Role of Thr(11) in the binding of omega-conotoxin MVIIC to N-type Ca2+ channels

As replacement of Thr(11) of omega-conotoxin MVIIC with Ala significantly reduced the affinity for both N- and P/Q-type calcium channels, we examined the effect of substitution at this position with other residues. Binding assays using rat cerebellar P2 membranes showed that the affinity is in the order of Leu>Val, aminobutyric acid, Thr>Asn&z.Gt;Ser, Ala, Asp, Phe, Tyr for N-type channels and Thr>Leu, Val, aminobutyric acid, Asn, Ser>Ala&z.Gt;Asp, Phe, Tyr for P/Q-type channels, suggesting that aliphatic amino acids with longer side chains are favorable for block of N-type channels. The effects of substitution were examined electrophysiologically in BHK cells expressing N-type Ca2+ channels. Inhibition of Ba2+ current by the analogs did not completely correlate with binding affinity, although binding to BHK cells was comparable to rat cerebellar membranes.

Binding of Ala-scanning analogs of omega-conotoxin MVIIC to N- and P/Q-type calcium channels

omega-Conotoxin MVIIC binds to P/Q-type calcium channels with high affinity and N-type channels with low affinity. To reveal the residues essential for subtype selectivity, we synthesized Ala-scanning analogs of MVIIC. Binding assays using rat cerebellar P(2) membranes suggested that Thr(11), Tyr(13) and Lys(2) are essential for binding to both N- and P/Q-type channels, whereas Lys(4) and Arg(22) are important for binding to P/Q-type channels. These results suggest that MVIIC interacts with P/Q-type channels via a large surface, in good agreement with previous observations using chimeric analogs.

Binding of six chimeric analogs of omega-conotoxin MVIIA and MVIIC to N- and P/Q-type calcium channels

Replacement of the N-terminal half of omega-conotoxin MVIIC, a peptide blocker of P/Q-type calcium channels, with that of omega-conotoxin MVIIA significantly increased the affinity for N-type calcium channels. To identify the residues essential for subtype selectivity, we examined single reverse mutations from MVIIA-type to MVIIC-type in this chimeric analog. A reverse mutation from Lys(7) to Pro(7) decreased the affinity for both P/Q- and N-type channels, whereas that from Leu(11) to Thr(11) increased the affinity for P/Q-type channels and decreased the affinity for N-type channels. The roles of these two residues were confirmed by synthesizing two MVIIC analogs in which Pro(7) and Thr(11) were replaced with Lys(7) and Leu(11), respectively.

Redistribution of presynaptic proteins during alpha-latrotoxin-induced release of neurotransmitter and membrane retrieval at the frog neuromuscular junction

Calcium-dependent exocytosis at the nerve terminal involves the synaptic core (SNARE) complex composed of the t-SNAREs syntaxin 1 and synaptosome-associated protein of 25 kDa (SNAP-25), and the v-SNARE vesicle-associated membrane protein (VAMP/synaptobrevin), a stable heterotrimer which can associate with the putative calcium sensor protein, synaptotagmin. The distribution of these proteins at the frog neuromuscular junction was examined by immunofluorescent staining and confocal microscopy following exocytosis induced by alpha-latrotoxin. Experiments were performed under conditions in which synaptic vesicle recycling was either maintained in balance with exocytosis, or completely blocked, or during recovery from block of endocytosis. When endocytosis was maintained, protein distribution was essentially identical to that of unstimulated nerve terminals, in which syntaxin 1 and SNAP-25 are localized to the presynaptic active zones coincident with the postsynaptic folds that contain a high density of acetylcholine receptors (AChRs). Block of endocytosis led to complete incorporation of vesicle proteins into the plasmalemma, and t-SNARE distribution was no longer restricted to active zones. Five minutes after the onset of recovery, both synaptic vesicle proteins and t-SNARE proteins were concentrated into small spots, in a similar pattern to that obtained following endocytosis of the vital styryl dye FM1-43. These findings are consistent with a model in which following sustained exocytosis, t-SNARE trafficking involves internalization and transit via a vesicular compartment before recycling to the presynaptic plasma membrane.

Direct interaction of the calcium sensor protein synaptotagmin I with a cytoplasmic domain of the alpha1A subunit of the P/Q-type calcium channel

Synaptotagmins are synaptic vesicle proteins containing two calcium-binding C2 domains which are involved in coupling calcium influx through voltage-gated channels to vesicle fusion and exocytosis of neurotransmitters. The interaction of synaptotagmins with native P/Q-type calcium channels was studied in solubilized synaptosomes from rat cerebellum. Antibodies against synaptotagmins I and II, but not IV co-immunoprecipitated [125I]omega-conotoxin MVIIC-labelled calcium channels. Direct interactions were studied between in vitro-translated [35S]synaptotagmin I and fusion proteins containing cytoplasmic loops of the alpha1A subunit (BI isoform). Gel overlay revealed the association of synaptotagmin I with a single region (residues 780-969) located in the intracellular loop connecting homologous domains II and III. Saturable calcium-independent binding occurred with equilibrium dissociation constants of 70 nM and 340 nM at 4 degrees C and pH 7.4, and association was blocked by addition of excess recombinant synaptotagmin I. Direct synaptotagmin binding to the pore-forming subunit of the P/Q-type channel may optimally locate the calcium-binding sites that initiate exocytosis within a zone of voltage-gated calcium entry.

Cysteine string proteins associated with secretory granules of the rat neurohypophysis

The properties and subcellular distribution of cysteine string proteins (csps) were analyzed in peptidergic nerve terminals of the rat neurohypophysis. Polyclonal antibodies raised against recombinant rat brain csp recognized a 36 kDa protein in isolated neurosecretosomes from the post-pituitary. After chemical deacylation, a single 27 kDa form was detected that displayed identical properties to csps in a whole-brain synaptosomal fraction. Immunoisolation demonstrated that synaptophysin and csps were located in the same vesicles. Density gradient centrifugation of postsynaptosomal supernatants of neurohypophysial homogenates revealed that csps and VAMP were present in two distinct vesicle populations. Synaptophysin was only detected in the slowly migrating population corresponding to small synaptic vesicles, whereas arginine vasopressin was present in the more rapidly sedimenting population indicating that it contains large dense core vesicles (LDCVs). Immobilized antibodies against csp, synaptotagmin, or VAMP captured vesicular arginine vasopressin confirming the association of these proteins with LDCVs. Co-immunoprecipitation assays with proteins solubilized from neurohypophysial or whole-brain nerve terminals failed to reveal complexes containing csp and [125I]omegaGVIA receptors. These results indicate that csps in the CNS are associated with both small synaptic vesicles and LDCVs. However, they do not provide support for the hypothesis that protein complexes implicated in exocytosis, which interact with presynaptic N-type calcium channels, contain csps.

Developmental regulation of synaptotagmin I, II, III, and IV mRNAs in the rat CNS

Synaptotagmin I is an abundant synaptic vesicle protein that has an essential function in mediating Ca2+-triggered neurotransmitter release. We have analyzed the distribution of four neural synaptotagmin isoforms during postnatal development of the rat CNS by in situ hybridization. Synaptotagmin I, II, III, and IV genes have distinct patterns of spatiotemporal expression except in cerebellum granule cells, where the four transcripts were detected during the formation of parallel fiber/Purkinje cell synapses. Throughout development synaptotagmin I mRNAs were widely expressed in brain, whereas synaptotagmin II transcripts were predominant in spinal cord. At all stages synaptotagmin III mRNAs were expressed uniformly in most neurons examined, although at a low level. Synaptotagmin I, II, and III gene expressions mainly increased during development and persisted in adulthood, mirroring neuronal differentiation. Conversely, synaptotagmin IV transcripts were predominant during perinatal development in a heterogeneous population of neurons and subsequently were expressed uniformly at a low level. Intense labeling was observed in the hippocampal CA3 field and in the subiculum, but not in the CA1 field, of the newborn rat. In cerebral cortex, lamina-specific labeling was detected with a high expression in cell layer V. Only a small number of Purkinje cell clusters were labeled in the flocculus and paraflocculus of the cerebellum. Heterogeneous sets of neurons expressing synaptotagmin IV gene also were observed in spinal cord. We thus speculate that synaptotagmin IV may a play a role in the development of the mammalian nervous system.

Distribution of components of the SNARE complex in relation to transmitter release sites at the frog neuromuscular junction

At the frog neuromuscular junction, neurotransmitter release sites are regularly spaced at 1 micron intervals along the nerve terminal, directly facing postsynaptic folds which contain a high density of acetylcholine receptors. Immunostaining and laser confocal scanning microscopy were used to compare the distribution of presynaptic proteins implicated in exocytosis with that of fluorescent alpha-bungarotoxin. Syntaxin, synaptosome-associated 25 kDa protein and calcium channels were located predominantly at release sites. Synaptobrevin (vesicle-associated membrane protein) was distributed in the cytoplasm of the nerve terminal, presumably in the packets of microvesicles associated with each active zone. N-Ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment proteins (alpha beta SNAP) displayed a diffuse distribution throughout the terminal cytoplasm and also colocalized in distinct concentrated zones adjacent to the presynaptic membrane.

Tyr13 is essential for the binding of omega-conotoxin MVIIC to the P/Q-type calcium channel

An analog of omega-conotoxin MVIIC (Y13A-MVIIC) was synthesized by replacing Tyr13 with Ala to study the role of Tyr13 residue conserved in many omega-conotoxins. Y13A-MVIIC has an overall conformation similar to that of the native toxin, but an enormously reduced ability to displace 125I-omega-conotoxin MVIIC binding to rat cerebellar P2 membranes. These results suggest that Tyr13 is essential for the activity of omega-conotoxins at P/Q-type calcium channels.

Mouse motor nerve terminal immunoreactivity to synaptotagmin II during sustained quantal transmitter release

An antibody directed against the lumenal NH2-terminus of synaptotagmin II was used to examine the distribution of this vesicular protein either after spontaneous acetylcholine release or after sustained release induced by La3+ or alpha-latrotoxin, in conditions that prevent endocytosis. The detection of the epitope was examined in the presence or absence of Triton X-100. We show that, in resting conditions of transmitter release, permeabilization of nerve terminal membranes is required for obvious detection of synaptotagmin Ii immunoreactivity whereas during sustained rates of quantal release, permeabilization is not necessary. These data indicate that, in the latter conditions, synaptotagmin II is incorporated into the terminal axolemma and its intravesicular domain exposed at the extracellular nerve terminal surface.

Antigens associated with N- and L-type calcium channels in Lambert-Eaton myasthenic syndrome

In Lambert-Eaton myasthenic syndrome neurotransmitter release is reduced by an autoimmune response directed against the calcium channel complex of the nerve terminal. Autoantibodies were detected by immunoprecipitation assays using solubilized receptors labeled with ligands selective for N-type (125I-omega conotoxin GVIA) and L-type ([3H]PN200-110) calcium channels. Sera with a high antibody titer (> 3 nM) against rat brain N-type channels contained autoantibodies that immunoprecipitated neuronal and muscle L-type channels. These IgG fractions stained a 55-kDa protein in immunoblots of purified skeletal muscle dihydropyridine receptor, suggesting that they contain autoantibodies against the beta subunit of the calcium channel. A distinct antibody population in the same fractions reacted with a nerve terminal 65-kDa protein that is unrelated to the beta subunit and displays properties similar to those of synaptotagmin.

Interaction of a synaptobrevin (VAMP)-syntaxin complex with presynaptic calcium channels

Nerve terminal protein complexes implicated in exocytosis were examined by immuno-isolation from rat brain synaptosomes. Immunoprecipitation with anti-syntaxin or anti-VAMP antibodies revealed a syntaxin-SNAP25-VAMP-synaptotagmin complex. Anti-VAMP antibodies also trapped a distinct VAMP-synaptophysin complex. A similar fraction (about 70%) of N-type calcium channels ([125I]omega conotoxin GVIA receptors), was immunoprecipitated by either anti-syntaxin or anti-VAMP antibodies, but not by anti-synaptophysin antibodies (< 4%). The majority of N- but not L-type calcium channels ([3H]PN200-110 receptors), appear to be associated with a synaptic vesicle prefusion complex.

Purification of the N-type calcium channel associated with syntaxin and synaptotagmin. A complex implicated in synaptic vesicle exocytosis

omega-Conotoxin-sensitive N-type calcium channels control neurotransmitter release at the nerve terminal and interact with proteins implicated in secretion. Solubilized omega-conotoxin receptors from rat brain synaptic membrane were immunoprecipitated by antibodies against calcium channel alpha 1 subunits, syntaxin, and a 105-kDa plasma membrane protein. A multimeric complex, composed of calcium channel subunits, and synaptic proteins that showed varying degrees of association, was purified by a procedure involving anti-syntaxin immunoaffinity chromatography. A 250-kDa N-type alpha 1 subunit, containing cAMP-dependent phosphorylation site(s), was identified by photoaffinity labeling with 125I-azidonitrobenzoyl omega-conotoxin and immunoblotting with sequence-directed antibodies. An immunologically related 210-kDa form of the alpha 1 subunit was detected that displayed different pharmacological and regulatory properties. Protein bands of 140, 70, 58, and 35 kDa comigrated with purified alpha 1 subunits upon sucrose gradient centrifugation, whereas the 105-kDa protein was removed. The 58- and 35-kDa bands contained, respectively, the synaptic vesicle protein synaptotagmin and syntaxin, a plasma membrane protein that binds synaptic vesicle proteins. Purified omega-contoxin receptors were quantitatively immunoprecipitated by anti-syntaxin antibodies. These proteins may constitute an isolated exocytotic complex in which the N-type calcium channel tightly interacts with a synaptic vesicle docking site.

Interaction of synaptotagmin with voltage gated calcium channels: a role in Lambert-Eaton myasthenic syndrome?

Plasma from patients with Lambert-Eaton myasthenic syndrome (LEMS), an autoimmune disease of neuromuscular transmission, contains antibodies that bind to the synaptic vesicle protein synaptotagmin. Synaptotagmin associates with calcium channels and appears to regulate synaptic vesicle docking at the plasma membrane prior to rapid neurotransmitter release. Autoantibodies directed against a synaptotagmin-calcium channel complex may be involved in the etiology of LEMS. In the majority of patients LEMS is associated with small cell lung cancer (SCLC). We have detected the expression of proteins of the secretory pathway, including synaptotagmin, syntaxin and N-type calcium channels, in a panel of SCLC tumor lines. These observations are compatible with the hypothesis that the initial autoimmune response in LEMS is triggered by the tumor.

Expression of synaptotagmin and syntaxin associated with N-type calcium channels in small cell lung cancer

The presence of synaptic proteins involved in excitation/secretion coupling was examined in ten small cell lung cancer lines. N-Type calcium channels (omega-conotoxin receptors), synaptotagmin (p65) and syntaxin (HPC-1) were detected in eight. Co-immunoprecipitation experiments indicated that syntaxin can form a complex with synaptotagmin and calcium channels. The expression of synaptotagmin in small cell lung cancer may elicit an autoimmune response that reduces transmitter release at the nerve terminal.

Omega-conotoxin sensitive calcium channels in cerebellar granule cells are not coupled to [3H]glutamate release

We have studied the biochemical and functional aspects of omega-conotoxin GVIA (omega-CgTx)-sensitive calcium channels in cerebellar granule cells in vitro. 125I-omega-Conotoxin GVIA (125I-omega-CgTx) binding sites were detected in intact cultured cerebellar granule cells and binding parameters were measured (Bmax: 134 fmol/mg protein; kinetic association constant kappa: 3.10(6) M-1.s-1). [3H]Glutamate release was assessed under different release paradigms (namely release triggered by calcium, voltage, and sodium channel agonists) and different times (15 s and 2 min). However, in all cases, [3H]glutamate release was found to be completely insensitive to omega-CgTx. Conversely, voltage-dependent release was inhibited in a dose-dependent fashion by cadmium chloride, with total inhibition at 10(-4) M. These results indicate that N-type calcium channels are not involved in glutamate secretion from granule neurons.

Synaptotagmin: a Lambert-Eaton myasthenic syndrome antigen that associates with presynaptic calcium channels

Plasma from patients with Lambert-Eaton myasthenic syndrome (LEMS), an autoimmune disease of neuromuscular transmission, contains antibodies that immunoprecipitate 125I-omega-conotoxin GVIA labeled-calcium channels solubilized from rat brain. These antibodies label a 58-kDa protein in Western blots of partially purified 125I-omega-conotoxin receptor preparations. Monoclonal antibody 1D12, produced by immunizing mice with synaptic membranes, has similar properties as these LEMS IgG. 1D12 antigen was purified by immunoaffinity chromatography and shown to bind LEMS IgG. The antigen was identified by immunoscreening a rat brain cDNA library with mAb 1D12 and found to have strong homology to the synaptic vesicle protein synaptotagmin. These antibodies immunoprecipitate calcium channels by binding to synpatotagmin, an associated protein. We suggest that the interaction between synaptotagmin and omega-conotoxin sensitive calcium channels plays a role in docking synaptic vesicles at the plasma membrane prior to rapid neurotransmitter release. Autoantibody binding to a synaptotagmin-calcium channel complex may be involved in the etiology of LEMS.

Cyclic AMP-dependent phosphorylation of two size forms of alpha 1 subunits of L-type calcium channels in rat skeletal muscle cells

Skeletal muscle dihydropyridine-sensitive calcium channels are in vitro substrates for cAMP-dependent protein kinase. In the present work, alpha 1 subunits were isolated from cultured skeletal muscle cells by immunoprecipitation with a specific monoclonal antibody under conditions where proteolysis and dephosphorylation were prevented. Two forms of alpha 1 subunit, 200 and 160 kDa, were identified by back phosphorylation in vitro with cAMP-dependent protein kinase, specific immunoprecipitation, and phosphopeptide mapping. Treatment of cells with forskolin, isoproterenol, calcitonin gene-related peptide, or 8-bromo-cAMP to increase intracellular cAMP reduced 32P incorporation into all phosphopeptides in vitro by 60-80% indicating that increases in cAMP caused endogenous phosphorylation of all sites on both alpha 1(200) and alpha 1(160) to nearly maximal levels. The extents of basal and stimulated phosphorylation in vivo were estimated by back phosphorylation methods to be 35-40% and 83-86%, respectively. In muscle cells metabolically labeled with 32P, 3 mol of phosphate were incorporated into alpha 1 subunits. Forskolin stimulated 32P incorporation into alpha 1 subunits 1.6-fold. Taken together, our results show that skeletal muscle cells contain two forms of the alpha 1 subunit which both are basally phosphorylated on cAMP-dependent phosphorylation sites and are further phosphorylated in response to agents that increase intracellular cAMP.

Photoaffinity labeling of the K+-channel-associated apamin-binding molecule in smooth muscle, liver and heart membranes

High-affinity binding sites for mono[125I]iodoapamin were detected in membranes (Kd = 59 pM, Bmax = 24 fmol/mg protein) and cultured cells (Kd = 69 pM, Bmax = 2.8 fmol/mg protein) from rat heart and in membranes from guinea-pig ileum (Kd = 67 pM, Bmax 42 fmol/mg protein) and liver (Kd = 15 pM, Bmax = 43 fmol/mg protein). Binding was stimulated by K+ ions (K0.5 = 0.3-0.5 mM). Covalent labeling with arylazide [125I]iodoapamin derivatives showed that smooth muscle, liver and heart binding molecules are associated with a 85-87-kDa polypeptide. A second strongly labeled 57-kDa component was identified in liver membranes only.

Subunit structure of dihydropyridine-sensitive calcium channels from skeletal muscle

Purified dihydropyridine-sensitive calcium channels from rabbit transverse-tubule membranes consist of three noncovalently associated classes of subunits: alpha (167 kDa), beta (54 kDa), and gamma (30 kDa). Cleavage of disulfide bonds reveals two distinct alpha polypeptides and an additional component, delta. The alpha 1 subunit, a 175-kDa polypeptide that is not N-glycosylated, contains the dihydropyridine binding site, cAMP-dependent protein kinase phosphorylation site(s), and substantial hydrophobic domain(s). alpha 2, a 143-kDa glycoprotein, has none of the properties characteristic of alpha 1 but binds lectins and contains about 25% N-linked carbohydrate. alpha 2 is disulfide-linked to delta, a 24- to 27-kDa glycopeptide. beta (54 kDa) contains a cAMP-dependent phosphorylation site but is not N-glycosylated and does not have a hydrophobic domain. gamma (30 kDa) has a carbohydrate content of about 30% and extensive hydrophobic domain(s). Precipitation with affinity-purified anti-alpha 1 antibodies or alpha 2-specific lentil lectin-agarose demonstrated that alpha 1 alpha 2 beta gamma delta behaves as a complex in the presence of digitonin or 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, whereas the alpha 2 delta complex dissociates from alpha 1 beta gamma in the presence of Triton X-100. A model for subunit interaction and membrane insertion is proposed on the basis of these observations.

Detection and photoaffinity labeling of the Ca2+-activated K+ channel-associated apamin receptor in cultured astrocytes from rat brain

Apamin, an 18-amino acid bee venom peptide, is a specific blocker of a class of Ca2+ activated K+ channels. Mono 125I-iodoapamin was used to detect the K+ channel-associated receptor site in cultured astrocytes from rat brain. Specific high-affinity binding to intact glial cells with a Kd of about 90 pM at 1 degree C and pH 7.5 was demonstrated by equilibrium and kinetic methods. The average receptor capacity was 3 fmol/mg cell protein which is 2 to 3-fold lower than in primary cultured neurons. Binding was stimulated by K+ ions, but to a lesser extent than with neuronal receptors. Photoaffinity labeling of receptor/ion channel components using an arylazide derivative of 125I-monoiodoapamin revealed the presence of the 86- and 33-kDa polypeptides, previously detected in neurones. However a 59-kDa peptide which is present in synaptic membrane preparations from adult rat brain, but not in cultured neurons, was also clearly labeled in intact astrocytes. This indicates that the 59-kDa polypeptide is not a proteolytic fragment of the 86-kDa chain but an associated subunit which is only accessible to photolabeling in certain apamin receptor preparations. Apamin-sensitive Ca2+-activated K+ channels in astrocytes may be one of the pathways by which glial cells redistribute K+ in the central nervous system (CNS).

Solubilization of the apamin receptor associated with a calcium-activated potassium channel from rat brain

The apamin binding protein was solubilized from rat brain synaptic membranes using sodium cholate. Receptor yield and stability depended closely on the detergent/protein ratio. In optimum conditions the receptor retained high affinity for mono 125I-iodoapamin with Kd = 40 pM at pH 7.5 and 1 degree C and a binding capacity of 17 fmol/mg protein. 125I-apamin binding was stimulated by K+ ions with a K0.5 = 0.6 mM, demonstrating that the regulatory K+ site is also part of the soluble complex. Other ions could be substituted for K+ with an affinity sequence Tl+ = K+ = Rb+ greater than Cs+ greater than NH4+ greater than Li+ or Na+. Binding was inhibited by the neuromuscular blockers gallamine and tubocurarine and by the K+ channel blockers quinidine and tetraethylammonium chloride but not by 4-aminopyridine, in agreement with known pharmacological profile for inhibition of apamin-sensitive K+ permeability. Increasing the K+ concentration did not reverse inhibition by tetraethylammonium ions demonstrating that it does not bind competitively to the regulatory cationic site. Analysis of the covalently labeled apamin binding protein/sodium cholate complex by density gradient centrifugation indicated a high molecular weight with S20,w = 20 S.

Molecular structure of rat brain apamin receptor: differential photoaffinity labeling of putative K+ channel subunits and target size analysis

Two photoreactive apamin derivatives were prepared with an aryl azide [[(azidonitrophenyl)amino]acetate (ANPAA)] group coupled at different positions on the neurotoxin molecule. These ligands were used to identify membrane components in the environment of the neuronal binding site that is associated with a Ca2+-activated K+ channel. 125I-[alpha-ANPAA-Cys1] apamin labeled a single Mr 86 000 chain in cultured neurons whereas two bands corresponding to Mr 86 000 and 59,000 were detected in synaptic membrane preparations, suggesting that the Mr 59,000 polypeptide may be a degradation product. 125I-[epsilon-ANPAA-Lys4]apamin however incorporated uniquely into two smaller components with Mr 33,000 and 22,000 in both cultured neurons and synaptic membranes. Randomly modified 125I-ANPAA-apamin gave a cross-linking profile equivalent to the sum of those obtained with the two defined derivatives. The apamin binding site seems to be located at the frontier between three or more putative K+ channel subunits which are only accessible from limited regions of the receptor-associated photoprobe. Irradiation of frozen rat brain membranes with high-energy electrons led to a reduction in 125I-apamin receptor capacity, yielding a target size for the functional binding unit of Mr 84,000-115,000, which could be constituted by the Mr 86,000 subunit alone or by the Mr 86,000 subunit in conjuction with one of the two smaller subunits.

Photoaffinity labeling of components of the apamin-sensitive K+ channel in neuronal membranes

An azidonitrophenylaminoacetyl mono[125I]iodoapamin derivative was prepared which showed specific binding to rat neuronal membranes. UV photolysis lead to the irreversible occupation of binding sites. Photo-labeling of intact primary cultured rat neurones followed by membrane solubilization, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and autoradiography revealed the covalent incorporation of radioactivity into 3 main components with Mr = 86,000, 30,000, and 23,000. Labeling was completely prevented by a competing excess of native apamin. Similar studies on purified synaptic membranes from the rat brain showed another labeling pattern with major bands corresponding to Mr = 86,000 and 59,000. Although the reasons for the partial discrepancy between cultured embryonic neurons and an adult brain membrane fraction are not yet clear, we conclude that these proteins are intimately associated with the apamin binding site and are probably components of a type of Ca2+-activated K+ channel.

Interactions of the neurotoxin apamin with a Ca2+-activated K+ channel in primary neuronal cultures

Mono[125I]iodoapamin bound to specific sites on cultured rat embryonic neurons. The dissociation constant for the receptor-neurotoxin complex measured at equilibrium was 60-120 pM at pH 7.2 and 4 degrees C, with a maximal binding capacity of 3-8 fmol/mg of cell protein. Apamin inhibited calcium ionophore-induced 86Rb+ release from cell cultures. The dose effect curve for this pharmacological test corresponded closely to the displacement of 125I-apamin by native apamin in binding experiments. Formation of the 125I-apamin receptor complex requires exogenous K+. Reduced binding in the absence of K+ was due to diminished binding capacity rather than a lower affinity. The apamin receptor seems to be associated with a cell surface K+ site which shows 50% occupancy at 1.6 mM, and which could be involved in the regulation of channel activity. Apamin sites were present at the earliest developmental stage tested and their number did not evolve during 8 days in culture. In the same period, however, alpha-scorpion toxin binding increased by a factor of 10. The ontogenesis of Ca2+-activated K+ channels does not seem to occur in parallel with that of voltage-sensitive Na+ channels.

Inhibitory effects of tunicamycin and 2-deoxyglucose on thyroglobulin synthesis

The kinetic incorporation of labelled sugars and amino acids by rat thyroid hemilobes was measured in the presence of 2-deoxyglucose and tunicamycin, inhibitors of the glycosylation of glycoproteins. With either inhibitor the carbohydrate content of exocytosed thyroglobulin was only slightly decreased (less than 20% of control) whereas the rate of exocytosis was strongly inhibited (by 60-80%). As no intracellular accumulation or proteolysis of non-glycosylated molecules was detected, the reduced rate of thyroglobulin release seems essentially due to a decrease in protein synthesis. In a whole cell system (hemilobes), it is impossible to uncouple glycosylation and protein synthesis by incubation with tunicamycin; 50 micrograms/ml tunicamycin for 270 min inhibited total [3H]-glucosamine and 14C-labelled amino acid incorporation by 65% and 33% respectively. This can be contrasted with cell-free incubation of thyroid rough microsomes where glycosylation was blocked by the same tunicamycin concentration (90% inhibition of N-[3H]acetylglucosamine transfer from UDP-N-[3H]acetylglucosamine) whilst ongoing protein synthesis was not significantly modified (less than 4% inhibition). This clearly suggests that, in thyroid follicular cells, a regulatory link exists between the synthesis of the peptide moiety of a glycoprotein and its glycosylation.