Caveolins, a Family of Scaffolding Proteins for Organizing “Preassembled Signaling Complexes” at the Plasma Membrane

Caveolae are vesicular invaginations of the plasma membrane. The chief structural proteins of caveolae are the caveolins. Caveolins form a scaffold onto which many classes of signaling molecules can assemble to generate preassembled signaling complexes. In addition to concentrating these signal transducers within a distinct region of the plasma membrane, caveolin binding may functionally regulate the activation state of caveolae-associated signaling molecules. Because the responsibilities assigned to caveolae continue to increase, this review will focus on: (i) caveolin structure/function and (ii) caveolae-associated signal transduction. Studies that link caveolae to human diseases will also be considered. Molecular cloning has identified three distinct caveolin genes (1Glenney J.R. Soppet D. Proc. Natl. Acad. Sci. U. S. A. 1992; 89: 10517-10521Crossref PubMed Scopus (341) Google Scholar, 2Scherer P.E. Tang Z. Chun M. Sargiacomo M. Lodish H.F. Lisanti M.P. J. Biol. 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Caveolin-1 and -2 are most abundantly expressed in adipocytes, endothelial cells, and fibroblastic cell types, whereas the expression of caveolin-3 is muscle-specific. Caveolin proteins interact with themselves to form homo- and hetero-oligomers (7Sargiacomo M. Scherer P.E. Tang Z. Kubler E. Song K.S. Sanders M.C. Lisanti M.P. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 9407-9411Crossref PubMed Scopus (477) Google Scholar, 8Monier S. Parton R.G. Vogel F. Behlke J. Henske A. Kurzchalia T. Mol. Biol. Cell. 1995; 6: 911-927Crossref PubMed Scopus (401) Google Scholar, 9Scherer P. Lewis R. Volonté D. Engelman J. Galbiati F. Couet J. Kohtz D. van Donselaar E. Peters P. Lisanti M.P. J. Biol. Chem. 1997; 272: 29337-29346Abstract Full Text Full Text PDF PubMed Scopus (471) Google Scholar), which directly bind cholesterol (10Murata M. Peranen J. Schreiner R. Weiland F. Kurzchalia T. Simons K. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 10339-10343Crossref PubMed Scopus (765) Google Scholar) and require cholesterol for insertion into model lipid membranes (10Murata M. Peranen J. Schreiner R. Weiland F. Kurzchalia T. Simons K. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 10339-10343Crossref PubMed Scopus (765) Google Scholar, 11Li S. Song K.S. Lisanti M.P. J. Biol. Chem. 1996; 271: 568-573Abstract Full Text Full Text PDF PubMed Scopus (199) Google Scholar). Caveolin oligomers may also interact with glycosphingolipids (12Fra A.M. Masserini M. Palestini P. Sonnino S. Simons K. FEBS Lett. 1995; 375: 11-14Crossref PubMed Scopus (162) Google Scholar). These protein-protein and protein-lipid interactions are thought to be the driving force for caveolae formation (7Sargiacomo M. Scherer P.E. Tang Z. Kubler E. Song K.S. Sanders M.C. Lisanti M.P. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 9407-9411Crossref PubMed Scopus (477) Google Scholar). In addition, the caveolin gene family is structurally and functionally conserved from worms (Caenorhabditis elegans) to man (13Tang Z. Okamoto T. Boontrakulpoontawee P. Otsuka A.J. Katada T. Lisanti M.P. J. Biol. Chem. 1997; 272: 2437-2445Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar), supporting the idea that caveolins play an essential role. Caveolin-1 assumes an unusual topology. A central hydrophobic domain (residues 102–134) is thought to form a hairpin-like structure within the membrane. As a consequence, both the N-terminal domain (residues 1–101) and the C-terminal domain (residues 135–178) face the cytoplasm. A 41-amino acid region of the N-terminal domain (residues 61–101) directs the formation of caveolin homo-oligomers (7Sargiacomo M. Scherer P.E. Tang Z. Kubler E. Song K.S. Sanders M.C. Lisanti M.P. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 9407-9411Crossref PubMed Scopus (477) Google Scholar), whereas the 44-amino acid C-terminal domain acts as a bridge to allow these homo-oligomers to interact with each other, thereby forming a caveolin-rich scaffold (14Song K. Tang Z. Li S. Lisanti M. J. Biol. Chem. 1997; 272: 4398-4403Abstract Full Text Full Text PDF PubMed Scopus (150) Google Scholar). Recent co-immunoprecipitation and dual labeling experiments directly show that caveolin-1 and -2 form a stable hetero-oligomeric complex and are strictly co-localized (9Scherer P. Lewis R. Volonté D. Engelman J. Galbiati F. Couet J. Kohtz D. van Donselaar E. Peters P. Lisanti M.P. J. Biol. Chem. 1997; 272: 29337-29346Abstract Full Text Full Text PDF PubMed Scopus (471) Google Scholar). Caveolin-2 localization corresponds to caveolae membranes as visualized by immunoelectron microscopy (9Scherer P. Lewis R. Volonté D. Engelman J. Galbiati F. Couet J. Kohtz D. van Donselaar E. Peters P. Lisanti M.P. J. Biol. Chem. 1997; 272: 29337-29346Abstract Full Text Full Text PDF PubMed Scopus (471) Google Scholar). Thus, caveolin-2 may function as an “accessory protein” in conjunction with caveolin-1. A number of studies support the hypothesis that caveolin proteins provide a direct means for resident caveolae proteins to be sequestered within caveolae microdomains. These caveolin-interacting proteins include G-protein α subunits, Ha-Ras, Src family tyrosine kinases, endothelial NOS, 1The abbreviations used are: NOS, nitric oxide synthase; EGF-R, epidermal growth factor receptor; eNOS, endothelial NOS; PKC, protein kinase C; MAP, mitogen-activated protein; PDGF, platelet-derived growth factor; PtdIns, phosphatidylinositol; PFK-M, phosphofructokinase-M; GPI, glycosylphosphatidylinositol; GM1, II3NeuAcGgOse4Cer. EGF-R and related receptor tyrosine kinases, and protein kinase C isoforms (11Li S. Song K.S. Lisanti M.P. J. Biol. 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Cell Biol. 1994; 126: 353-363Crossref PubMed Scopus (343) Google Scholar, 22Smart E.J. Ying Y. Mineo C. Anderson R.G.W. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 10104-10108Crossref PubMed Scopus (676) Google Scholar, 23Song K.S. Li S. Okamoto T. Quilliam L. Sargiacomo M. Lisanti M.P. J. Biol. Chem. 1996; 271: 9690-9697Abstract Full Text Full Text PDF PubMed Scopus (918) Google Scholar, 24Schnitzer J.E. Liu J. Oh P. J. Biol. Chem. 1995; 270: 14399-14404Abstract Full Text Full Text PDF PubMed Scopus (325) Google Scholar, 25Robbins S.M. Quintrell N.A. Bishop J.M. Mol. Cell. Biol. 1995; 15: 3507-3515Crossref PubMed Scopus (228) Google Scholar, 26Liu P. Ying Y. Ko Y.-G. Anderson R.G.W. J. Biol. Chem. 1996; 271: 10299-10303Abstract Full Text Full Text PDF PubMed Scopus (337) Google Scholar, 27Mineo C. James G.L. Smart E.J. Anderson R.G.W. J. Biol. Chem. 1996; 271: 11930-11935Abstract Full Text Full Text PDF PubMed Scopus (403) Google Scholar, 28Li S. Seitz R. Lisanti M.P. J. Biol. Chem. 1996; 271: 3863-3868Abstract Full Text Full Text PDF PubMed Scopus (309) Google Scholar, 29Chang W.J. Ying Y.S. Rothberg K.G. Hooper N.M. Turner A.J. Gambliel H.A. De Gunzburg J. Mumby S.M. Gilman A.G. Anderson R.G.W. J. Cell Biol. 1994; 126: 127-138Crossref PubMed Scopus (311) Google Scholar, 30Ju H. Zou R. Venema V.J. Venema R.C. J. Biol. Chem. 1997; 272: 18522-18525Abstract Full Text Full Text PDF PubMed Scopus (525) Google Scholar, 31Couet J. Li S. Okamoto T. Ikezu T. Lisanti M.P. J. Biol. Chem. 1997; 272: 6525-6533Abstract Full Text Full Text PDF PubMed Scopus (805) Google Scholar, 32Couet J. Sargiacomo M. Lisanti M.P. J. Biol. Chem. 1997; 272: 30429-30438Abstract Full Text Full Text PDF PubMed Scopus (545) Google Scholar). G-proteins are dramatically enriched within caveolae membranes, where caveolin-1 directly interacts with the α subunits of G-proteins (18Li S. Okamoto T. Chun M. Sargiacomo M. Casanova J.E. Hansen S.H. Nishimoto I. Lisanti M.P. J. Biol. Chem. 1995; 270: 15693-15701Abstract Full Text Full Text PDF PubMed Scopus (559) Google Scholar). Mutational or pharmacological activation of Gsα prevents its co-fractionation with caveolin-1 and blocks its direct interaction with caveolin-1 in vitro, indicating that the inactive GDP-bound form of Gsα preferentially interacts with caveolin-1. G-protein binding activity is located within a 41-amino acid region of the cytoplasmic N-terminal domain of caveolin-1 (residues 61–101). A polypeptide derived from this region of caveolin-1 (residues 82–101) effectively suppresses the basal GTPase activity of purified G-proteins by inhibiting GDP/GTP exchange. In contrast, the analogous region of caveolin-2 possesses GTPase-activating protein activity with regard to heterotrimeric G-proteins (3Scherer P.E. Okamoto T. Chun M. Nishimoto I. Lodish H.F. Lisanti M.P. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 131-135Crossref PubMed Scopus (492) Google Scholar). However, both of these activities (GDI and GAP) actively hold or place G-proteins in the inactiveGDP-liganded conformation (3Scherer P.E. Okamoto T. Chun M. Nishimoto I. Lodish H.F. Lisanti M.P. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 131-135Crossref PubMed Scopus (492) Google Scholar). Ha-Ras and Src family tyrosine kinases also directly interact with caveolin-1 (20Li S. Couet J. Lisanti M.P. J. Biol. Chem. 1996; 271: 29182-29190Abstract Full Text Full Text PDF PubMed Scopus (674) Google Scholar, 23Song K.S. Li S. Okamoto T. Quilliam L. Sargiacomo M. Lisanti M.P. J. Biol. Chem. 1996; 271: 9690-9697Abstract Full Text Full Text PDF PubMed Scopus (918) Google Scholar). Using a detergent-free procedure and a polyhistidine-tagged form of caveolin-1 for affinity purification of caveolin-rich membranes, G-proteins, Src family kinases, and Ha-Ras were all found to co-fractionate and co-elute with caveolin-1. Wild-type Ha-Ras also interacted with recombinant caveolin-1 in vitro. Ras binding activity was localized to a 41-amino acid membrane-proximal region (61–101) of the cytosolic N-terminal domain of caveolin-1, i.e. the same caveolin-1 region responsible for interacting with G-protein α subunits. Reconstituted caveolin-rich membranes interacted with a soluble recombinant form of wild-type Ha-Ras but failed to interact with mutationally activated soluble Ha-Ras (G12V) (23Song K.S. Li S. Okamoto T. Quilliam L. Sargiacomo M. Lisanti M.P. J. Biol. Chem. 1996; 271: 9690-9697Abstract Full Text Full Text PDF PubMed Scopus (918) Google Scholar). Thus, a single amino acid change (G12V) that constitutively activates Ras prevents this interaction. Recombinant overexpression of caveolin in intact cells was sufficient to functionally recruit a non-farnesylated mutant of Ras (C186S) onto membranes (23Song K.S. Li S. Okamoto T. Quilliam L. Sargiacomo M. Lisanti M.P. J. Biol. Chem. 1996; 271: 9690-9697Abstract Full Text Full Text PDF PubMed Scopus (918) Google Scholar). This is consistent with the hypothesis that direct interaction with caveolin-1 promotes the sequestration of inactive Ha-Ras within caveolae microdomains. Caveolin-1 interacts with wild-type Src (c-Src) but does not form a stable complex with mutationally activated Src (v-Src) (20Li S. Couet J. Lisanti M.P. J. Biol. Chem. 1996; 271: 29182-29190Abstract Full Text Full Text PDF PubMed Scopus (674) Google Scholar). Thus, caveolin prefers the inactive conformation of Gα subunits, Ha-Ras and c-Src. Deletion mutagenesis indicates that the Src-interacting domain of caveolin is located within residues 61–101. A caveolin peptide derived from this region (residues 82–101) functionally suppressed the autoactivation of purified recombinant c-Src tyrosine kinase and a related Src family kinase, Fyn. Co-expression of caveolin-1 with c-Src shows that caveolin-1 dramatically suppresses the tyrosine kinase activity of c-Src. Thus, it appears that caveolin-1 functionally interacts with wild-type c-Src via caveolin residues 82–101. Several independent co-immunoprecipitation and domain-mapping studies demonstrate that eNOS interacts directly with caveolin-1 residues 82–101 (30Ju H. Zou R. Venema V.J. Venema R.C. J. Biol. Chem. 1997; 272: 18522-18525Abstract Full Text Full Text PDF PubMed Scopus (525) Google Scholar, 34Michel T. Feron O. J. Clin. Invest. 1997; 100: 2146-2152Crossref PubMed Scopus (846) Google Scholar, 36Garcı́a-Cardeña G. Fan R. Stern D.F. Liu J. Sessa W.C. J. Biol. Chem. 1996; 271: 27237-27240Abstract Full Text Full Text PDF PubMed Scopus (428) Google Scholar, 37Venema V. Ju H. Zou R. Venema R. J. Biol. Chem. 1997; 272: 28187-28190Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar, 38Garcia-Cardena G. Martasek P. Siler-Masters B.S. Skidd P.M. Couet J. Li S. Lisanti M.P. Sessa W.C. J. Biol. Chem. 1997; 272: 25437-25440Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). In support of these data, recombinant co-expression of caveolin-1 with eNOS can inhibit NOS activity in vivo (38Garcia-Cardena G. Martasek P. Siler-Masters B.S. Skidd P.M. Couet J. Li S. Lisanti M.P. Sessa W.C. J. Biol. Chem. 1997; 272: 25437-25440Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). Caveolin has also been shown to interact with other NOS isoforms (37Venema V. Ju H. Zou R. Venema R. J. Biol. Chem. 1997; 272: 28187-28190Abstract Full Text Full Text PDF PubMed Scopus (221) Google Scholar, 38Garcia-Cardena G. Martasek P. Siler-Masters B.S. Skidd P.M. Couet J. Li S. Lisanti M.P. Sessa W.C. J. Biol. Chem. 1997; 272: 25437-25440Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). In summary, a short cytosolic domain derived from the N-terminal region of caveolin-1: (i) is required to form multivalent homo-oligomers of caveolin; (ii) mediates the interaction of caveolin-1 with Gα subunits, Ha-Ras, Src family tyrosine kinases, and eNOS; (iii) a peptide encoding this region can functionally inactivate the enzymatic activity of G-protein, Src family kinases, and eNOS but does not affect the activity of Ha-Ras; and (iv) it is membrane-proximal, suggesting that this caveolin domain may be involved in other potential protein-protein interactions. As a consequence, this caveolin-derived protein domain has been termed the caveolin scaffolding domain (Fig. 1). What is the mechanism by which the caveolin scaffolding domain recognizes this diverse group of signal transducers? Perhaps the caveolin scaffolding domain recognizes a common sequence motif within caveolin-binding signaling molecules. To investigate this possibility, we have used the caveolin scaffolding domain as a receptor to select caveolin-binding peptide ligandsfrom random peptide sequences displayed at the surface of bacteriophage. Two related caveolin-binding motifs (ΦXΦXXXXΦ and ΦXXXXΦXXΦ, where Φ is aromatic amino acid Trp, Phe, or Tyr) were elucidated, and these motifs exist within most caveolae-associated proteins (31Couet J. Li S. Okamoto T. Ikezu T. Lisanti M.P. J. Biol. Chem. 1997; 272: 6525-6533Abstract Full Text Full Text PDF PubMed Scopus (805) Google Scholar). Thus, caveolin-binding motifs mediate the interaction of caveolin-binding proteins with the scaffolding domain of caveolin. These caveolin-binding motifs are present within most Gα subunits and the kinase domains of many distinct families of tyrosine and serine/threonine protein kinases (Src family kinases; PKCα; MAP kinase; EGF-R; insulin receptor; and PDGF receptor). As many known caveolae or caveolin-associated proteins contain caveolin-binding motifs (see Table II in Ref. 31Couet J. Li S. Okamoto T. Ikezu T. Lisanti M.P. J. Biol. Chem. 1997; 272: 6525-6533Abstract Full Text Full Text PDF PubMed Scopus (805) Google Scholar), this may be a general mechanism for caveolin-mediated sequestration and inactivation of a diverse group of signaling molecules within caveolae membranes for regulated activation by receptor ligands. Thus, the caveolin scaffolding domain may function like other modular protein domains (Src homology-2, Src homology-3, Pleckstrin homology, WW, and others) to generate preassembled membrane-bound oligomeric complexes that contain signaling molecules and cytoskeletal elements. In essence, caveolin may act as molecular “Velcro” to nucleate the formation of signal transduction complexes, holding these molecules in the off state (Fig. 1). Additional molecular mapping studies have identified functional caveolin-binding sequence motifs within the catalytic region of G-protein α-subunits and the kinase domains of EGF-R and PKC (32Couet J. Sargiacomo M. Lisanti M.P. J. Biol. Chem. 1997; 272: 30429-30438Abstract Full Text Full Text PDF PubMed Scopus (545) Google Scholar). Interaction of the caveolin scaffolding domain with these caveolin-binding sequence motifs inhibits the kinase activity of EGF-R and PKC, suggesting that caveolin may indeed function as a general kinase inhibitor (32Couet J. Sargiacomo M. Lisanti M.P. J. Biol. Chem. 1997; 272: 30429-30438Abstract Full Text Full Text PDF PubMed Scopus (545) Google Scholar, 39Oka N. Yamamoto M. Schwencke C. Kawabe J. Ebina T. Ohno S. Couet J. Lisanti M.P. Ishikawa Y. J. Biol. Chem. 1997; 272: 33416-33421Abstract Full Text Full Text PDF PubMed Scopus (239) Google Scholar). eNOS contains a well conserved predicted caveolin-binding motif (FSAAPFSGW) within its catalytic domain (31Couet J. Li S. Okamoto T. Ikezu T. Lisanti M.P. J. Biol. Chem. 1997; 272: 6525-6533Abstract Full Text Full Text PDF PubMed Scopus (805) Google Scholar, 38Garcia-Cardena G. Martasek P. Siler-Masters B.S. Skidd P.M. Couet J. Li S. Lisanti M.P. Sessa W.C. J. Biol. Chem. 1997; 272: 25437-25440Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar). Does this represent a functional caveolin-binding motif? Two independent lines of evidence suggest that this region binds to caveolin-1 directly. First, caveolin competes with calmodulin for binding at this site (30Ju H. Zou R. Venema V.J. Venema R.C. J. Biol. Chem. 1997; 272: 18522-18525Abstract Full Text Full Text PDF PubMed Scopus (525) Google Scholar, 40Michel J. Feron O. Sacks D. Michel T. J. Biol. Chem. 1997; 272: 15583-15586Abstract Full Text Full Text PDF PubMed Scopus (511) Google Scholar, 41Michel J. Feron O. Sase K. Prabhakar P. Michel T. J. Biol. Chem. 1997; 272: 25907-25912Abstract Full Text Full Text PDF PubMed Scopus (270) Google Scholar). This may have functional significance, as calmodulin binding activates eNOS activity, whereas caveolin binding represses eNOS activity (30Ju H. Zou R. Venema V.J. Venema R.C. J. Biol. Chem. 1997; 272: 18522-18525Abstract Full Text Full Text PDF PubMed Scopus (525) Google Scholar, 40Michel J. Feron O. Sacks D. Michel T. J. Biol. Chem. 1997; 272: 15583-15586Abstract Full Text Full Text PDF PubMed Scopus (511) Google Scholar,41Michel J. Feron O. Sase K. Prabhakar P. Michel T. J. Biol. Chem. 1997; 272: 25907-25912Abstract Full Text Full Text PDF PubMed Scopus (270) Google Scholar). This suggests that caveolin-1 and calmodulin have a reciprocal relationship with respect to eNOS functioning (40Michel J. Feron O. Sacks D. Michel T. J. Biol. Chem. 1997; 272: 15583-15586Abstract Full Text Full Text PDF PubMed Scopus (511) Google Scholar, 41Michel J. Feron O. Sase K. Prabhakar P. Michel T. J. Biol. Chem. 1997; 272: 25907-25912Abstract Full Text Full Text PDF PubMed Scopus (270) Google Scholar).Second, Sessa and colleagues (38Garcia-Cardena G. Martasek P. Siler-Masters B.S. Skidd P.M. Couet J. Li S. Lisanti M.P. Sessa W.C. J. Biol. Chem. 1997; 272: 25437-25440Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar) have performed site-directed mutagenesis to modify the predicted caveolin-binding motif (fromFSAAPFSGW to ASAAPASGA) within eNOS. It is known from in vitro studies that aromatic residues (Trp, Phe, or Tyr) are required for the proper recognition of the caveolin-binding motif (31Couet J. Li S. Okamoto T. Ikezu T. Lisanti M.P. J. Biol. Chem. 1997; 272: 6525-6533Abstract Full Text Full Text PDF PubMed Scopus (805) Google Scholar). In their work, Sessa and colleagues (38Garcia-Cardena G. Martasek P. Siler-Masters B.S. Skidd P.M. Couet J. Li S. Lisanti M.P. Sessa W.C. J. Biol. Chem. 1997; 272: 25437-25440Abstract Full Text Full Text PDF PubMed Scopus (694) Google Scholar) show that mutation of the caveolin-binding motif within eNOS blocks the ability of caveolin-1 to inhibit eNOS activity in vivo. These findings provide the first demonstration that a caveolin-binding motif is relevant and functional in vivo. Direct interaction of caveolin with signaling molecules leads to their inactivation (18Li S. Okamoto T. Chun M. Sargiacomo M. Casanova J.E. Hansen S.H. Nishimoto I. Lisanti M.P. J. Biol. Chem. 1995; 270: 15693-15701Abstract Full Text Full Text PDF PubMed Scopus (559) Google Scholar). Since many signaling molecules can cause cellular transformation when constitutively activated, it is reasonable to speculate that caveolin may possess transformation suppressor activity. Consistent with this hypothesis, both caveolae and caveolin are most abundantly expressed in terminally differentiated cells: adipocytes, endothelial cells, and muscle cells. In addition, caveolin-1 mRNA and protein expression are lost or reduced during cell transformation by activated oncogenes such as v-abl and Ha-ras (G12V); caveolae are absent from these cell lines (42Koleske A.J. Baltimore D. Lisanti M.P. Proc. Natl. Acad. Sci . U. S. A. 1995; 92: 1381-1385Crossref PubMed Scopus (472) Google Scholar). The potential “transformation suppressor” activity of caveolin-1 has recently been evaluated by using inducible expression in oncogenically transformed cells. Induction of caveolin-1 expression in v-Abl- and Ha-Ras (G12V)-transformed NIH 3T3 cells abrogated the anchorage-independent growth of these cells in soft agar and resulted in the de novo formation of caveolae (43Engelman J.A. Wykoff C.C. Yasuhara S. Song K.S. Okamoto T. Lisanti M.P. J. Biol. 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