Compartmentation of G-protein-coupled receptors and their signalling components in lipid rafts and caveolae

G-protein-coupled receptors (GPCRs) and post-GPCR signalling components are expressed at low overall abundance in plasma membranes, yet they evoke rapid, high-fidelity responses. Considerable evidence suggests that GPCR signalling components are organized together in membrane microdomains, in particular lipid rafts, enriched in cholesterol and sphingolipids, and caveolae, a subset of lipid rafts that also possess the protein caveolin, whose scaffolding domain may serve as an anchor for signalling components. Caveolae were originally identified based on their morphological appearance but their role in compartmentation of GPCR signalling has been primarily studied by biochemical techniques, such as subcellular fractionation and immunoprecipitation. Our recent studies obtained using both microscopic and biochemical methods with adult cardiac myocytes show expression of caveolin not only in surface sarcolemmal domains but also at, or close to, internal regions located at transverse tubules/sarcoplasmic reticulum. Other results show co-localization in lipid rafts/caveolae of AC (adenylyl cyclase), in particular AC6, certain GPCRs, G-proteins and eNOS (endothelial nitric oxide synthase; NOS3), which generates NO, a modulator of AC6. Existence of multiple caveolin-rich microdomains and their expression of multiple modulators of signalling strengthen the evidence that caveolins and lipid rafts/caveolae organize and regulate GPCR signal transduction in eukaryotic cells.