Functional domains of the mouse β₃-adrenoceptor associated with differential G-protein coupling

Localization of G-protein-coupled receptors within membrane microdomains is associated with differential signalling pathway activation. We have shown that two mouse β₃-AR (β₃-adrenoceptor) isoforms encoded by alternatively spliced mRNAs differ in their signalling properties; the β_3a-AR couples only with G_s, whereas the β_3b-AR couples with both G_s and G_i. Our previous studies indicated that the β_3a-AR is restrained from coupling with G_i due to the interaction of residues in the C-terminus with other protein(s). We have investigated the hypothesis that the β_3a-AR interacts with caveolin. Disruption of caveolae in CHO (Chinese-hamster ovary)-K1 cells expressing wild-type β_3a-ARs with filipin III, or mutation of a putative caveolin-binding site in the β_3a-AR, causes cAMP accumulation to become PTX (pertussis toxin)-sensitive. Likewise, filipin treatment of mouse brown adipocytes that express endogenous β_3a-ARs produces a substantial reduction in agonist-stimulated cAMP production that is rescued by pre-treatment with PTX. These studies suggest that β_3a-ARs may be restricted to caveolae and that localization of the receptor may play a specific role in G-protein-mediated signalling.