Mutational analysis of caveolin-induced vesicle formation. Expression of caveolin-1 recruits caveolin-2 to caveolae membranes

Caveolae are vesicular organelles with a characteristic uniform diameter in the range of 50-100 nm. Although recombinant expression of caveolin-1 is sufficient to drive caveolae formation, it remains unknown what controls the uniform diameter of these organelles. One hypothesis is that specific caveolin-caveolin interactions regulate the size of caveolae, as caveolin-1 undergoes two stages of self-oligomerization. To test this hypothesis directly, we have created two caveolin-1 deletion mutants that lack regions of caveolin-1 that are involved in directing the self-assembly of caveolin-1 oligomers. More specifically, Cav-1 delta61-100 lacks a region of the N-terminal domain that directs the formation of high molecular mass caveolin-1 homo-oligomers, while Cav-1 deltaC lacks a complete C-terminal domain that is required to allow caveolin homo-oligomers to interact with each other, forming a caveolin network. It is important to note that these two mutants retain an intact transmembrane domain. Our current results show that although Cav-1 delta61-100 and Cav-1 deltaC are competent to drive vesicle formation, these vesicles vary widely in their size and shape with diameters up to 500-1000 nm. In addition, caveolin-induced vesicle formation appears to be isoform-specific. Recombinant expression of caveolin-2 under the same conditions failed to drive the formation of vesicles, while caveolin-3 expression yielded caveolae-sized vesicles. These results are consistent with the previous observation that in transformed NIH 3T3 cells that lack caveolin-1 expression, but continue to express caveolin-2, no morphologically distinguishable caveolae are observed. In addition, as caveolin-2 alone exists mainly as a monomer or homo-dimer, while caveolins 1 and 3 exist as high molecular mass homo-oligomers, our results are consistent with the idea that the formation of high molecular mass oligomers of caveolin are required to regulate the formation of uniform caveolae-sized vesicles. In direct support of this notion, regulated induction of caveolin-1 expression in transformed NIH 3T3 cells was sufficient to recruit caveolin-2 to caveolae membranes. The ability of caveolin-1 to recruit caveolin-2 most likely occurs through a direct interaction between caveolins 1 and 2, as caveolins 1 and 2 are normally co-expressed and interact with each other to form high molecular mass hetero-oligomers containing both caveolins 1 and 2.

Dissecting the interaction between nitric oxide synthase (NOS) and caveolin. Functional significance of the nos caveolin binding domain in vivo

Endothelial nitric oxide synthase (eNOS) is a dually acylated peripheral membrane protein that targets to the Golgi region and caveolae of endothelial cells. Recent evidence has shown that eNOS can co-precipitate with caveolin-1, the resident coat protein of caveolae, suggesting a direct interaction between these two proteins. To test this idea, we examined the interactions of eNOS with caveolin-1 in vitro and in vivo. Incubation of endothelial cell lysates or purified eNOS with glutathione S-transferase (GST)-caveolin-1 resulted in the direct interaction of the two proteins. Utilizing a series of GST-caveolin-1 deletion mutants, we identified two cytoplasmic domains of caveolin-1 that interact with eNOS, the scaffolding domain (amino acids 61-101) and to a lesser extent the C-terminal tail (amino acids 135-178). Incubation of pure eNOS with peptides derived from the scaffolding domains of caveolin-1 and -3, but not the analogous regions from caveolin-2, resulted in inhibition of eNOS, inducible NOS (iNOS), and neuronal NOS (nNOS) activities. These results suggest a common mechanism and site of inhibition. Utilizing GST-eNOS fusions, the site of caveolin binding was localized between amino acids 310 and 570. Site-directed mutagenesis of the predicted caveolin binding motif within eNOS blocked the ability of caveolin-1 to suppress NO release in co-transfection experiments. Thus, our data demonstrate a novel functional role for caveolin-1 in mammalian cells as a potential molecular chaperone that directly inactivates NOS. This suggests that the direct binding of eNOS to caveolin-1, per se, and the functional consequences of eNOS targeting to caveolae are likely temporally and spatially distinct events that regulate NO production in endothelial cells. Additionally, the inactivation of eNOS and nNOS by the scaffolding domain of caveolin-3 suggests that eNOS in cardiac myocytes and nNOS in skeletal muscle are likely subject to negative regulation by this muscle-specific caveolin isoform.

Recombinant expression of the MAL proteolipid, a component of glycolipid-enriched membrane microdomains, induces the formation of vesicular structures in insect cells

The MAL proteolipid has been identified as a component of glycolipid-enriched membrane microdomains resistant to detergent solubilization in epithelial Madin-Darby canine cells, as well as in T lymphocytes and in myelin-forming cells. To study the function of the MAL proteolipid we have ectopically expressed a tagged form of MAL in both mammalian and insect cellular backgrounds. Immunofluorescence analysis in transiently transfected COS-7 cells showed the presence of MAL in large vesicular structures, and biochemical analysis identified MAL in the fraction of membranes resistant to Triton X-100 solubilization. Electron microscopic analysis showed that the expression of MAL in Sf21 cells morphologically resulted in the intracellular accumulation of large vesicles with a diameter from 200 to greater than 700 nm that were absent in uninfected or control infected cultures. Thus, ectopic expression of MAL in this heterologous expression system was sufficient to drive the formation of vesicles with a size similar to that of the vesicles detected in mammalian cells. These vesicles were clearly different from the caveolae-like vesicles induced by caveolin expression, as evidenced by co-infection experiments using a recombinant caveolin baculovirus. Taken together, these results suggest that the MAL proteolipid might play a role as a component of the machinery of vesiculation of glycolipid-enriched membranes.

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.

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.

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.

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.

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.

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.

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.

Evidence for a regulated interaction between heterotrimeric G proteins and caveolin

Caveolae are flask-shaped plasma membrane specializations. A 22-kDa protein, caveolin, is a principal component of caveolar membranes in vivo. As recent evidence suggests that caveolae may participate in G protein-coupled signaling events, we have investigated the potential interaction of caveolin with heterotrimeric G proteins. Using cell fractionation techniques, we found that mutational or pharmacologic activation of Gs alpha prevents its cofractionation with caveolin. In a second independent approach, we directly examined the interaction of G proteins with caveolin. For this purpose, we recombinantly expressed caveolin as a glutathione S-transferase fusion protein. Using an in vitro binding assay, we found that caveolin interacts with G protein alpha subunits (Gs, Go, and Gi). Mutational or pharmacologic activation (with guanosine 5'-O-(thiotriphosphate)) of G alpha subunits prevents this interaction, indicating that the inactive GDP-bound form of G alpha subunits preferentially interacts with caveolin. This G protein binding activity is located within a 41-amino acid region of caveolin's cytoplasmic N-terminal domain (residues 61-101). Further functional analysis shows that a polypeptide derived from this region of caveolin (residues 82-101) effectively suppresses the basal activity of purified G proteins, apparently by inhibiting GDP/GTP exchange. This caveolin sequence is homologous to a region of the Rab GDP dissociation inhibitor, a known inhibitor of GDP/GTP exchange for Rab proteins. These data suggest that caveolin could function to negatively regulate the activation state of heterotrimeric G proteins.

Neuromuscular junctions contain NP185: the multifunctional protein is located at the presynaptic site

The NP185 polypeptide (AP3) is a multifunctional component isolated from brain endocytic vesicles, which binds to tubulin and clathrin light chains, decoated vesicles, synaptic vesicles, and the synaptosomal plasma membrane (Su et al., 1991). The NP185 molecules are expressed during avian cerebellar synaptogenesis and appear to function in CNS regions rich in synaptic terminals (Perry et al., 1991). In this report we describe double-labelling experiments with avian embryonic striated muscle fibers demonstrating the exclusive presence of the brain-specific protein at the neuromuscular junction. We used indirect rhodamine immunofluorescence labeling with a monoclonal antibody (mAb-8G8) to mark the location of NP185 in muscle combined with fluorescein-alpha-bungarotoxin to mark the postsynaptic location of the acetylcholine receptors (AChRs). We show that the distribution of both NP185 and AChRs has an overall correlation, but the location of NP185 is circumscribed to presynaptic structures adjacent but not overlapping with postsynaptic structures displaying the AchRs. To confirm the identity of NP185, the molecule was extracted from both tissues, partially purified, immunoprecipitated, and identified in Western blots with the mAb 8G8. The mAb reacted with an identical 185 kD protein band purified from both tissues. Based on its properties and specific neuronal location, the NP185 molecule may function in motor nerve terminals by screening membrane proteins, identifying areas of the synaptic plasma membrane, and to anchor these elements with structural proteins for their recycling and transport within the neuronal cellular compartments.

Neuronal protein NP185 is developmentally regulated, initially expressed during synaptogenesis, and localized in synaptic terminals

Evidence is presented here that demonstrates the presence of NP185 (AP3) in neuronal cells, specifically within syn-aptic terminals of the central nervous system and in the peripheral nervous system, particularly in the neuro-muscular junction of adult chicken muscle. Biochemical results obtained in our laboratories indicate that NP185 is associated with brain synaptic vesicles, with clathrin-coated vesicles, and with the synaptosomal plasma membrane. Also, NP185 binds to tubulin and clathrin light chains and the binding is regulated by phosphorylation (Su et al., 1991). Based on these properties and the data reported here, we advance the postulate that NP185 fulfills multiple functions in synaptic terminals. One function is that of a plasma membrane docking or channel protein, another of a signaling molecule for brain vesicles to reach the synaptic terminal region, and a third is that of a recycling molecule by binding to protein components on the lipid bilayer of the synaptic plasma membrane during the process of endocytosis. In support of these premises, a thorough study of NP185 using the developing chick brain, adult mouse brain, and chicken straited muscle was begun by temporally and spatially mapping the expression and localization of NP185 in evolving and mature nerve endings. To achieve these objectives, monoclonal antibodies to NP185 were used for immunocytochemistry in tissue sections of chicken and mouse cerebella. The distribution of NP185 was compared with those of other cytoskeletal and cytoplasmic proteins of axons and synapses, namely synaptophysin, vimentin, neurofilament NF68, and the intermediate filaments of glial cells (GFAP). The data indicate that expression of NP185 temporally coincides with synaptogenesis, and that the distribution of this protein is specific for synaptic terminal buttons of the CNS and the PNS.