 1 (SCD1) form a stable ternary complex. The formation of the complexes significantly enhances electron transfer rates, and that the single TM of cyt b5 and b5R mediated assembly of the complexes. These results reveal a novel functional role of TMs in cyt b5 and b5R and suggest that an electron transport chain composed of a stable ternary complex may be a general feature in oxidoreductases that require the participation of cyt b5 and b5R.