Segregation of heterotrimeric G proteins in cell surface microdomains -: Gq binds caveolin to concentrate in caveolae, whereas Gi and Gs target lipid rafts by default

Select lipid-anchored proteins such as glycosylphosphatidylinositol (GPI)-anchored proteins and nonreceptor tyrosine kinases may preferentially partition into sphingomyelin-rich and cholesterol-rich plasmalemmal microdomains, thereby acquiring resistance to detergent extraction. Two such domains, caveolae and lipid rafts, are morphologically and biochemically distinct, contain many signaling molecules, and may function in compartmentalizing cell surface signaling. Subfractionation and confocal immunofluorescence microscopy reveal that, in lung tissue and in cultured endothelial and epithelial cells, heterotrimeric G proteins (G(i), G(q), G(s), and G(beta gamma)) target discrete cell surface microdomains. G(q) specifically concentrates in caveolae, whereas G(i) and G(s) concentrate much more in lipid rafts marked by GPI-anchored proteins (5 ' nucleotidase and folate receptor). G(q), apparently without G(beta gamma) subunits, stably associates with plasmalemmal and cytosolic caveolin. G(i) and G(s) interact with G(beta gamma) subunits but not caveolin. G(i) and G(s), unlike G(q), readily move out of caveolae. Thus, caveolin may function as a scaffold to trap, concentrate, and stabilize G(q) preferentially within caveolae over lipid rafts. In N2a cells lacking caveolae and caveolin, G(q), G(i), and G(s) all concentrate in lipid rafts as a complex with G(beta gamma). Without effective physiological interaction with caveolin, G proteins tend by default to segregate in lipid rafts. The ramifications of the segregated microdomain distribution and the G(q)-caveolin complex without G(beta gamma) for trafficking, signaling, and mechanotransduction are discussed.