A role for G-proteins in directing G-protein-coupled receptor-caveolae localization.

Caveolae are membrane domains that may influence cell signaling by sequestering specific proteins such as G-protein-coupled receptors (GPCRs). While previous reports largely show that Gα(q) subunits, but not other G-proteins, interact strongly with the caveolae protein, Caveolin-1 (Cav1), the inclusion of GPCRs in caveolae is controversial. Here, we have used fluorescence methods to determine the effect of caveolae on the physical and functional properties of two GPCRs that have been reported to reside in caveolae, bradykinin receptor type 2 (B(2)R), which is coupled to Gα(q), and the μ-opioid receptor (μOR), which is coupled to Gα(i). While caveolae do not affect cAMP signals mediated by μOR, they prolong Ca(2+) signals mediated by B(2)R. In A10 cells that endogenously express B(2)R and Cav1, downregulation of Cav1 ablates the prolonged recovery seen upon bradykinin stimulation in accord with the idea that the presence of caveolae prolongs Gα(q) activation. Immunofluorescence and Förster resonance energy transfer (FRET) studies show that a significant fraction of B(2)R resides at or close to caveolae domains while none or very little μOR resides in caveolae domains. The level of FRET between B(2)R and caveolae is reduced by downregulation of Gα(q) or by addition of a peptide that interferes with Gα(q)-Caveolin-1 interactions, suggesting that Gα(q) promotes localization of B(2)R to caveolae domains. Our results lead to the suggestion that Gα(q) can localize its associated receptors to caveolae domains to enhance their signals.