VIP21-caveolin, a membrane protein constituent of the caveolar coat, oligomerizes in vivo and in vitro

Src tyrosine kinases, Galpha subunits, and H-Ras share a common membrane-anchored scaffolding protein, caveolin. Caveolin binding negatively regulates the auto-activation of Src tyrosine kinases

Caveolae are plasma membrane specializations present in most cell types. Caveolin, a 22-kDa integral membrane protein, is a principal structural and regulatory component of caveolae membranes. Previous studies have demonstrated that caveolin co-purifies with lipid modified signaling molecules, including Galpha subunits, H-Ras, c-Src, and other related Src family tyrosine kinases. In addition, it has been shown that caveolin interacts directly with Galpha subunits and H-Ras, preferentially recognizing the inactive conformation of these molecules. However, it is not known whether caveolin interacts directly or indirectly with Src family tyrosine kinases. Here, we examine the structural and functional interaction of caveolin with Src family tyrosine kinases. Caveolin was recombinantly expressed as a glutathione S-transferase fusion. Using an established in vitro binding assay, we find that caveolin interacts with wild-type Src (c-Src) but does not form a stable complex with mutationally activated Src (v-Src). Thus, it appears that caveolin prefers the inactive conformation of Src. Deletion mutagenesis indicates that the Src-interacting domain of caveolin is located within residues 82-101, a cytosolic membrane-proximal region of caveolin. A caveolin peptide derived from this region (residues 82-101) functionally suppressed the auto-activation of purified recombinant c-Src tyrosine kinase and Fyn, a related Src family tyrosine kinase. We further analyzed the effect of caveolin on c-Src activity in vivo by transiently co-expressing full-length caveolin and c-Src tyrosine kinase in 293T cells. Co-expression with caveolin dramatically suppressed the tyrosine kinase activity of c-Src as measured via an immune complex kinase assay. Thus, it appears that caveolin structurally and functionally interacts with wild-type c-Src via caveolin residues 82-101. Besides interacting with Src family kinases, this cytosolic caveolin domain (residues 82-101) has the following unique features. First, it is required to form multivalent homo-oligomers of caveolin. Second, it interacts with G-protein alpha-subunits and down-regulates their GTPase activity. Third, it binds to wild-type H-Ras. Fourth, it is membrane-proximal, suggesting that it may be involved in other potential protein-protein interactions. Thus, we have termed this 20-amino acid stretch of caveolin residues the caveolin scaffolding domain.

Baculovirus-based expression of mammalian caveolin in Sf21 insect cells. A model system for the biochemical and morphological study of caveolae biogenesis

Caveolae were originally defined morphologically as 50-100 nm noncoated vesicular organelles located at or near the plasma membrane. Caveolin, a vesicular integral membrane protein of 21 kDa, is a principal protein component of caveolae membranes in vivo. Caveolin interacts with itself to form high molecular mass oligomers, suggesting that it might play a structural role in the formation of caveolae membranes. However, it remains controversial whether recombinant expression of caveolin is necessary or sufficient to generate caveolae membranes in vivo. To directly address this issue, we have taken a different experimental approach by exploiting a heterologous expression system. Here, we have recombinantly expressed mammalian caveolin in Sf21 insect cells using baculovirus-based vectors. Two isoforms of caveolin have been identified that differ at their extreme N terminus; alpha-caveolin contains residues 1-178, and beta-caveolin contains residues 32-178. After recombinant expression in Sf21 insect cells, both alpha- and beta-caveolin formed SDS-resistant high molecular mass oligomers of the same size as native caveolin. Morphologically, expression of either caveolin isoform resulted in the intracellular accumulation of a homogeneous population of caveolae-sized vesicles with a diameter between 50 and 120 nm (80.3 +/- 14.8 nm). This indicates that each caveolin isoform can independently generate these structures and that caveolin residues 1-31 are not required for this process. Using caveolin as a marker protein and a detergent-free procedure to purify caveolae from mammalian cells, we purified these recombinant caveolin-induced vesicles from insect cells. These purified recombinant vesicles: (i) have the same buoyant density as mammalian caveolae; (ii) appear as approximately 50-100 nm membranous structures by whole-mount electron microscopy; and (iii) contain approximately 95% of the recombinantly expressed caveolin protein by Western blotting. Immuno-labeling of these structures with anti-caveolin IgG confirmed that they contain caveolin. Thus, ectopic overexpression of caveolin in this heterologous system is sufficient to drive the formation of caveolae-like vesicles. Further functional analysis demonstrated that caveolin was capable of interacting with a known caveolin-interacting protein, Ha-Ras, when coexpressed in insect cells by co-infection with two recombinant baculoviruses. Taken together, our results demonstrate that baculovirus-based expression of caveolin in insect cells provides an attractive experimental system for studying the biogenesis of caveolae.

Caveolae and caveolins

In the past year, we have witnessed considerable progress towards an understanding of the workings of caveolae. Highlights include the identification of new caveolin family members, the characterization of VIP21-caveolin as a cholesterol-binding oligomeric protein, and evidence for functional interactions between caveolins and heterotrimeric G proteins. In addition, novel systems for caveolae purification and for studying caveolae biogenesis are starting to reveal insights into the molecular basis of caveolae formation and function.

Phosphorylation of caveolin by src tyrosine kinases. The alpha-isoform of caveolin is selectively phosphorylated by v-Src in vivo

Caveolae are flask-shaped plasma membrane specializations that are thought to exist in most cell types. A 22-kDa protein, caveolin, is an integral membrane component of caveolae membranes in vivo. Previous studies have demonstrated that caveolin is phosphorylated on tyrosine by oncogenic viral Src (v-Src) and that caveolin is physically associated as a hetero-oligomeric complex with normal cellular Src (c-Src) and other Src family tyrosine kinases. Caveolin contains eight conserved tyrosine residues that may serve as potential substrates for Src. Here, we have begun to study the phosphorylation of caveolin by Src family tyrosine kinases both in vitro and in vivo. Using purified recombinant components, we first reconstituted the phosphorylation of caveolin by Src kinase in vitro. Microsequencing of Src-phosphorylated caveolin revealed that phosphorylation occurs within the extreme N-terminal region of full-length caveolin between residues 6 and 26. This region contains three tyrosine residues at positions 6, 14, and 25. Deletion mutagenesis demonstrates that caveolin residues 1-21 are sufficient to support this phosphorylation event, implicating tyrosine 6 and/or 14. In vitro phosphorylation of caveolin-derived synthetic peptides and site-directed mutagenesis directly show that tyrosine 14 is the principal substrate for Src kinase. In support of these observations, tyrosine 14 is the only tyrosine residue within caveolin that bears any resemblance to the known recognition motifs for Src family tyrosine kinases. In order to confirm or refute the relevance of these in vitro studies, we next analyzed the tyrosine phosphorylation of endogenous caveolin in v-Src transformed NIH 3T3 cells. In vivo, two isoforms of caveolin are known to exist: alpha-caveolin contains residues 1-178 and beta-caveolin contains residues 32-178. Only alpha-caveolin underwent tyrosine phosphorylation in v-Src transformed NIH 3T3 cells, although beta-caveolin is well expressed in these cells. As beta-caveolin lacks residues 1-31 (and therefore tyrosine 14), these in vivo studies directly demonstrate the validity of our in vitro studies. Because alpha- and beta-caveolin are known to assume a distinct but overlapping subcellular distribution within a single cell, v-Src phosphorylation of alpha-caveolin may only affect a subpopulation of caveolae that contain alpha-caveolin.

And still they are moving.... dynamic properties of caveolae

Caveolae are structures found on the surface of many mammalian cells. In the last few years the biogenesis and the function of these organelles have been intensively investigated but many challenging questions remain. One of these is whether caveolae are statically attached to the cytoplasmic surface of the plasma membrane or are moving to other intracellular organelles. Also the cycling of the caveolar coat component, VIP21-caveolin, is a subject of intensive discussion. The solution to these problems could give an insight into the understanding of caveolar function.

Expression of caveolin-3 in skeletal, cardiac, and smooth muscle cells. Caveolin-3 is a component of the sarcolemma and co-fractionates with dystrophin and dystrophin-associated glycoproteins

Caveolae are microdomains of the plasma membrane that have been implicated in signal transduction. Caveolin, a 21-24-kDa integral membrane protein, is a principal component of the caveolae membrane. Recently, we and others have identified a family of caveolin-related proteins; caveolin has been retermed caveolin-1. Caveolin-3 is most closely related to caveolin-1, but caveolin-3 mRNA is expressed only in muscle tissue types. Here, we examine (i) the expression of caveolin-3 protein in muscle tissue types and (ii) its localization within skeletal muscle fibers by immunofluorescence microscopy and subcellular fractionation. For this purpose, we generated a novel monoclonal antibody (mAb) probe that recognizes the unique N-terminal region of caveolin-3, but not other members of the caveolin gene family. A survey of tissues and muscle cell types by Western blot analysis reveals that the caveolin-3 protein is selectively expressed only in heart and skeletal muscle tissues, cardiac myocytes, and smooth muscle cells. Immunolocalization of caveolin-3 in skeletal muscle fibers demonstrates that caveolin-3 is localized to the sarcolemma (muscle cell plasma membrane) and coincides with the distribution of another muscle-specific plasma membrane marker protein, dystrophin. In addition, caveolin-3 protein expression is dramatically induced during the differentiation of C2C12 skeletal myoblasts in culture. Using differentiated C2C12 skeletal myoblasts as a model system, we observe that caveolin-3 co-fractionates with cytoplasmic signaling molecules (G-proteins and Src-like kinases) and members of the dystrophin complex (dystrophin, alpha-sarcoglycan, and beta-dystroglycan), but is clearly separated from the bulk of cellular proteins. Caveolin-3 co-immunoprecipitates with antibodies directed against dystrophin, suggesting that they are physically associated as a discrete complex. These results are consistent with previous immunoelectron microscopic studies demonstrating that dystrophin is localized to plasma membrane caveolae in smooth muscle cells.

Oligomerization of VIP21-caveolin in vitro is stabilized by long chain fatty acylation or cholesterol

VIP21-caveolin is one of the components which form the cytoplasmic surface of caveolae. In vivo, this integral membrane protein is found in homo-oligomers with molecular masses of approximately 200, 400 and 600 kDa. These oligomers are also formed by the addition of cytosol to the in vitro synthesized and membrane inserted VIP21-caveolin. Here we show that long chain fatty acyl coenzyme A esters can completely substitute for cytosol in inducing 200 kDa and 400 kDa complexes, whereas 25-hydroxy-cholesterol can produce the 200 kDa oligomer. In order to understand whether acylation of VIP21-caveolin itself is a prerequisite for oligomerization, we studied a mutant protein lacking all three cysteines. When analyzed by velocity sucrose gradient centrifugation in the presence of the non-ionic detergent octylglucoside, both palmitoylated and non-palmitoylated VIP21-caveolin formed oligomers that were indistinguishable. However, only the oligomers of the non-palmitoylated protein are disrupted when analyzed by SDS-PAGE without boiling. These data suggest that the protein domains of VIP21-caveolin are the primary determinants of oligomerization, but that palmitoylation of cysteine residues can increase the stability of the oligomers.

Co-purification and direct interaction of Ras with caveolin, an integral membrane protein of caveolae microdomains. Detergent-free purification of caveolae microdomains

Caveolae are plasma membrane specializations that have been implicated in signal transduction. Caveolin, a 21-24-kDa integral membrane protein, is a principal structural component of caveolae membranes in vivo. G protein alpha subunits are concentrated in purified preparations of caveolae membranes, and caveolin interacts directly with multiple G protein alpha subunits, including G(s), G(o), and G(i2). Mutational or pharmacologic activation of G alpha subunits prevents the interaction of caveolin with G proteins, indicating that inactive G alpha subunits preferentially interact with caveolin. Here, we show that caveolin interacts with another well characterized signal transducer, Ras. Using a detergent-free procedure for purification of caveolin-rich membrane domains and a polyhistidine tagged form of caveolin, we find that Ras and other classes of lipid-modified signaling molecules co-fractionate and co-elute with caveolin. The association of Ras with caveolin was further evaluated using two distinct in vitro binding assays. Wild-type H-Ras interacted with glutathione S-transferase (GST)-caveolin fusion proteins but not with GST alone. Using a battery of GST fusion proteins encoding distinct regions of caveolin, Ras binding activity was localized to a 41-amino acid membrane proximal region of the cytosolic N-terminal domain of caveolin. In addition, reconstituted caveolin-rich membranes (prepared with purified recombinant caveolin and purified lipids) interacted with a soluble form of wild-type H-Ras but failed to interact with mutationally activated soluble H-Ras (G12V). Thus, a single amino acid change (G12V) that constitutively activates Ras prevents or destabilizes this interaction. These results clearly indicate that (i) caveolin is sufficient to recruit soluble Ras onto lipid membranes and (ii) membrane-bound caveolin preferentially interacts with inactive Ras proteins. In direct support of these in vitro studies, we also show that recombinant overexpression of caveolin in intact cells is sufficient to functionally recruit a nonfarnesylated mutant of Ras (C186S) onto membranes, overcoming the normal requirement for lipid modification of Ras. Taken together, these observations suggest that caveolin may function as a scaffolding protein to localize or sequester certain caveolin-interacting proteins, such as wild-type Ras, within caveolin-rich microdomains of the plasma membrane.

Molecular cloning of caveolin-3, a novel member of the caveolin gene family expressed predominantly in muscle

Caveolin, a 21-24-kDa integral membrane protein, is a principal component of caveolar membranes in vivo. Caveolin interacts directly with heterotrimeric G-proteins and can functionally regulate their activity. Recently, a second caveolin gene has been identified and termed caveolin-2. Here, we report the molecular cloning and expression of a third member of the caveolin gene gamily, caveolin-3. Caveolin-3 is most closely related to caveolin-1 based on protein sequence homology; caveolin-1 and caveolin-3 are approximately 65% identical and approximately 85% similar. A single stretch of eight amino acids (FED-VIAEP) is identical in caveolin-1, -2, and -3. This conserved region may represent a "caveolin signature sequence" that is characteristic of members of the caveolin gene family. Caveolin-3 mRNA is expressed predominantly in muscle tissue-types (skeletal muscle, diaphragm, and heart) and is selectively induced during the differentiation of skeletal C2C12 myoblasts in culture. In many respects, caveolin-3 is similar to caveolin-1: (i) caveolin-3 migrates in velocity gradients as a high molecular mass complex; (ii) caveolin-3 colocalizes with caveolin-1 by immunofluorescence microscopy and cell fractionation studies; and (iii) a caveolin-3-derived polypeptide functionally suppresses the basal GTPase activity of purified heterotrimeric G-proteins. Identification of a muscle-specific member of the caveolin gene family may have implications for understanding the role of caveolin in different muscle cell types (smooth, cardiac, and skeletal) as previous morphological studies have demonstrated that caveolae are abundant in these cells. Our results also suggest that other as yet unknown caveolin family members are likely to exist and may be expressed in a regulated or tissue-specific fashion.

Identification, sequence, and expression of caveolin-2 defines a caveolin gene family

Caveolin, a 21- to 24-kDa integral membrane protein, is a principal component of caveolae membranes. Caveolin interacts directly with heterotrimeric guanine nucleotide binding proteins (G proteins) and can functionally regulate their activity. Here, an approximately 20-kDa caveolin-related protein, caveolin-2, was identified through microsequencing of adipocyte-derived caveolin-enriched membranes; caveolin was retermed caveolin-1. Caveolins 1 and 2 are similar in most respects. mRNAs for both caveolin-1 and caveolin-2 are most abundantly expressed in white adipose tissue and are induced during adipocyte differentiation. Caveolin-2 colocalizes with caveolin-1, indicating that caveolin-2 also localizes to caveolae. However, caveolin-1 and caveolin-2 differ in their functional interactions with heterotrimeric G proteins, possibly explaining why caveolin-1 and -2 are coexpressed within a single cell.

Expression and characterization of recombinant caveolin. Purification by polyhistidine tagging and cholesterol-dependent incorporation into defined lipid membranes

Caveolin, a 22-24-kDa integral membrane protein, is a principal component of caveolar membranes in vivo. Caveolin has been proposed to function as a scaffolding protein to organize and concentrate signaling molecules within caveolae. Because of its unusual membrane topology, both the N- and C-terminal domains of caveolin remain entirely cytoplasmic and are not subject to luminal modifications that are accessible to other integral membrane proteins. Under certain conditions, caveolin also exists in a soluble form as a cytosolic protein in vivo. These properties make caveolin an attractive candidate for recombinant expression in Escherichia coli. Here, we successfully expressed recombinant full-length caveolin in E.coli. A polyhistidine tag was placed at its extreme C terminus for purification by Ni(2+)-nitrilotriacetic acid affinity chromatography. Specific antibody probes demonstrated that recombinant caveolin contained a complete N and C terminus. Recombinant caveolin remained soluble in solutions containing the detergent octyl glucoside and formed high molecular mass oligomers like endogenous caveolin. By electron microscopy, recombinant caveolin homo-oligomers appeared as individual spherical particles that were indistinguishable from endogenous caveolin homo-oligomers visualized by the same technique. As recombinant caveolin behaved as expected for endogenous caveolin, this provides an indication that recombinant caveolin can be used to dissect the structural and functional interaction of caveolin with other protein and lipid molecules in vitro. Recombinant caveolin was efficiently incorporated into lipid membranes as assessed by floatation in sucrose density gradients. This allowed us to use defined lipid components to assess the possible requirements for insertion of caveolin into membranes. Using a purified synthetic form of phosphatidylcholine (1,2-dioleoylphosphorylcholine), we observed that incorporation of caveolin into membranes was cholesterol-dependent; the addition of cholesterol dramatically increased the incorporation of caveolin into these phosphatidylcholine-based membranes by approximately 25-30-fold. This fits well with in vivo studies demonstrating that cholesterol plays an essential role in maintaining the structure and function of caveolae. Further functional analysis of these reconstituted caveolin-containing membranes showed that they were capable of recruiting a soluble recombinant form of G(i)2 alpha. This is in accordance with previous studies demonstrating that caveolin specifically interacts directly with multiple G protein alpha-subunits. Thus, recombinant caveolin incorporated into defined lipid membranes provides an experimental system in which the structure, function, and biogenesis of caveolin-rich membrane domains can be dissected in vitro.

M-caveolin, a muscle-specific caveolin-related protein

Caveolae, small invaginations of the plasma membrane, are a characteristic feature of many mammalian cells. The best-characterised caveolar protein is the integral membrane protein, VIP21-caveolin. We now describe a novel homologue of VIP21-caveolin, M-caveolin, which is expressed exclusively in muscle. M-caveolin was shown to be expressed in differentiated myotubes but not myoblasts. Epitope-tagged M-caveolin expressed in non-muscle cells was targetted to surface caveolae where it colocalized with endogenous VIP21-caveolin. M-caveolin may play a specialised role in the caveolae of muscle cells.

M-caveolin, a muscle-specific caveolin-related protein

Caveolae, small invaginations of the plasma membrane, are a characteristic feature of many mammalian cells. The best-characterised caveolar protein is the integral membrane protein, VIP21-caveolin. We now describe a novel homologue of VIP21-caveolin, M-caveolin, which is expressed exclusively in muscle. M-caveolin was shown to be expressed in differentiated myotubes but not myoblasts. Epitope-tagged M-caveolin expressed in non-muscle cells was targetted to surface caveolae where it colocalized with endogenous VIP21-caveolin. M-caveolin may play a specialised role in the caveolae of muscle cells.

A photo-reactive derivative of ganglioside GM1 specifically cross-links VIP21-caveolin on the cell surface

Previous studies have shown that sphingolipids may be enriched in caveolae, plasmalemmal invaginations implicated in endocytosis and signal transduction. We synthesised a radiolabeled derivative of ganglioside GM1 bearing a photo-reactive cross-linker at the end of its acyl chain. After insertion in the plasma membrane of cultured A431 or MDCK cells and photoactivation, the main protein cross-linked by the GM1 derivative was VIP21-caveolin, an essential structural component of caveolae. This result shows close proximity between GM1 molecules and VIP21-caveolin in the caveolar membrane and strongly implicates sphingolipid segregation in the biogenesis of caveolae.

VIP21/caveolin is a cholesterol-binding protein

VIP21/caveolin is localized to both caveolae and apical transport vesicles and presumably cycles between the cell surface and the Golgi complex. We have studied the lipid interactions of this protein by reconstituting Escherichia coli-expressed VIP21/caveolin into liposomes. Surprisingly, the protein reconstituted only with cholesterol-containing lipid mixtures. We demonstrated that the protein binds at least 1 mol of cholesterol per mole of protein and that this binding promotes formation of protein oligomers. These findings suggest that VIP21/caveolin, through its cholesterol-binding properties, serves a specific function in microdomain formation during membrane trafficking.

De novo formation of caveolae in lymphocytes by expression of VIP21-caveolin

Caveolae are plasma membrane invaginations, which have been implicated in endothelial transcytosis, endocytosis, potocytosis, and signal transduction. In addition to their well-defined morphology, caveolae are characterized by the presence of an integral membrane protein termed VIP21-caveolin. We have recently observed that lymphocytes have no detectable VIP21-caveolin and lack plasma membrane invaginations resembling caveolae. Here we transiently express VIP21-caveolin in a lymphocyte cell line using the Semliki Forest virus expression system and show de novo formation of plasma membrane invaginations containing VIP21-caveolin. These invaginations appear homogeneous in size and morphologically indistinguishable from caveolae of nonlymphoid cells. Moreover, the glycosylphosphatidylinositol-anchored protein. Thy1, patched by antibodies, redistributes to the newly formed caveolae. Our results show that VIP21-caveolin is a key structural component required for caveolar biogenesis.