Solubilization of proteins from bovine brain coated vesicles by protein perturbants and Triton X-100

Interplay between phosphorylation and palmitoylation mediates plasma membrane targeting and sorting of GAP43

A combination of biochemical, genetic, and imaging approaches is used to show that phosphorylation and lipidation exhibit a complex interplay in sorting of GAP43. Palmitoylation tags GAP43 for global sorting by inducing piggybacking on exocytic vesicles, whereas phosphorylation locally regulates plasma membrane targeting of palmitoylated GAP43.

Phosphorylation and lipidation provide posttranslational mechanisms that contribute to the distribution of cytosolic proteins in growing nerve cells. The growth-associated protein GAP43 is susceptible to both phosphorylation and S-palmitoylation and is enriched in the tips of extending neurites. However, how phosphorylation and lipidation interplay to mediate sorting of GAP43 is unclear. Using a combination of biochemical, genetic, and imaging approaches, we show that palmitoylation is required for membrane association and that phosphorylation at Ser-41 directs palmitoylated GAP43 to the plasma membrane. Plasma membrane association decreased the diffusion constant fourfold in neuritic shafts. Sorting to the neuritic tip required palmitoylation and active transport and was increased by phosphorylation-mediated plasma membrane interaction. Vesicle tracking revealed transient association of a fraction of GAP43 with exocytic vesicles and motion at a fast axonal transport rate. Simulations confirmed that a combination of diffusion, dynamic plasma membrane interaction and active transport of a small fraction of GAP43 suffices for efficient sorting to growth cones. Our data demonstrate a complex interplay between phosphorylation and lipidation in mediating the localization of GAP43 in neuronal cells. Palmitoylation tags GAP43 for global sorting by piggybacking on exocytic vesicles, whereas phosphorylation locally regulates protein mobility and plasma membrane targeting of palmitoylated GAP43.

Epidermal growth factor variations in amniotic membrane used for ex vivo tissue constructs

INTRODUCTION

The amniotic membrane (AM) is used for engineering ex vivo tissue constructs used in ocular surface reconstruction. Epidermal growth factor (EGF) content of the AM is believed to play a key role in supporting corneal epithelial cell expansion on AM. This study investigated EGF content in AM in relation to intra- and inter-donor variations and the effect of processing and preservation (handling).

METHODS

Fifteen human AM, both fresh and handled, were analyzed for EGF gene and protein expression by real-time polymerase chain reaction and ELISA, respectively.

RESULTS

EGF gene expression was predominantly seen in the AM epithelium (p<0.01). Similarly, EGF protein too was predominantly seen in the epithelial layer (p<0.01) for fresh and handled samples. EGF protein content varied between membranes (inter-donor) and at different sites within the same membrane (intra-donor). The highest EGF protein concentration was noted in the AM apical and mid-region epithelium. Significant EGF protein loss (p<0.01) was observed after handling.

CONCLUSION

There is a considerable variation in EGF content between and within donors. This is further affected by handling of the AM. Such variations could affect the clinical efficacy of tissue constructs. Current use of AM for ex vivo expansion of epithelial cells is not standardized and remains an area of concern.

Site-directed antibodies to low-voltage-activated calcium channel CaV3.3 (alpha1I) subunit also target neural cell adhesion molecule-180

Synthetic peptides of defined amino acid sequence are commonly used as unique antigens for production of antibodies to more complex target proteins. We previously showed that an affinity-purified, site-directed polyclonal antibody (CW90) raised against a peptide antigen (CNGRMPNIAKDVFTKM) anticipated to be specific to a T-type voltage-dependent Ca(2+) channel subunit identified recombinant rat alpha1I/Ca(V)3.3 and two endogenous mouse proteins distinct in their developmental expression and apparent molecular mass (neonatal form 260 kDa, mature form 190 kDa) [Yunker AM, Sharp AH, Sundarraj S, Ranganathan V, Copeland TD, McEnery MW (2003) Immunological characterization of T-type voltage-dependent calcium channel Ca(V)3.1 (alpha 1G) and Ca(V)3.3 (alpha 1I) isoforms reveal differences in their localization, expression, and neural development. Neuroscience 117:321-335]. In the present study, we further characterize the biochemical properties of the CW90 antigens. We show for the first time that recombinant alpha1I/Ca(V)3.3 is modified by N-glycosylation. Using peptide:N-glycosidase F (PNGase F), an enzyme that removes polysaccharides attached at Asn residues, and endoneuraminidase-N (Endo-N), which specifically removes polysialic acid modifications, we reveal that differential glycosylation fully accounts for the large difference in apparent molecular mass between neonatal and adult CW90 antigens and that the neonatal form is polysialylated. As very few proteins are substrates for Endo-N, we carried out extensive analyses and herein present evidence that CW90 reacts with recombinant alpha1I/Ca(V)3.3 as well as endogenous neural cell adhesion molecule-180 (NCAM-180). We demonstrate the basis for CW90 cross-reactivity is a five amino acid epitope (AKDVF) present in both alpha1I/Ca(V)3.3 and NCAM-180. To extend these findings, we introduce a novel polyclonal anti-peptide antibody (CW678) that uniquely recognizes NCAM-180 and a new antibody (CW109) against alpha1I/Ca(V)3.3. Western blot analyses obtained with CW678, CW109 and CW90 on a variety of samples confirm that the endogenous CW90 signals are fully attributed to the two developmental forms of NCAM-180. Using CW678, we present novel data on differentiation-dependent NCAM-180 expression in human neuroblastoma IMR32 cells. These results strongly suggest the need for careful analyses to validate anti-peptide antibodies when targeting membrane proteins of low abundance.

Tau interacts with Golgi membranes and mediates their association with microtubules

Tau, a microtubule-associated protein enriched in the axon, is known to stabilize and promote the formation of microtubules during axonal outgrowth. Several studies have reported that tau was associated with membranes. In the present study, we further characterized the interaction of tau with membranous elements by examining its distribution in subfractions enriched in either Golgi or endoplasmic reticulum membranes isolated from rat brain. A subfraction enriched with markers of the medial Golgi compartment, MG160 and mannosidase II, presented a high tau content indicating that tau was associated with these membranes. Electron microscope morphometry confirmed the enrichment of this subfraction with Golgi membranes. Double-immunogold labeling experiments conducted on this subfraction showed the direct association of tau with vesicles labeled with either an antibody directed against MG160 or TGN38. The association of tau with the Golgi membranes was further confirmed by immunoisolating Golgi membranes with an anti-tau antibody. Immunogold labeling confirmed the presence of tau on the Golgi membranes in neurons in vivo. Overexpression of human tau in primary hippocampal neurons induced the formation of large Golgi vesicles that were found in close vicinity to tau-containing microtubules. This suggested that tau could serve as a link between Golgi membranes and microtubules. Such role for tau was demonstrated in an in vitro reconstitution assay. Finally, our results showed that some tau isoforms present in the Golgi subfraction were phosphorylated at the sites recognized by the phosphorylation-dependent antibodies PHF-1 and AT-8.

Cell confluence-dependent remodeling of endothelial membranes mediated by cholesterol

The plasma membranes of endothelial cells reaching confluence undergo profound structural and functional modifications, including the formation of adherens junctions, crucial for the regulation of vascular permeability and angiogenesis. Adherens junction formation is accompanied by the tyrosine dephosphorylation of adherens junctions proteins, which has been correlated with the strength and stability of adherens junctions. Here we show that cholesterol is a critical determinant of plasma membrane remodeling in cultures of growing cow pulmonary aortic endothelial cells. Membrane cholesterol increased dramatically at an early stage in the formation of confluent cow pulmonary aortic endothelial cell monolayers, prior to formation of intercellular junctions. This increase was accompanied by the redistribution of caveolin from a high density to a low density membrane compartment, previously shown to require cholesterol, and increased binding of the annexin II-p11 complex to membranes, consistent with other studies indicating cholesterol-dependent binding of annexin II to membranes. Furthermore, partial depletion of cholesterol from confluent cells with methyl-beta-cyclodextrin both induced tyrosine phosphorylation of multiple membrane proteins, including adherens junctions proteins, and disrupted adherens junctions. Both effects were dramatically reduced by prior complexing of methyl-beta-cyclodextrin with cholesterol. Our results reveal a novel physiological role for cholesterol regulating the formation of adherens junctions and other plasma membrane remodeling events as endothelial cells reach confluence.

Clathrin and adaptors

Clathrin and adaptors are components of clathrin-coated pits and vesicles. The AP-1 adaptor complex is associated with clathrin-coated vesicles budding from the TGN, while the AP-2 adaptor complex is associated with clathrin-coated vesicles budding from the plasma membrane. The clathrin forms a polyhedral lattice and is believed to be the driving force behind membrane invagination leading to vesicle budding. The adaptors attach the clathrin to the membrane and also interact with the cytoplasmic domains of selected transmembrane proteins, causing these proteins to become concentrated in clathrin-coated vesicles. Clathrin-coated vesicles budding from the TGN have been implicated in the sorting of newly synthesised lysosomal enzymes, while clathrin-coated vesicles budding from the plasma membrane facilitate the receptor-mediated endocytosis of ligands, such as low density lipoproteins and transferrin. A novel adaptor-related complex, AP-3, has recently been identified, which is recruited onto membranes of the TGN and a more peripheral compartment but does not appear to be associated with clathrin. Genetic studies indicate that AP-3 plays a role in the sorting of proteins to lysosomes and lysosome-related organelles.

Clathrin light chain and synaptotagmin I in rat sympathetic neurons

Clathrin light chain (clathrin LC) and synaptotagmin I in sympathetic neurons in rat superior cervical ganglia (SCG) were studied using immunofluorescence and confocal microscopy. The distributions of clathrin LC and synaptotagmin I were compared with that of tyrosine hydroxylase (TH) and neuropeptide Y (NPY) in double label experiments. The influence of preganglionic regulation on the expression of clathrin LC and synaptotagmin I in post-ganglionic adrenergic neurons was investigated after cutting the cervical sympathetic trunk. In SCGs and irides of control animals, the calthrin LC- and synaptotagmin-I-positive structures were present in a granular pattern in nerve fibers and varicose terminals. In principal neurons, the two proteins were present in a perinuclear network (the Golgi complex). After decentralization, the synaptotagmin-I- and clathrin LC-positive granules normally present in preganglionic nerve terminals outlining the neuronal somata were no longer observed on day 1, but reappeared, and were increased above control in number and intensity, in axon bundles in the ganglia, on day 3 and up to day 28 post-decentralization. In irides, the fluorescence intensity and density of clathrin LC- and synaptotagmin-I-positive nerve terminals in the dilator plate, were semi-quantified using the confocal microscopy software. It was found that both proteins increased shortly after decentralization. Immunoblot data confirmed the immunohistochemical/confocal microscopy observations. Fast axonal transport of clathrin LC- and synaptotagmin I in preganglionic sympathetic neurons was demonstrated in crush-operated cervical sympathetic trunk. Both proteins rapidly accumulated proximally as well as distally to the crush, demonstrating fast anterograde and retrograde axonal transport (recycling). Thus, clathrin LC and synaptotagmin I are normally present in pre- as well as post-ganglionic sympathetic neurons. The colocalization of clathrin LC with synaptotagmin I in the Golgi complex of the adrenergic neurons may imply that clathrin participates in the synthesis/sorting of the fast transported materials in these neurons. Possible explanations for the increase of the two proteins after decentralization are discussed.

Isoform-specific, calcium-regulated interaction of the synaptic vesicle proteins SV2 and synaptotagmin

The identification and functional characterization of proteins localized to synaptic vesicles has contributed significantly to our understanding of neurotransmission. Studies of synaptic vesicle protein interactions have both led to the identification of novel synaptic proteins and suggested hypotheses of protein function. Synaptic vesicle protein 2 (SV2), is an integral membrane glycoprotein present in all synaptic vesicles. There are two characterized isoforms, SV2A and SV2B. Despite their homology to transporter proteins, the function of the SV2s remains unknown. In an effort to determine SV2 function and identify cofactors required for SV2 activity, we examined the protein interactions of SV2 using a combination of cross-linking, immunoprecipitation, and recombinant protein affinity chromatography. We report that SV2 is part of a large protein complex that contains the synaptic vesicle protein synaptotagmin. The interaction between SV2 and synaptotagmin is direct, specific to SV2A, and inhibited by calcium with an EC50 of approximately 10 microM. Interaction is mediated by the cytoplasmic amino terminus of SV2A and the C2B domain of synaptotagmin. Our observations suggest a regulatory relationship between these two proteins.

Interaction of tau with the neural plasma membrane mediated by tau's amino-terminal projection domain

The neuronal microtubule-associated protein tau is required for the development of cell polarity in cultured neurons. Using PC12 cells that stably express tau and tau amino-terminal fragments, we report that tau interacts with the neural plasma membrane through its amino-terminal projection domain. In differentiated PC12 transfectants, tau is found in growth cone-like structures in a nonmicrotubule-dependent manner. In hippocampal neurons, tau is differentially extracted by detergent and enriched in the growth cone and the distal axon when membrane is left intact. In PC12 transfectants, overexpression of tau's amino-terminal fragment, but not of full-length tau, suppresses NGF-induced process formation. Our data suggest that tau's amino-terminal projection domain has an important role in neuritic development and establishes tau as a mediator of microtubule-plasma membrane interactions.

Sorting of synaptophysin into special vesicles in nonneuroendocrine epithelial cells

Synaptophysin is a major transmembrane glycoprotein of a type of small vesicle with an electron-translucent content (SET vesicles), including the approximately 50-nm presynaptic vesicles in neuronal cells, and of similar, somewhat larger (< or = approximately 90 nm) vesicles (SLMV) in neuroendocrine (NE) cells. When certain epithelial non-NE cells, such as human hepatocellular carcinoma PLC cells, were cDNA transfected to synthesize synaptophysin, the new molecules appeared in specific SET vesicles. As this was in contrast to other reports that only NE cells were able to sort synaptophysin away from other plasma membrane proteins into presynaptic- or SLMV-type vesicles, we have further characterized the vesicles containing synaptophysin in transfected PLC cells. Using fractionation and immunoisolation techniques, we have separated different kinds of vesicles, and we have identified a distinct type of synaptophysin-rich, small (30-90-nm) vesicle that contains little, if any, protein of the constitutive secretory pathway marker hepatitis B surface antigen, of the fluid phase endocytosis marker HRP, and of the plasma membrane recycling endosomal marker transferrin receptor. In addition, we have found variously sized vesicles that contained both synaptophysin and transferrin receptor. A corresponding result was also obtained by direct visualization, using double-label immunofluorescence microscopy for the endocytotic markers and synaptophysin in confocal laser scan microscopy and in double-immunogold label electron microscopy. We conclude that diverse non-NE cells of epithelial nature are able to enrich the "foreign" molecule synaptophysin in a category of SET vesicles that are morphologically indistinguishable from SLMV of NE cells, including one type of vesicle in which synaptophysin is sorted away from endosomal marker proteins. Possible mechanisms of this sorting are discussed.

Influence of the clathrin coat on the membrane lipidic organization of endocytic vesicles: a fluorescence study

Endocytic coated vesicles (CV) were purified from bovine brain, and uncoated vesicles (UV) were obtained from the latter by dialysis against 1 M Tris. Membrane dynamics were explored in both vesicle populations using two complementary fluorescence approaches: diphenylhexatriene fluorescence anisotropy to account for rotational lipid movements, and pyrene excimerization with a phosphatidylcholine pyrene derivative for translational motion. It was concluded that membrane fluidity was considerably higher in UV than in CV, and that adding bulk coat proteins (adaptors+clathrin) to uncoated vesicles re-established the low fluidity found in coated vesicles. However, adding coat protein constituents separately had no effect.

A novel cholinergic-specific antigen (Chol-2) in mammalian brain

Three new antisera have been raised in sheep against cholinergic electromotor presynaptic plasma membranes prepared from the electric organs of the electric ray, Torpedo marmorata. They all recognized one or more cholinergic-specific antigens in the mammalian nervous system by the following criteria: they sensitized the cholinergic subpopulation of rat-brain synaptosomes--and only this subpopulation--to lysis by the complement system and, in an immunocytochemical study, selectively stained choline acetyltransferase-positive cholinergic neurons in the rat spinal cord. However, two of the three antisera failed to recognize Chol-1 alpha and -beta, two closely related minor gangliosides already identified as the cholinergic-specific antigens recognized by previous anti-Torpedo presynaptic plasma membrane antisera or indeed any other ganglioside and the third recognized only Chol-1 alpha. A further investigation of the antigen(s) recognized by the most antigenic of the new antisera indicated that it is proteinaceous in nature, but has epitopes in common with electric organ gangliosides.

Clathrin-coated vesicles in nervous tissue are involved primarily in synaptic vesicle recycling

The recycling of synaptic vesicles in nerve terminals is thought to involve clathrin-coated vesicles. However, the properties of nerve terminal coated vesicles have not been characterized. Starting from a preparation of purified nerve terminals obtained from rat brain, we isolated clathrin-coated vesicles by a series of differential and density gradient centrifugation steps. The enrichment of coated vesicles during fractionation was monitored by EM. The final fraction consisted of greater than 90% of coated vesicles, with only negligible contamination by synaptic vesicles. Control experiments revealed that the contribution by coated vesicles derived from the axo-dendritic region or from nonneuronal cells is minimal. The membrane composition of nerve terminal-derived coated vesicles was very similar to that of synaptic vesicles, containing the membrane proteins synaptophysin, synaptotagmin, p29, synaptobrevin and the 116-kD subunit of the vacuolar proton pump, in similar stoichiometric ratios. The small GTP-binding protein rab3A was absent, probably reflecting its dissociation from synaptic vesicles during endocytosis. Immunogold EM revealed that virtually all coated vesicles carried synaptic vesicle proteins, demonstrating that the contribution by coated vesicles derived from other membrane traffic pathways is negligible. Coated vesicles isolated from the whole brain exhibited a similar composition, most of them carrying synaptic vesicle proteins. This indicates that in nervous tissue, coated vesicles function predominantly in the synaptic vesicle pathway. Nerve terminal-derived coated vesicles contained AP-2 adaptor complexes, which is in agreement with their plasmalemmal origin. Furthermore, the neuron-specific coat proteins AP 180 and auxilin, as well as the alpha a1 and alpha c1-adaptins, were enriched in this fraction, suggesting a function for these coat proteins in synaptic vesicle recycling.

Neuronal specific protein NP185 is enriched in nerve endings: binding characteristics for clathrin light chains, synaptic vesicles, and synaptosomal plasma membrane

The neuronal specific protein NP185, found associated with brain clathrin-coated vesicles, formed a complex with unphosphorylated, but not with phosphorylated, clathrin light chains. The NP185-clathrin light chain complex was associated with casein kinase II activity, which, in the presence of polylysine, phosphorylated clathrin light chain b but not the NP185. The dissociation of this complex with 50% ethylene glycol pH 11.5 suggests that NP185 binds to hydrophobic domains of clathrin light chains. When NP185 molecules were retained by monoclonal antibody-linked Sepharose beads, they bound synaptic vesicles, decoated vesicles and synaptosomal plasma membrane. Immunohistochemistry on mouse cerebellar tissue sections using 8G8, a monoclonal antibody raised against NP185, showed neuronal specific labeling closely following synaptic distribution. In immunoblots, NP185 shares similar epitopes to those detected in another assembly polypeptide, AP-180, an indication that both proteins are identical. It appears that NP185 plays a specific role in nerve ending functions through its ability to induce clathrin to polymerize into cages, its interaction with synaptic vesicles, with the plasma membrane and with clathrin coat components.

Novel proteins mediate an interaction between clathrin-coated vesicles and polymerizing actin filaments

A monoclonal antibody, A-7C11, was generated which reacts with two polypeptides of 40 kDa and 80 kDa associated with the coat proteins of purified brain clathirn-coated vesicles. The 40-kDa antigen was purified and found to display actin-binding properties. Negative-staining electron microscopy showed that one of the antigens reactive with A-7C11 appears to mediate the association of isolated clathrin-coated vesicles with assembling actin filaments in vitro. Immunofluorescence microscopy of cultured fibroblasts with A-7C11 revealed the antigens aligned with both actin filaments and as punctate structures near the plasma membrane. The data suggest that the interaction between clathrin-coated vesicles and the actin cytoskeleton is mediated by antigens identified by monoclonal antibody A-7C11.

Incorporation of spin-labeled fatty acids into bovine brain clathrin coated vesicles

Stearic acids with a nitroxide radical at selected positions have been incorporated in the phospholipid bilayers of clathrin coated vesicles, uncoated vesicles and sonicated liposomes made from the lipids extracted from the uncoated vesicles. The extent of incorporation was found minimum for stearic acids labeled on C-12 and for bilayers of uncoated vesicles. The ESR spectra of the spin-labeled fatty acids incorporated in the bilayers showed a pronounced temperature dependence (without discontinuity) and a decrease in the hyperfine splitting as the nitroxide group was inserted deeper in the hydrophobic core of the membranes. An abrupt phospholipid phase transition or a phase separation could be excluded. The presence of the external proteins (the clathrin coat) on the membranes was not found to noticeably influence the gradient of flexibility of the fatty acid chains of the phospholipids. The influence of the internal proteins embedded in the bilayers was evidenced by a detailed analysis of the ESR spectra of (7,8)SA in terms of two components: one component arising from the labels surrounded exclusively by phospholipids, the other component arising from labels of reduced mobility perturbed by the vicinity of the proteins. These results support the persistence of lipidic domains in the endocytic vesicles despite the accumulation of receptors which follows their formation.

P29: a novel tyrosine-phosphorylated membrane protein present in small clear vesicles of neurons and endocrine cells

A novel membrane protein from rat brain synaptic vesicles with an apparent 29,000 Mr (p29) was characterized. Using monospecific polyclonal antibodies, the distribution of p29 was studied in a variety of tissues by light and electron microscopy and immunoblot analysis. Within the nervous system, p29 was present in virtually all nerve terminals. It was selectively associated with small synaptic vesicles and a perinuclear region corresponding to the area of the Golgi complex. P29 was not detected in any other subcellular organelles including large dense-core vesicles. The distribution of p29 in various subcellular fractions from rat brain was very similar to that of synaptophysin and synaptobrevin. The highest enrichment occurred in purified small synaptic vesicles. Outside the nervous system, p29 was found only in endocrine cell types specialized for peptide hormone secretion. In these cells, p29 had a distribution very similar to that of synaptophysin. It was associated with microvesicles of heterogeneous size and shape that are primarily concentrated in the centrosomal-Golgi complex area. Secretory granules were mostly unlabeled, but their membrane occasionally contained small labeled evaginations. Immunoisolation of subcellular organelles from undifferentiated PC12 cells with antisynaptophysin antibodies led to a concomitant enrichment of p29, synaptobrevin, and synaptophysin, further supporting a colocalization of all three proteins. P29 has an isoelectric point of approximately 5.0 and is not N-glycosylated. It is an integral membrane protein and all antibody binding sites are exposed on the cytoplasmic side of the vesicles. Two monoclonal antibodies raised against p29 cross reacted with synaptophysin, indicating the presence of related epitopes. P29, like synaptophysin, was phosphorylated on tyrosine residues by endogenous tyrosine kinase activity in intact vesicles.

Topogenesis and sorting of synaptophysin: synthesis of a synaptic vesicle protein from a gene transfected into nonneuroendocrine cells

Diverse nonneuroendocrine (non-NE) cells were forced to express synaptophysin (SY), the major and typical transmembrane glycoprotein of small (30-80 nm) neurotransmitter vesicles of NE cells, using microinjection of RNA synthesized in vitro from cDNA or transient and stable transfections with cDNA brought under SV40 promoter control. The glycoprotein synthesized in non-NE cells is indistinguishable from SY of NE cells and is integrated with similar, if not identical, orientation in the membranes of a specific, novel type of small cytoplasmic vesicle that structurally resembles synaptic vesicles and in which SY is the only major protein detected. A non-N-glycosylated form of SY generated by site-directed mutagenesis showed the same behavior and specific distribution in small vesicles. The results show that the information contained in this protein alone is sufficient to secure its sorting into a special type of vesicle in a heterotypic context, i.e., in the absence of other NE-specific components.

Interaction of clathrin coat proteins with unilamellar and multilamellar vesicles of phosphatidylcholine

The binding of clathrin and accessory coat proteins to small unilamellar vesicles and to liposomes of uncharged phospholipids has been followed by chromatography, 31P-NMR, ESR and fluorescence anisotropy. At pH 6.5 and at an ionic strength value (0.1 M Mes) close to that used during the purification of clathrin-coated vesicles, the proteins do not restore the characteristic network found around the natural vesicles. Instead, a limited fusion leads to enlarged structures in which the perturbation of the dynamics of the phospholipids decreases gradually with the depth in the membrane. While the rate of motion of the outer polar heads is lowered, the order parameter of doxyl groups located either under or in the vicinity of the glycerol backbone is not affected by the proteins. In the inner core of the membrane, the main thermotropic transition of the hydrocarbon chains is unchanged. All the effects are the results of interactions limited to the membrane surface. The electrostatic nature of these interactions is evidenced when the embedded spin labels have a charge protruding at the membrane surface. An 'anchoring' effect appears which is due to the charged groups of the proteins. The lateral diffusion of the probes is reduced and, at low ionic strength, a cationic derivative no longer detects the thermotropic transition of the hydrocarbon chains. These results indicate that, although it is known that clathrin and accessory proteins bind to membranes by a series of protein-protein interactions, this system is not devoid of lipid-protein interactions, at least when it is not organized as in the natural system.

Highly basic 30- and 32-kilodalton proteins associated with synapse formation on polylysine-coated beads in enriched neuronal cell cultures

Neuronal proteins involved in axonal outgrowth and synapse formation were examined in an enriched neuronal cell culture system of the cerebellum. In rat cerebellar cell cultures, 98.9% of the cells are neurons and the remaining 1.1% of the cells are flat nonneuronal cells. These enriched neuronal cultures, examined with two-dimensional gel electrophoresis, showed protein patterns similar to those of neonatal cerebellum, but very different patterns from glial enriched cultures. High levels of a neuronal membrane acidic 29-kilodalton (kD) protein were found. It has been shown previously that neuronal cultures incubated with polylysine-coated beads will develop numerous presynaptic elements on the bead surface. We report here that isolation of the beads from enriched neuronal cell cultures incubated with [35S]methionine showed, with two-dimensional nonequilibrium pH gradient gel electrophoresis (2D-NEPHGE), levels of a basic 32-kD protein (pI 8) note detected in cultures alone, and increased levels of a 30-kD protein (pI 10). When culture medium was examined with 2D-NEPHGE, three acidic proteins were identified that were secreted by the cultured neurons. In summary, a neuronal enriched cell culture system was used with isolated polylysine-coated beads to identify basic 30-kD and 32-kD proteins that may be involved in synapse formation.

Characterization of coated-vesicle adaptors: their reassembly with clathrin and with recycling receptors

Adaptors sort out those receptors that participate in assembly of coated pits from those that are excluded. Two distinct adaptor units have so far been identified: (1) adaptors restricted to plasma membrane coated pits (HA-II type, named according to their elution position during hydroxylapatite chromatography) and (2) adaptors restricted to Golgi region coated pits (HA-I type). Adaptors contain a heterodimer of two 100-kDa polypeptides, a beta-adaptin (possibly carrying an essentially common clathrin-binding domain) and a distinct alpha- or gamma-adaptin characteristic of the type of adaptor and its specific location. Each adaptor in constructed from four different polypeptides. Thus HA-II adaptors contain a beta-adaptin and an alpha-adaptin in combination with a 50-kDa protein and a 16-kDa polypeptide. The HA-I adaptors contain a beta-adaptin and a gamma-adaptin in combination with a 47-kDa protein and a 19-kDa polypeptide. Both types of adaptors and also a 180-kDa polypeptide will promote the assembly of clathrin to form coats, the size range of which appears to be relatively restricted compared to cages made from clathrin alone. The HA-II adaptors, characteristic of plasma membrane coated pits, bind to the cytoplasmic tail of the LDL receptor. They also assemble with the mannose 6-phosphate receptor in vitro in the absence of membrane. When clathrin is included, the adaptors promote the assembly of coats containing bound receptor.

Immunoelectron microscopic localization of synaptophysin in a Golgi subcompartment of developing hypothalamic neurons

Synaptophysin, previously identified as an integral membrane glycoprotein (mol. wt 38,000) characteristic of presynaptic vesicles of mature neurons, provides a molecular marker to study the origin, formation and traffic of synaptic vesicles. Using the monoclonal antibody SY38 against this polypeptide we have localized synaptophysin by immunofluorescence and electron microscope immunoperoxidase methods in cultured mouse hypothalamic neurons taken from 16-day-old fetuses which achieve synaptogenesis after 10-12 days in vitro. We have compared the localization of synaptophysin in perikarya and nerve endings as a function of age (2-19 days in vitro) and of treatment of mature neurons with nocodazole. Using immunofluorescence microscopy, synaptophysin was already detected in neuronal soma at 2 days in vitro, where the initiation of neurite development is observed. At the electron microscope level, virtually all mature synaptic boutons and varicosities showed an extensive synaptophysin labeling of synaptic vesicles at 12-13 days in culture whereas neurites showed only very few labeled vesicles. In neuronal soma taken before synapse formation (6 days in vitro), synaptophysin was selectively localized in membranes of the innermost cisternae of the Golgi zone and in vesicles of variable size and shape in the core of the Golgi zone. In contrast, after synapse formation, synaptophysin labeling was barely detected in the Golgi zone of neurons but a very strong labeling of synaptic vesicles in synaptic boutons was observed. Treatment of mature neurons (12 days in vitro) with nocodazole (10(-5) M) resulted in a conspicuous synaptophysin staining of the innermost trans-Golgi cisternae and numerous vesicles in the cytoplasm. Furthermore, an accumulation of labeled synaptic vesicles on the presynaptic membrane of nerve terminals was found. The data suggest that synaptophysin is released from the Golgi apparatus in a vesicular form, after glycosylation, and is then transported to nerve endings by a mechanism which requires integrity of microtubules.

Pituitary gonadotropin content in gonadectomized rats. Immunoassay measurements influenced by extraction solvent and testosterone replacement

Increasing interest in events regulating gonadotropin synthesis and secretion led to the reexamination, by radioimmunoassay (RIA), of the pituitary content of luteinizing hormone (LH) and follicle stimulating hormone (FSH). In a preliminary experiment, the importance of the composition of the solvent used to homogenize pituitaries on the detection of LH and FSH was demonstrated by RIA. Experiment 1 demonstrated that a drop in pituitary FSH occurred acutely following gonadectomy in males, but not in females. Experiment 2 tested whether this decline resulted from loss of testosterone or inhibin. Males were castrated and treated daily with oil or testosterone propionate, porcine follicular fluid, porcine serum, or testosterone plus follicular fluid for 2 to 14 days. Castration lowered pituitary FSH at 2, 4, and 6 days. Follicular fluid suppressed serum FSH, but not LH, and did not prevent the fall in pituitary FSH. Testosterone blocked the rise of serum LH and FSH and prevented the decline in pituitary FSH.

Identification and characterization of the major proteins of mammalian brain synaptic vesicles

Highly purified rat and cow brain synaptic vesicles contain major proteins with molecular weights of approximately 74,000, 60,000, 57,000, 40,000, 38,000, and 34,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The presence of the major proteins on synaptic vesicles was confirmed by immunoprecipitation of intact rat brain synaptic vesicles with a synaptic vesicle-specific monoclonal antibody. The 40,000-Mr protein appeared to be identical to the 38,000-Mr integral membrane glycoprotein, p38 or synaptophysin, previously identified as a major component of mammalian synaptic vesicles. The isoelectric point of the 75,000-Mr proteins from either rat or cow brain synaptic vesicles is 5.0, and the pI of the 57,000-Mr protein is approximately 5.1 in both species. The similarity in size and charge of several major proteins in rat and cow synaptic vesicles suggests a high degree of structure conservation of these proteins in diverse mammalian species and raises the possibility that a set of functions common to most or all mammalian synaptic vesicles is mediated by these proteins.

Distribution of [3H]dihydrotetrabenazine binding in bovine striatal subsynaptic fractions: enrichment of higher affinity binding in a synaptic vesicle fraction

[3H]Dihydrotetrabenazine bound to a single class of binding sites in bovine striatal synaptic vesicles with an apparent dissociation constant of 3-9 nM. This is comparable to the inhibitory potency of dihydrotetrabenazine in catecholamine transport assays. In contrast to these results, [3H]dihydrotetrabenazine bound to at least two classes of sites in all other subsynaptic fractions investigated. The higher affinity class of sites was comparable in affinity to that of synaptic vesicles, whereas the lower affinity sites exhibited an apparent dissociation constant of 95-400 nM. Higher affinity sites were most abundant in the synaptic vesicle fraction, and little higher affinity binding was observed in mitochondrial and myelin fractions, or in highly purified synaptic plasma membranes. Lower affinity binding was not enriched in any subsynaptic fraction and was the only class of binding sites detected in homogenates of liver and diaphragm. The distribution of the presynaptic vesicle marker synaptophysin corresponded with that of higher affinity but not lower affinity binding. These results are consistent with the expectation that the higher affinity sites are associated primarily with synaptic vesicles and other neuronal entities that are in communication with these organelles.

Membrane contents of distinct subpopulations of coated vesicles determined by scanning transmission electron microscopy

In order to investigate the heterogeneity of clathrin-coated vesicles purified from rat liver, and to quantitate rigorously their membrane contents, we have analyzed scanning transmission electron micrographs of unstained coated vesicles before and after extraction with the non-ionic detergent Triton X-100, as well as of vesicles whose coats had been removed by dialysis against 10 mM or 100 mM Tris (pH 8.2). Their respective distributions of particle masses were thus determined and compared, in light of complementary biochemical quantitations of lipid and protein. Smaller coated particles, 25-45 MDa in mass and 60-80 nm in diameter, lose no mass when extracted with Triton, and disappear when their coats are dissociated. We conclude that they do not contain membrane vesicles, although they have dense, presumably proteinaceous, cores. They may represent particles generated during tissue homogenization or, possibly, a storage form of clathrin. The remaining 70% contain bona fide vesicles: these particles are 75-150 nm in diameter, and their average mass is about 80 MDa, of which 48 MDa is contributed by coat proteins, 10-12 MDa by phospholipid and cholesterol, and 20-22 MDa by vesicle-associated proteins. Their vesicles are of two types: smaller, denser, vesicles that contain substantial amounts of internalized material, and larger, less dense, vesicles that do not. The distinction between them may, in view of other findings, reflect a difference between coated vesicles derived respectively from the Golgi and the plasma membrane.

Cholinergic vesicle specific proteoglycan: stability in isolated vesicles and in synaptosomes during induced transmitter release

Exposure of synaptosomes isolated from the electric organ of Torpedo marmorata to conditions that promote the release of acetylcholine does not cause the co-release of a vesicle specific proteoglycan. Proteoglycan within synaptosomes is quite stable during various incubation conditions as measured by immune dot blotting. Isolated vesicles from Torpedo also retain their proteoglycan immunoreactivity when exposed to a variety of incubation conditions. Lysis of vesicles in H2O, treatment with pH 11.5 buffer, or exposure to high ionic strength (2 M KCl) results in the loss of acetylcholine or ATP while the proteoglycan is retained by vesicle membranes. Only treatment with Nonidet P-40 releases proteoglycan from vesicles or synaptosomes and free proteoglycan immunoreactivity is then susceptible to degradation by trypsin or heparinase. These results suggest that the proteoglycan is an integral component of vesicle membranes and is at least in the synaptosomal preparation not subject to extensive co-release with acetylcholine or ATP.

The synaptic vesicle and the cytoskeleton

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Instability of bovine brain clathrin-coated vesicles on Sephacryl S-1000 gel chromatography

Clathrin-coated vesicles have been isolated from bovine brain. To allow their further use in biophysical studies, the homogeneity of the preparations has been fully characterized after chromatography on Sephacryl S-1000, which is employed in many studies. It is demonstrated here that clathrin-coated vesicles are not stable on the gels and that their instability is increased in preparations using gels that are not presaturated with phospholipids. In addition, some fractionation occurs during chromatography. It is proposed that the slower migrating fractions contain mainly empty clathrin coats. Changes that occurred during the chromatography step are the result of reversible and irreversible events and are probably related to the assembly/disassembly cycle of clathrin observed in vitro.

Four synaptic vesicle-specific proteins: identification by monoclonal antibodies and distribution in the nervous tissue and the adrenal medulla

Synaptic vesicles from the guinea-pig cerebrum were isolated and administered to mice for the production of monoclonal antibodies (MAb). Four vesicle-associated proteins in the guinea-pig nervous tissue were specifically and differentially recognized by MAbs thus obtained. These proteins had molecular weights of 30,000, 36,000, 38,000 and 65,000 Da and were named SVPs (synaptic vesicle proteins) 30, 36, 38 and 65, respectively. Immunohistochemistry demonstrated that all SVPs were localized in the synaptic regions throughout the central nervous system and in the adrenal medulla. Nerve terminals in skeletal muscle, smooth muscle and sympathetic ganglion contained SVPs 36 and 38. Immunoelectron microscopy of the cerebellar cortex confirmed the localization of SVPs in the synaptic vesicles and the adjacent membranes of the presynaptic nerve terminals. Fractionation of the cerebral tissue and treatment with various agents showed that SVPs were localized in the synaptic vesicles and the synaptic plasma membrane and that SVPs were integrated within the membrane and liberated only after solubilization of the membrane.

Protein p38: an integral membrane protein specific for small vesicles of neurons and neuroendocrine cells

An intrinsic membrane protein of brain synaptic vesicles with Mr 38,000 (p38, synaptophysin) has recently been partially characterized (Jahn, R., W. Schiebler, C. Ouimet, and P. Greengard, 1985, Proc. Natl. Acad. Sci. USA, 83:4137-4141; Wiedenmann, B., and W. W. Franke, 1985, Cell, 41:1017-1028). We have now studied the presence of p38 in a variety of tissues by light and electron microscopy immunocytochemistry and by immunochemistry. Our results indicate that, within the nervous system, p38, like the neuron-specific phosphoprotein synapsin I, is present in virtually all nerve terminals and is selectively associated with small synaptic vesicles (SSVs). No p38 was detectable on large dense-core vesicles (LDCVs). p38 and synapsin I were found to be present in similar concentrations throughout the brain. Outside the nervous system, p38 was found in a variety of neuroendocrine cells, but not in any other cell type. In neuroendocrine cells p38 was localized on a pleiomorphic population of small, smooth-surfaced vesicles, which were interspersed among secretory granules and concentrated in the Golgi area, but not on the secretory granules themselves. Immunoblot analysis of endocrine tissues and cell lines revealed a band with a mobility slightly different from that of neuronal p38. This difference was attributable to a difference in glycosylation. The finding that p38, like synapsin I, is a component of SSVs of virtually all neurons, but not of LDCVs, supports the idea that SSVs and LDCVs are organelles of two distinct pathways for regulated neuronal secretion. In addition, our results indicate the presence in a variety of neuroendocrine cells of an endomembrane system, which is related to SSVs of neurons but is distinct from secretory granules.

Regulation of Plasma Membrane beta-Glucan Synthase from Red Beet Root by Phospholipids : Reactivation of Triton X-100 Extracted Glucan Synthase by Phospholipids

Extraction of red beet root plasma membranes with the detergent Triton X-100 at a level of 2.0% (weight/volume) resulted in the depletion of over 90% of total membrane phospholipid and the reduction of glucan synthase activity by 80 to 90%. Reconstitution of the delipidated Triton X-100, 100,000g fraction in the presence of phospholipids restored glucan synthase activity. The most effective phospholipid was phosphatidyl-ethanolamine, which restored 110 to 144% of the original activity at 0.5% (weight/volume). Glucan synthase in the phospholipid-reactivated Triton X-100-treated fraction was enriched 9-fold in specific activity relative to microsomal membranes but was unstable in digitonin. These results support the hypothesis that glucan synthase activity is regulated by its phospholipid environment.

Sorting and transepithelial transport of adsorbed protein tracers: effects of temperature

The epithelium of the guinea pig yolk sac is involved in the selective transport of macromolecules to the fetus. We studied the compartments involved in sorting and transepithelial transport of protein tracers and the effect of lowered temperature (18 degrees C) on these events. Explants of yolk sac were incubated with a mixture of cationized ferritin (CF) and horseradish peroxidase (HRP, Sigma type VI). At 4 degrees C, both tracers were bound to the cell surface and binding of an HRP-gold complex was shown to be inhibited by mannan. At 37 degrees C and 18 degrees C, both tracers were taken up into tubules and vesicles in the apical cytoplasm. Usually the tubules contained a mixture of tracers, but they often showed a polarized distribution with CF and HRP at opposite ends. The vesicles also contained mixtures of the tracers, but some contained only one. In addition, there were some irregularly shaped vacuoles composed of saccules that contained either a mixture, HRP alone, or CF alone. These results suggest that these adsorbed ligands are binding to unique microdomains of the endocytic complex. After 20 min at 37 degrees C coated vesicles 100 nm in diameter were located in the apical cytoplasm and coated vesicles of the same size were located at the lateral cell membrane. Usually they contained only HRP or CF, although occasional mixtures were seen. At 18 degrees C, HRP was transported across the cells in 100 nm vesicles. However, transport of CF was completely inhibited at the lower temperature.(ABSTRACT TRUNCATED AT 250 WORDS)

Subpopulations of liver coated vesicles resolved by preparative agarose gel electrophoresis

Rat liver clathrin coated vesicles (CVs) were separated into several distinct subpopulations using non-sieving concentrations of agarose, which allowed the separation of species differing primarily in surface charge. Using preparative agarose electrophoresis (Kedersha, N. L., and L. H. Rome, 1986, Anal. Biochem., in press), the CVs were recovered and analyzed for differences in morphology, coat protein composition, and stripped vesicle protein composition. Coat proteins from different populations appeared identical on SDS PAGE, and triskelions stripped from the different populations showed the same mobility on the agarose gel, suggesting that the mobility differences observed in intact CVs were due to differences in the surface charge of underlying vesicles rather than to variations in their clathrin coats. Several non-coat polypeptides appeared to segregate exclusively with different populations as resolved by two-dimensional electrophoresis. Stripped CVs also exhibited considerable heterogeneity when analyzed by Western blotting: the fast-migrating population was enriched in the mannose 6-phosphate receptor, secretory acetylcholine esterase, and an Mr 195,000 glycoprotein. The slow-migrating population of CVs was enriched in the asialoglycoprotein receptor, and it appeared to contain all detectable concanavalin A-binding polypeptides as well as the bulk of detectable WGA-binding proteins. When CVs were prepared from 125I-asialoorosomucoid-perfused rat liver, ligand was found in the slow-migrating CVs, suggesting that these were endocytic in origin. Morphological differences were also observed: the fast-migrating population was enriched in smaller CVs, whereas the slow-migrating population exhibited an enrichment in larger CVs. As liver consists largely of hepatocytes, these subpopulations appear to originate from the same cell type and probably represent CVs of different intracellular origin and destination.

Identification of a distinct 9S form of soluble clathrin in cultured cells and tissues

We have used a monoclonal antibody (CHC5.9) to identify clathrin (Mr 180,000; 'heavy chain') in coated vesicles, triskelion structures prepared in vitro and in high-speed supernatants (HSS) of cell homogenates from a variety of tissues and species (e.g., brain and liver from rat, cow and man; Xenopus ovaries). HSS proteins were subjected to sucrose density gradient centrifugation and gel filtration, and the fractions obtained were assayed for clathrin by enzyme-linked immunosorbent assay (ELISA) and polyacrylamide gel electrophoresis (PAGE), followed by immunoblotting. The native soluble clathrin identified in such fractions was indistinguishable from triskelions produced in vitro from purified bovine brain clathrin by several criteria, e.g. by its sedimentation coefficient (9S) and elution profile on gel filtration using Sephacryl S 300. No other major forms of soluble clathrin were detected. The results indicate that cells contain a soluble pool of clathrin and that the predominant molecular form of this soluble clathrin has properties similar to those of the triskelion obtained by dissociation studies in vitro. We hypothesize that this distinct 9S form represents a major oligomeric subunit involved in assembly and disassembly of clathrin polyhedron coats in the living cell.

Brain clathrin light chain 2 can be phosphorylated by a coated vesicle kinase

A protein kinase activity was observed in coated vesicles, prepared from bovine brain, that had clathrin-associated protein 2 (CAP2, also known as clathrin light chain 2) as its principal substrate. Coated vesicles were purified by sucrose density gradient centrifugation followed by Sephacryl S-1000 column chromatography, and all buffers utilized in these procedures contained a mixture of proteolysis inhibitors to maintain CAP2 kinase activity. Incubation of vesicles with [gamma-32P]ATP in the presence of 7 microM polylysine resulted in an overall increase in the incorporation of phosphate. NaDodSO4/PAGE revealed that the principal recipient of this additional phosphate was CAP2 (Mr 33,000), the faster-migrating component of the clathrin coat-associated proteins, whereas CAP1 (Mr 36,000) was not phosphorylated. A number of other proteins, in the Mr 140,000 and 100,000 regions, were phosphorylated to a lesser extent. Polyarginine and polyethylenimine also supported CAP2 phosphorylation, but arginine and lysine were ineffective. The phosphorylated protein was identified as CAP2 because addition of exogenous CAPs resulted in increased incorporation of label into Mr 33,000 polypeptides and because heat treatment of labeled vesicles followed by ultracentrifugation resulted in recovery of labeled Mr 33,000 protein in the supernatant. Phosphorylation of CAP2 may play a regulatory role in clathrin coat/coated vesicle functions.

Identification and localization of synaptophysin, an integral membrane glycoprotein of Mr 38,000 characteristic of presynaptic vesicles

A polypeptide of Mr 38,000 has been identified as a specific component of the membrane of presynaptic vesicles, using the monoclonal antibody SY38. This protein, which is acidic (isoelectric at approximately pH 4.8) and glycosylated, appears to be an integral membrane protein, as suggested by its solubilization with the nonionic detergent Triton X-100 and the finding that the epitope recognized by antibody SY38 is located on the cytoplasmic surface of those vesicles. It is found in presynaptic vesicles of neurons of the brain, spinal cord, and retina as well as at neuromuscular junctions. It is also found in the adrenal medulla. Its occurrence in diverse vertebrate species indicates its stability during evolution. This protein, for which we propose the name synaptophysin*, provides a molecular marker for the presynaptic vesicle membrane and may be involved in synaptic vesicle formation and exocytosis.