Transmembrane helices mediate the formation of a stable ternary complex of b 5 R, cyt b 5 , and SCD1.

Mammalian cytochrome b 5 (cyt b 5 ) and cytochrome b 5 reductase (b 5 R) are electron carrier proteins for membrane-embedded oxidoreductases. Both b 5 R and cyt b 5 have a cytosolic domain and a single transmembrane (TM) helix. The cytosolic domains of b 5 R and cyt b 5 contain cofactors required for electron transfer, but it is not clear if the TM helix has function beyond being an anchor to the membrane. Here we show that b 5 R and cyt b 5 form a stable binary complex, and so do cyt b 5 and stearoyl-CoA desaturase-1 (SCD1). We also show that b 5 R, cyt b 5 and SCD1 form a stable ternary complex. We demonstrate that the TM helices are required for the assembly of stable binary and ternary complexes where electron transfer rates are greatly enhanced. These results reveal a role of the TM helix in cyt b 5 and b 5 R, and suggest that an electron transport chain composed of a stable ternary complex may be a general feature in membrane-embedded oxidoreductases that require cyt b 5 and b 5 R.