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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.
Guan et al. (Sun,) studied this question.