Colocalization of beta-adrenergic receptors and caveolin within the plasmamembrane

The rapid amplification of beta-adrenergic receptor signaling involves the sequential activation of multiple signaling molecules ranging from the rece ptor to adenylyl cyclase. The prevailing view of the agonist-induced intera ction between signaling molecules is based on random collisions between pro teins that diffuse freely in the plasma membrane. The recent identification of G protein alpha- and beta gamma-subunits in caveolae and their function al interaction with caveolin suggests that caveolae may participate in G pr otein-coupled signaling. We have investigated the potential interaction of beta-adrenergic receptors with caveolin under resting conditions. beta 1- a nd beta 2-adrenergic receptors were recombinantly overexpressed in COS-7 ce lls. Caveolae were isolated using the detergent-free sucrose gradient centr ifugation method. beta 1- and beta 2-adrenergic receptors were localized in the same gradient fractions as caveolin, where Gs alpha- and beta gamma-su bunits were detected as well. Immunofluorescence microscopy demonstrated th e colocalization of beta-adrenergic receptors with caveolin, indicating a n onrandom distribution of beta-adrenergic receptors in the plasma membrane. Using polyhistidine-tagged recombinant proteins, P-adrenergic receptors wer e copurified with caveolin, suggesting that they were physically bound. Our results suggest that, in addition to clathrin-coated pits, caveolae may ac t as another plasma membrane microdomain to compartmentalize beta-adrenergi c receptors. (C) 1999 Wiley-Liss, Inc.