Receptor number and caveolar co-localization determine receptor coupling efficiency to adenylyl cyclase

Recent evidence suggests that many signaling molecules localize in microdomains of the plasma membrane, particularly caveolae. In this study, overexpression of adenylyl cyclase was used as a functional probe of G protein-coupled receptor (GPCR) compartmentation. We found that three endogenous receptors in neonatal rat cardiomyocytes couple with different levels of efficiency to the activation of adenylyl cyclase type 6 (AC6), which localizes to caveolin-rich membrane fractions. Overexpression of AC6 enhanced the maximal cAMP response to beta (1)-adrenergic receptor (beta (1)AR)-selective activation 3.7-fold, to beta (2)AR-selective activation only 1.6-fold and to prostaglandin E-2 (PGE(2)) not at all. Therefore, the rank order of efficacy in coupling to AC6 is beta (1)AR > beta (2)AR > prostaglandin E-2 receptor (EP2R). beta (2)AR coupling efficiency was greater when we overexpressed the receptor or blocked its desensitization by expressing beta ARKct, an inhibitor of G protein-coupled receptor kinase activation, but was not significantly greater when cells were treated with pertussis toxin. Assessment of receptor and AC expression indicated co-localization of AC5/6,beta (1)AR, and beta (2)AR, but not EP2R, in caveolin-rich membranes and caveolin-3 immunoprecipitates, likely explaining the observed activation of AC6 by beta AR subtypes but lack thereof by PGE(2). When cardiomyocytes were stimulated with a beta AR agonist, beta (2)AR were no longer found in caveolin-3 immunoprecipitates; an effect that was blocked by expression of beta ARKet. Thus, agonist-induced translocation of beta (2)AR out of caveolae causes a sequestration of receptor from effector and likely contributes to the lower efficacy of beta (2)AR coupling to AC6 as compared with beta (1)AR, which do not similarly translocate. Therefore, spatial co-localization is a ey determinant of efficiency of coupling by particular extracellular signals to activation of GPCR-linked effectors.