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.