Enhancement of membrane insertion and function in a type IIIb membrane protein following introduction of a cleavable signal peptide.

The human beta 2 adrenergic receptor is a type IIIb membrane protein. It has a putative seven-transmembrane topology but lacks an amino-terminal cleavable signal sequence. The mechanism by which the amino terminus of the beta 2 receptor is translocated across the endoplasmic reticulum membrane is unknown. Furthermore, it is not known if translocation as a type IIIb protein is essential for the proper folding. Our studies indicate that conversion of beta 2 receptor from a type IIIb to a type IIIa membrane protein by introducing an NH2-terminal cleavable signal sequence enhances translocation of the receptor into the endoplasmic reticulum membrane, thereby facilitating expression of functional receptor.