Expression systems and methods for the expression of functional membrane polypeptides such as human cytochrome b5 are provided. The systems include recombinant expression vectors capable of expressing soluble fusion proteins that include a solubilizing agent, a linker, and a membrane polypeptide, as well as one or more cleavers, e.g. proteases, capable of cleaving the linker to release the membrane polypeptide. When the fusion protein is expressed, the linker is cleaved by the cleaver to allow association of the membrane polypeptide with a membrane.