ORGANIZED ENDOTHELIAL-CELL SURFACE SIGNAL-TRANSDUCTION IN CAVEOLAE DISTINCT FROM GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED PROTEIN MICRODOMAINS

Regulated signal transduction in discrete microdomains of the cell sur face is an attractive hypothesis for achieving spatial and temporal sp ecificity in signaling. A procedure for purifying caveolae separately from other similarly buoyant microdomains including those rich in glyc osylphosphatidylinositol-anchored proteins has been developed (Schnitz er, J. E., McIntosh, D. P., Dvorak, A. M., Liu, J., and Oh, P. (1995) Science 269, 1435-1439) and used here to show that caveolae contain ma ny signaling molecules including select kinases (platelet-derived grow th factor (PDGF) receptors, protein kinase C, phosphatidylinositol 3-k inase, and Src-like kinases), phospholipase C, sphingomyelin, and even phosphoinositides. More importantly, two different techniques reveal that caveolae function as signal transducing subcompartments of the pl asma membrane. PDGF rapidly induces phosphorylation of endothelial cel l plasmalemmal proteins residing in caveolae as detected by membrane s ubfractionation and confocal immunofluorescence microscopy. This PDGF signaling cascade is halted when the caveolar compartment is disassemb led by filipin. Finally, in vitro kinase assays show that caveolae con tain most of the intrinsic tyrosine kinase activity of the plasma memb rane. As signal transducing organelles, caveolae organize a distinct s et of signaling molecules to permit direct regionalized signal transdu ction within their boundaries.