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

Association of phosphofructokinase-M with caveolin-3 in differentiated skeletal myotubes. Dynamic regulation by extracellular glucose and intracellular metabolites

Caveolin-3 is a member of the caveolin family of proteins that is primarily expressed in striated muscle cell types (skeletal and cardiac). Here, we show that an approximately 80-kDa protein specifically co-immunoprecipitates with caveolin-3 expressed in differentiated skeletal C2C12 myotubes. Microsequence analysis of this approximately 80-kDa polypeptide revealed its identity as a key regulatory enzyme in the glycolytic pathway, namely phosphofructokinase-M (PFK-M). Pulse-chase experiments demonstrate that PFK-M associates with caveolin-3 with a significant time lag after the biosynthesis of PFK-M. In addition, we show that this interaction is (i) highly regulated by the extracellular concentration of glucose and (ii) can be stabilized by a number of relevant intracellular metabolites, such as fructose 1,6-bisphosphate and fructose 2,6-bisphosphate, which are known allosteric activators of PFK. While the bulk of these experiments were performed in C2C12 cells, identical results were obtained using mouse skeletal muscle extracts. Taken together, our results suggest that glucose-dependent plasma membrane recruitment of activated PFK-M by caveolin-3 could have important implications for understanding the mechanisms that regulate energy metabolism in skeletal muscle fibers.

Direct interaction of endothelial nitric-oxide synthase and caveolin-1 inhibits synthase activity

Endothelial nitric-oxide synthase (eNOS) and caveolin-1 are associated within endothelial plasmalemmal caveolae. It is not known, however, whether eNOS and caveolin-1 interact directly or indirectly or whether the interaction affects eNOS activity. To answer these questions, we have cloned the bovine caveolin-1 cDNA and have investigated the eNOS-caveolin-1 interaction in an in vitro binding assay system using glutathione S-transferase (GST)-caveolin-1 fusion proteins and baculovirus-expressed bovine eNOS. We have also mapped the domains involved in the interaction using an in vivo yeast two-hybrid system. Results obtained using both in vitro and in vivo protein interaction assays show that both N- and C-terminal cytosolic domains of caveolin-1 interact directly with the eNOS oxygenase domain. Interaction of eNOS with GST-caveolin-1 fusion proteins significantly inhibits enzyme catalytic activity. A synthetic peptide corresponding to caveolin-1 residues 82-101 also potently and reversibly inhibits eNOS activity by interfering with the interaction of the enzyme with Ca2+/calmodulin (CaM). Regulation of eNOS in endothelial cells, therefore, may involve not only positive allosteric regulation by Ca2+/CaM, but also negative allosteric regulation by caveolin-1.

Dynamic targeting of the agonist-stimulated m2 muscarinic acetylcholine receptor to caveolae in cardiac myocytes

In cardiac myocytes, as well as specialized conduction and pacemaker cells, agonist binding to muscarinic acetylcholine receptors (mAchRs) results in the activation of several signal transduction cascades including the endothelial isoform of nitric-oxide synthase (eNOS) expressed in these cells. Recent evidence indicates that, as in endothelial cells, eNOS in cardiac myocytes is localized to plasmalemma caveolae, specialized lipid microdomains that contain caveolin-3, a muscle-specific isoform of the scaffolding protein caveolin. In this report, using a detergent-free method for isolation of sarcolemmal caveolae from primary cultures of adult rat ventricular myocytes, we demonstrated that the muscarinic cholinergic agonist carbachol promotes the translocation of mAchR into low density gradient fractions containing most myocyte caveolin-3 and eNOS. Following isopycnic centrifugation, the different gradient fractions were exposed to the muscarinic radioligand [3H]quinuclidinyl benzilate (QNB), and binding was determined after membrane filtration or immunoprecipitation. In a direct radioligand binding assay, we found that [3H]QNB binding can be detected in caveolin-enriched fractions only when cardiac myocytes have been previously exposed to carbachol. Furthermore, most of this [3H]QNB binding can be specifically immunoprecipitated by an antibody to the m2 mAchR, indicating that the translocation of this receptor subtype is responsible for the [3H]QNB binding detected in the low density fractions. Moreover, the [3H]QNB binding could be quantitatively immunoprecipitated from the light membrane fractions with a caveolin-3 antibody (but not a control IgG1 antibody), confirming that the m2 mAchR is targeted to caveolae after carbachol treatment. Importantly, atropine, a muscarinic cholinergic antagonist, did not induce translocation of m2 mAchR to caveolae and prevented receptor translocation in response to the agonist carbachol. Thus, dynamic targeting of sarcolemmal m2 mAchR to caveolae following agonist binding may be essential to initiate specific downstream signaling cascades in these cells.

Increased expression of caveolin-1 in heterozygous Niemann-Pick type II human fibroblasts

Human Niemann-Pick type II fibroblasts, which encompass the panethnic type C (NPC) and Nova Scotia Acadian type D (NPD) variants, exhibit altered expression of caveolin-1 protein when examined by immunoblotting using an anti-caveolin-1 monoclonal antibody. Unexpectedly, caveolin-1 in heterozygous fibroblasts was significantly elevated as much as 10-fold compared to caveolin-1 in normal and homozygous affected fibroblasts. Homozygous NPC fibroblasts expressed caveolin-1 levels similar to those in normal fibroblasts, while the expression of caveolin-1 in homozygous NPD fibroblasts was slightly elevated. Northern analysis indicates that normal fibroblasts and NPC heterozygous fibroblasts have similar amounts of caveolin-1 mRNA, while NPC homozygous fibroblasts have significantly less caveolin-1 mRNA. In contrast, heterozygous and homozygous NPD fibroblasts exhibit increased levels of caveolin-1 mRNA. These novel findings suggest that caveolin-1 containing subcellular structures are involved in the pathophysiology of Niemann-Pick type II disease. Furthermore, altered caveolin-1 protein expression may serve as a useful marker for the diagnosis of carriers of NPC or NPD.

Caveolin-1 detergent solubility and association with endothelial nitric oxide synthase is modulated by tyrosine phosphorylation

Caveolin-1 and endothelial nitric oxide synthase (eNOS) are associated within endothelial caveolae. We have shown previously that eNOS is translocated to the detergent-insoluble, cytoskeletal fraction of bovine aortic endothelial cells (BAEC) in response to bradykinin (BK)-stimulation or tyrosine phosphatase inhibition. In the present study, we have examined whether caveolin-1 is similarly translocated in response to these or other stimuli. Exposure of BAEC to the eNOS-activating agonists, BK, histamine, or ATP produces transient increases in the amounts of detergent-insoluble caveolin-1. Increases in insolubility are blocked by tyrosine kinase inhibitors and are potently mimicked by tyrosine phosphatase inhibitors. Increased insolubility is accompanied by an increased association of caveolin-1 with eNOS and inhibition of eNOS catalytic activity. Agonist-activation of eNOS in endothelial cells thus appears to involve tyrosine phosphorylation-dependent changes in the interaction of eNOS with caveolin-1. Increased interaction of eNOS with caveolin-1 may deactivate the enzyme subsequent to its activation by Ca2+/calmodulin.

Recombinant expression of caveolin-1 in oncogenically transformed cells abrogates anchorage-independent growth

Caveolae are plasma membrane-attached vesicular organelles. Caveolin-1, a 21-24-kDa integral membrane protein, is a principal component of caveolae membranes in vivo. Both caveolae and caveolin are most abundantly expressed in terminally differentiated cells: adipocytes, endothelial cells, and muscle cells. Conversely, caveolin-1 mRNA and protein expression are lost or reduced during cell transformation by activated oncogenes such as v-abl and H-ras (G12V); caveolae are absent from these cell lines. However, its remains unknown whether down-regulation of caveolin-1 protein and caveolae organelles contributes to their transformed phenotype. Here, we have expressed caveolin-1 in oncogenically transformed cells under the control of an inducible-expression system. Regulated induction of caveolin-1 expression was monitored by Western blot analysis and immunofluorescence microscopy. Our results indicate that caveolin-1 protein is expressed well using this system and correctly localizes to the plasma membrane. Induction of caveolin-1 expression in v-Abl-transformed and H-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 as seen by transmission electron microscopy. Consistent with its antagonism of Ras-mediated cell transformation, caveolin-1 expression dramatically inhibited both Ras/MAPK-mediated and basal transcriptional activation of a mitogen-sensitive promoter. Using an established system to detect apoptotic cell death, it appears that the effects of caveolin-1 may, in part, be attributed to its ability to initiate apoptosis in rapidly dividing cells. In addition, we find that caveolin-1 expression levels are reversibly down-regulated by two distinct oncogenic stimuli. Taken together, our results indicate that down-regulation of caveolin-1 expression and caveolae organelles may be critical to maintaining the transformed phenotype in certain cell populations.

Reciprocal regulation of endothelial nitric-oxide synthase by Ca2+-calmodulin and caveolin

The endothelial nitric-oxide synthase (eNOS) is a key determinant of vascular homeostasis. Like all known nitric-oxide synthases, eNOS enzyme activity is dependent on Ca2+-calmodulin. eNOS is dynamically targeted to specialized cell surface signal-transducing domains termed plasmalemmal caveolae and interacts with caveolin, an integral membrane protein that comprises a key structural component of caveolae. We have previously reported that the association between eNOS and caveolin is quantitative and tissue-specific (Feron, O., Belhassen, L., Kobzick, L., Smith, T. W., Kelly, R. A., and Michel, T. (1996) J. Biol. Chem. 271, 22810-22814). We now report that in endothelial cells the interaction between eNOS and caveolin is importantly regulated by Ca2+-calmodulin. Addition of calmodulin disrupts the heteromeric complex formed between eNOS and caveolin in a Ca2+-dependent fashion. In addition, overexpression of caveolin markedly attenuates eNOS enzyme activity, but this inhibition is reversed by purified calmodulin. Caveolin overexpression does not affect the activity of the other NOS isoforms, suggesting eNOS-specific inhibition of NO synthase by caveolin. We propose a model of reciprocal regulation of eNOS in endothelial cells wherein the inhibitory eNOS-caveolin complex is disrupted by binding of Ca2+-calmodulin to eNOS, leading to enzyme activation. These findings may have broad implications for the regulation of Ca2+-dependent signal transduction in plasmalemmal caveolae.

Functional rafts in cell membranes

A new aspect of cell membrane structure is presented, based on the dynamic clustering of sphingolipids and cholesterol to form rafts that move within the fluid bilayer. It is proposed that these rafts function as platforms for the attachment of proteins when membranes are moved around inside the cell and during signal transduction.

Flotillin and epidermal surface antigen define a new family of caveolae-associated integral membrane proteins

Caveolae are plasmalemmal microdomains that are involved in vesicular trafficking and signal transduction. We have sought to identify novel integral membrane proteins of caveolae. Here we describe the identification and molecular cloning of flotillin. By several independent methods, flotillin behaves as a resident integral membrane protein component of caveolae. Furthermore, we have identified epidermal surface antigen both as a flotillin homologue and as a resident caveolar protein. Significantly, flotillin is a marker for the Triton-insoluble, buoyant membrane fraction in brain, where to date mRNA species for known caveolin gene family members have not been detected.

Physical modulation of intracellular signaling processes by locational regulation

Recent observations in the field of signal transduction suggest that where a protein is located within a cell can be as important as its activity measured in solution for activation of its downstream pathway. The physical organization of the cell can provide an additional layer of control upon the chemical reaction networks that govern ultimately perceived signals. Using the cytosol and plasma membrane as relevant compartmental distinctions, we analyze the effect of relocation on the rate of association with a membrane-associated target. We quantify this effect as an enhancement factor E in terms of measurable parameters such as the number of available targets, molecular diffusivities, and intrinsic reaction rate constants. We then employ two simple yet relevant example models to illustrate how relocation can affect the dynamics of signal transduction pathways. The temporal profiles and phase behavior of these models are investigated. We also relate experimentally observable aspects of signal transduction such as peak activation and the relative time scales of stimulus and response to quantitative aspects of the relocation mechanisms in our models. In our example schemes, nearly complete relocation of the cytosolic species in the signaling pair is required to generate meaningful activation of the model pathways when the association rate enhancement factor E is as low as 10; when E is 100 or greater, only a small fraction of the protein must be relocated.

Regulation by cAMP of post-translational processing and subcellular targeting of endothelial nitric-oxide synthase (type 3) in cardiac myocytes

Cardiac myocytes express the nitric-oxide synthase isoform originally identified in endothelial cells, termed eNOS or NOS3, where it plays a role in regulating myocyte responsiveness to both adrenergic and muscarinic cholinergic autonomic nervous system agonists. eNOS in endothelial cells has been shown to undergo extensive post-translational processing, and in cardiac myocytes as well as endothelial cells, eNOS has been shown to be targeted to plasmalemmal caveolae, a process that is dependent on myristoylation and palmitoylation. Other post-translational modifications essential for the correct subcellular targeting of eNOS have not been described previously. We demonstrate, using [35S]methionine pulse-chase experiments, that native eNOS in adult rat ventricular myocytes is initially translated as a nonpalmitoylated 150-kDa isoform, which is associated with cytosolic and intracellular membrane-enriched fractions. This is subsequently processed to a palmitoylated 135-kDa isoform, which is found only in a sarcolemma-enriched membrane fraction. Forskolin, an agent that elevates intracellular cAMP, rapidly inhibited processing of the 150-kDa isoform to the 135-kDa isoform and transport of eNOS to the sarcolemma, effects paralleled by protein kinase A-dependent phosphorylation of the larger eNOS isoform. Forskolin also decreased palmitoylation of the 135-kDa isoform, although it did not accelerate depalmitoylation of sarcolemmal eNOS, as determined by pulse-chase experiments with [3H]palmitate. Thus, post-translational processing of a 150-kDa isoform of myocyte eNOS appears to be necessary for intracellular trafficking of the enzyme to sarcolemmal caveolae. Both the post-translational processing and subcellular targeting of eNOS appear to be modified by changes in intracellular cAMP, an effect that may have important implications for cardiac myocyte responsiveness to autonomic agonists in vivo.

Assembly and intracellular targeting of the betagamma subunits of heterotrimeric G proteins

The assembly in living cells of heterotrimeric guanine nucleotide binding proteins from their constituent alpha, beta, and gamma subunits is a complex process, compounded by the multiplicity of the genes that encode them, and the diversity of receptors and effectors with which they interact. Monoclonal anti-beta antibodies (ARC5 and ARC9), raised against immunoaffinity purified beta gamma complexes, recognize beta subunits when not associated with gamma and can thus be used to monitor assembly of beta gamma complexes. Complex formation starts immediately after synthesis and is complete within 30 min. Assembly occurs predominantly in the cytosol, and association of beta gamma complexes with the plasma membrane fraction starts between 15-30 min of chase. Three pools of beta subunits can be distinguished based on their association with gamma subunits, their localization, and their detergent solubility. Association of beta and alpha subunits with detergent-insoluble domains occurs within 1 min of chase, and increases to reach a plateau of near complete detergent resistance within 30 min of chase. Brefeldin A treatment does not interfere with delivery of beta gamma subunits to detergent-insoluble domains, suggesting that assembly of G protein subunits with their receptors occurs distally from the BFA-imposed block of intracellular membrane trafficking and may occur directly at the plasma membrane.

Association of p75(NTR) with caveolin and localization of neurotrophin-induced sphingomyelin hydrolysis to caveolae

Caveolae are plasma membrane microdomains that are enriched in caveolin, the structural protein of caveolae, sphingomyelin, and other signaling molecules. We previously suggested that neurotrophin-induced p75(NTR)-dependent sphingomyelin hydrolysis may be localized to the plasma membrane. Therefore, we examined if caveolae were a major site of p75(NTR)-dependent sphingomyelin hydrolysis in p75(NTR)-NIH 3T3 fibroblasts. Caveolin-enriched membranes (CEMs) were prepared by either detergent or detergent-free extraction and separated from noncaveolar membranes by centrifugation through sucrose gradients. Immunoblot analysis of the individual gradient fractions indicated that caveolin and p75(NTR) were enriched in CEMs. The localization of p75(NTR) to CEMs was not an artifact of receptor overexpression in the fibroblasts because a similar distribution of p75(NTR) was evident from PC12 cells, which endogenously express p75(NTR). In the p75(NTR) fibroblasts, nerve growth factor induced a time-dependent hydrolysis of sphingomyelin only in CEMs with no hydrolysis detected in noncaveolar membranes. Intriguingly, endogenous p75(NTR) was found to co-immunoprecipitate with caveolin, suggesting that p75(NTR) may associate with caveolin in vivo. This interaction was confirmed in vitro by the co-immunoprecipitation of a glutathione S-transferase fusion protein expressing the cytoplasmic domain of p75(NTR) with caveolin. Collectively, these results demonstrate that neurotrophin-induced p75(NTR)-dependent sphingomyelin hydrolysis localizes to CEMs and suggest that the interaction of p75(NTR) with caveolin may affect signaling through p75(NTR).

Reversible palmitoylation of signaling proteins

Palmitoylation is unique among lipid modifications of proteins in that it is reversible and regulable. Recent advances in the study of palmitoylation include the following: the correlation of this modification with the localization of a signaling protein to specific membrane subdomains; the demonstration of a specific protein-protein interaction that is promoted by palmitoylation; and the identification, characterization, and purification of enzymes catalyzing this modification.

Identification of peptide and protein ligands for the caveolin-scaffolding domain. Implications for the interaction of caveolin with caveolae-associated proteins

Caveolin, a 21-24-kDa integral membrane protein, is a principal component of caveolae membranes. We have suggested that caveolin functions as a scaffolding protein to organize and concentrate certain caveolin-interacting proteins within caveolae membranes. In this regard, caveolin co-purifies with a variety of lipid-modified signaling molecules, including G-proteins, Src-like kinases, Ha-Ras, and eNOS. Using several independent approaches, it has been shown that a 20-amino acid membrane proximal region of the cytosolic amino-terminal domain of caveolin is sufficient to mediate these interactions. For example, this domain interacts with G-protein alpha subunits and Src-like kinases and can functionally suppress their activity. This caveolinderived protein domain has been termed the caveolin-scaffolding domain. However, it remains unknown how the caveolin-scaffolding domain recognizes these molecules. Here, we have used the caveolin-scaffolding domain as a receptor to select random peptide ligands from phage display libraries. These caveolin-selected peptide ligands are rich in aromatic amino acids and have a characteristic spacing in many cases. A known caveolin-interacting protein, Gi2alpha, was used as a ligand to further investigate the nature of this interaction. Gi2alpha and other G-protein alpha subunits contain a single region that generally resembles the sequences derived from phage display. We show that this short peptide sequence derived from Gi2alpha interacts directly with the caveolin-scaffolding domain and competitively inhibits the interaction of the caveolin-scaffolding domain with the appropriate region of Gi2alpha. This interaction is strictly dependent on the presence of aromatic residues within the peptide ligand, as replacement of these residues with alanine or glycine prevents their interaction with the caveolin-scaffolding domain. In addition, we have used this interaction to define which residues within the caveolin-scaffolding domain are critical for recognizing these peptide and protein ligands. Also, we find that the scaffolding domains of caveolins 1 and 3 both recognize the same peptide ligands, whereas the corresponding domain within caveolin-2 fails to recognize these ligands under the same conditions. These results serve to further demonstrate the specificity of this interaction. The implications of our current findings are discussed regarding other caveolin- and caveolae-associated proteins.

Retargeting of cytosolic proteins to the plasma membrane by the Lck protein tyrosine kinase dual acylation motif

Several members of the Src family of protein tyrosine kinases have a N-terminal dual acylation motif which specifies their myristoylation and S-acylation. These lipid modifications are necessary for correct intracellular localisation to the plasma membrane and to detergent-resistant glycolipid-enriched membrane domains (GEMs). Using chimaeras of the Lck dual acylation motif with two normally cytosolic proteins (chloramphenicol acetyl transferase and galectin-3), we show here that this motif is sufficient to encode correct lipid modification and to target these chimaeras to the plasma membrane, as demonstrated by subcellular fractionation and confocal immunofluorescence microscopy of transiently transfected COS cells. In addition, the chimaeras are resistant to extraction with cold non-ionic detergent, indicating targeting to GEM subdomains in the plasma membrane. The dual acylation motif has potential for targeting proteins to specific plasma membrane subdomains involved in signalling.

Compartmentalized activation of the high affinity immunoglobulin E receptor within membrane domains

The earliest known step in the activation of the high affinity IgE receptor, FcepsilonRI, is the tyrosine phosphorylation of its beta and gamma subunits by the Src family tyrosine kinase, Lyn. We report here that aggregation-dependent association of FcepsilonRI with specialized regions of the plasma membrane precedes its tyrosine phosphorylation and appears necessary for this event. Tyrosine phosphorylation of beta and gamma occurs in intact cells only for FcepsilonRI that associate with these detergent-resistant membrane domains, which are enriched in active Lyn. Furthermore, efficient in vitro tyrosine phosphorylation of FcepsilonRI subunits occurs only for those associated with isolated domains. This association and in vitro phosphorylation are highly sensitive to low concentrations of detergent, suggesting that lipid-mediated interactions with Lyn are important in FcepsilonRI activation. Participation of membrane domains accounts for previously unexplained aspects of FcepsilonRI-mediated signaling and may be relevant to signaling by other multichain immune receptors.

Mutational analysis of the properties of caveolin-1. A novel role for the C-terminal domain in mediating homo-typic caveolin-caveolin interactions

Caveolin is a principal structural component of caveolae membranes in vivo. Recently, a family of caveolin-related proteins has been identified; caveolin has been retermed caveolin-1. Caveolin family members share three characteristic properties: (i) detergent insolubility at low temperatures; (ii) self-oligomerization; and (iii) incorporation into low density Triton-insoluble fractions enriched in caveolae membranes. Here, we have used a deletion mutagenesis approach as a first step toward understanding which regions of caveolin-1 contribute to its unusual properties. Two caveolin-1 deletion mutants were created that lack either the C-terminal domain (Cav-1DeltaC) or the N-terminal domain (Cav-1DeltaN); these mutants were compared with the behavior of full-length caveolin-1 (Cav-1FL) expressed in parallel. Our results show that the N-terminal domain and membrane spanning segment are sufficient to form high molecular mass oligomers of caveolin-1. However, a complete caveolin-1 molecule is required for conveying detergent insolubility and incorporation into low density Triton-insoluble complexes. These data indicate that homo-oligomerization and an intact transmembrane are not sufficient to confer detergent insolubility, suggesting an unknown role for the C-terminal domain in this process. To better understand the role of the C-terminal domain, this region of caveolin-1 (residues 135-178) was expressed as a glutathione S-transferase fusion protein in Escherichia coli. Purified recombinant glutathione S-transferase-C-Cav-1 was found to stably interact with full-length caveolin-1 but not with the two caveolin-1 deletion mutants. These results suggest that the C-terminal domain interacts with both the N-terminal and C-terminal domains of an adjacent caveolin-1 homo-oligomer. This appears to be a specific homo-typic interaction, because the C-terminal domain of caveolin-1 failed to interact with full-length forms of caveolin-2 and caveolin-3. Homo-typic interaction of the C-terminal domain with an adjacent homo-oligomer could provide a mechanism for clustering caveolin-1 homo-oligomers while excluding other caveolin family members. This type of lateral segregation event could promote caveolae membrane formation and contribute to the detergent insolubility of caveolins-1, -2, and -3.

Signal transduction from multiple Ras effectors

Ras proteins activate a signaling cascade through direct binding of the serine/threonine kinase Raf. They also activate additional signaling pathways that are essential for full biological activity. Candidate effectors for these pathways include RalGDS and phosphatidyl inositol 3' kinase, as well as several other Ras binding proteins the biochemical and biological properties of which are poorly understood.

Identification, sequence, and expression of an invertebrate caveolin gene family from the nematode Caenorhabditis elegans. Implications for the molecular evolution of mammalian caveolin genes

Caveolae are vesicular organelles that represent an appendage of the plasma membrane. Caveolin, a 21-24-kDa integral membrane protein, is a principal component of caveolae membranes in vivo. Caveolin has been proposed to function as a plasma membrane scaffolding protein to organize and concentrate signaling molecules within caveolae, including heterotrimeric G proteins (alpha and betagamma subunits). In this regard, caveolin interacts directly with Galpha subunits and can functionally regulate their activity. To date, three cDNAs encoding four subtypes of caveolin have been described in vertebrates. However, evidence for the existence of caveolin proteins in less complex organisms has been lacking. Here, we report the identification, cDNA sequence and genomic organization of the first invertebrate caveolin gene, Cavce (for caveolin from Caenorhabditis elegans). The Cavce gene, located on chromosome IV, consists of two exons interrupted by a 125-nucleotide intron sequence. The region of Cavce that is strictly homologous to mammalian caveolins is encoded by a single exon in Cavce. This suggests that mammalian caveolins may have evolved from the second exon of Cavce. Cavce is roughly equally related to all three known mammalian caveolins and, thus, could represent a common ancestor. Remarkably, the invertebrate Cavce protein behaves like mammalian caveolins: (i) Cavce forms a high molecular mass oligomer, (ii) assumes a cytoplasmic membrane orientation, and (iii) interacts with G proteins. A 20-residue peptide encoding the predicted G protein binding region of Cavce possesses "GDP dissociation inhibitor-like activity" with the same potency as described earlier for mammalian caveolin-1. Thus, caveolin appears to be structurally and functionally conserved from worms to man. In addition, we find that there are at least two caveolin-related genes expressed in C. elegans, defining an invertebrate caveolin gene family. These results establish the nematode C. elegans as an invertebrate model system to study caveolae and caveolin in vivo.

Understanding covalent modifications of proteins by lipids: where cell biology and biophysics mingle

Much effort has been expended on the in vitro characterization of enzymes that covalently attach lipids to proteins. Less information is available about properties conferred on modified proteins by their attached lipid groups, but biophysical studies of simple model systems have begun to shed light on this issue. Recent evidence suggests that the specificity of lipid modifications may be dependent upon the intracellular compartmentalization of the lipid and protein substrates of lipidating enzymes. The function and targeting of their lipidated products appear to be regulated dynamically through addition or subtraction of lipid moieties, other covalent or noncovalent modifications, as well as several devices that at this point can only be inferred. This field of research illustrates the necessity of integrating cell-biological and biophysical perspectives.

Identification of Triton X-100 insoluble membrane domains in the yeast Saccharomyces cerevisiae. Lipid requirements for targeting of heterotrimeric G-protein subunits

Low density Triton X-100 insoluble (LDTI) membrane domains are found in most mammalian cell types. Previous biochemical and immunolocalization studies have revealed the presence of G-protein coupled receptors and heterotrimeric G-protein subunits (Galpha and Gbetagamma subunits) within these structures, implicating mammalian LDTI membrane domains in G-protein coupled signaling. Here, we present biochemical evidence that similar LDTI structures exist in a genetically tractable organism, the yeast Saccharomyces cerevisiae. Yeast LDTI membranes were purified based on the known biochemical properties of mammalian LDTI membranes: (i) their Triton X-100 insolubility; and (ii) their discrete buoyant density in sucrose gradients. As with purified mammalian LDTI membranes, these yeast LDTI membranes harbor the subunits of the heterotrimeric G-proteins (Galpha and Gbetagamma subunits). Other plasma membrane marker proteins (the plasma membrane H+-ATPase and a GPI-linked protein Gas1p) are preferentially excluded from these purified fractions. Mutational and genetic analyses were performed to define the requirements for the targeting of G-protein subunits to these yeast membrane domains. We find that the targeting of Galpha is independent of myristoylation, whereas targeting of Ggamma requires prenylation. Perhaps surprisingly, the targeting of Gbeta to this membrane domain did not require coexpression of Ggamma. It should now be possible to dissect the function of LDTI membrane domains using yeast as a model genetic system.

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.

Endothelial nitric oxide synthase is regulated by tyrosine phosphorylation and interacts with caveolin-1

The regulation of endothelial nitric oxide synthase (eNOS) by phosphorylation is poorly understood. Here, we demonstrate that eNOS is tyrosine-phosphorylated in bovine aortic endothelial cells (BAEC) using 32P metabolic labeling followed by phosphoamino acid analysis and by phosphotyrosine specific Western blotting. Treatment of BAEC with hydrogen peroxide and the protein tyrosine phosphatase inhibitor, sodium orthovanadate, increases eNOS tyrosine phosphorylation. Utilizing a novel immunoNOS assay, the increase in tyrosine phosphorylation is associated with a 50% decrease in the specific activity of the enzyme. Because eNOS is localized in plasmalemma caveolae, we examined if tyrosine phosphorylated eNOS interacts with caveolin-1, the coat protein of caveolae. Immunoprecipitation of eNOS from bovine lung microvascular endothelial cells resulted in the co-precipitation of caveolin-1. Conversely, immunoprecipitation of caveolin-1 resulted in the co-precipitation of tyrosine-phosphorylated eNOS. Thus, tyrosine phosphorylation is a novel regulatory mechanism for eNOS and caveolin-1 is the first eNOS-associated protein. Collectively, these observations provide a novel regulatory mechanism for eNOS and suggest that tyrosine phosphorylation may influence its activity, subcellular trafficking, and interaction with other caveolin-interacting proteins in caveolae.

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.