Compartmentation of cyclic adenosine 3 ',5 '-monophosphate signaling in caveolae

The cAMP-signaling pathway is composed of multiple components ranging from receptors, G proteins, and adenylyl cyclase to protein kinase A. A common v iew of the molecular interaction between them is that these molecules are d isseminated on the plasma lipid membrane and random collide with each other to transmit signals. A limitation to this idea, however, is that a signali ng cascade involving multiple components may not occur rapidly. Caveolae an d their principal component, caveolin, have been implicated in transmembran e signaling, particularly in G protein-coupled signaling. We examined wheth er caveolin interacts with adenylyl cyclase, the membrane-bound enzyme that catalyzes the conversion of ATP to cAMP. When overexpressed in insect cell s, types ill, IV, and V adenylyl cyclase were localized in caveolin-enriche d membrane fractions. Caveolin was coimmunoprecipitated with adenylyl cycla se in tissue homogenates and copurified with a polyhistidine-tagged form of adenylyl cyclase by Ni-nitrilotriacetic acid resin chromatography in insec t cells, suggesting the colocalization of adenylyl cyclase and caveolin in the same microdomain. Further, the regulatory subunit of protein kinase A ( RII alpha, but not RI alpha) was also enriched in the same fraction as cave olin. Gs alpha was found in both caveolin-enriched and non-caveolin-enriche d membrane fractions. Our data suggest that the cAMP-signaling cascade occu rs within a restricted microdomain of the plasma membrane in a highly organ ized manner.