Colocalization of β‐adrenergic receptors and caveolin within the plasma membrane

The rapid amplification of β‐adrenergic receptor signaling involves the sequential activation of multiple signaling molecules ranging from the receptor to adenylyl cyclase. The prevailing view of the agonist‐induced interaction between signaling molecules is based on random collisions between proteins that diffuse freely in the plasma membrane. The recent identification of G protein α‐ and βγ‐subunits in caveolae and their functional interaction with caveolin suggests that caveolae may participate in G protein‐coupled signaling. We have investigated the potential interaction of β‐adrenergic receptors with caveolin under resting conditions. β1‐ and β2‐adrenergic receptors were recombinantly overexpressed in COS‐7 cells. Caveolae were isolated using the detergent‐free sucrose gradient centrifugation method. β1‐ and β2‐adrenergic receptors were localized in the same gradient fractions as caveolin, where Gsα‐ and βγ‐subunits were detected as well. Immunofluorescence microscopy demonstrated the colocalization of β‐adrenergic receptors with caveolin, indicating a nonrandom distribution of β‐adrenergic receptors in the plasma membrane. Using polyhistidine‐tagged recombinant proteins, β‐adrenergic receptors were copurified with caveolin, suggesting that they were physically bound. Our results suggest that, in addition to clathrin‐coated pits, caveolae may act as another plasma membrane microdomain to compartmentalize β‐adrenergic receptors. J. Cell. Biochem. 75:64–72, 1999. © 1999 Wiley‐Liss, Inc.