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

Mammalian cytochrome b (cyt b) and cytochrome b reductase (bR) are electron carrier proteins for membrane-embedded oxidoreductases. Both bR and cyt b have a cytosolic domain and a single transmembrane (TM) helix. The cytosolic domains of bR and cyt b 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 bR and cyt b form a stable binary complex, and so do cyt b and stearoyl-CoA desaturase-1 (SCD1). We also show that bR, cyt b 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 and bR, 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 and bR.