cDNAs for the postsynaptic 43-kDa protein of Torpedo electric organ encode two proteins with different carboxyl termini

alpha-Actinin interacts with rapsyn in agrin-stimulated AChR clustering

AChR is concentrated at the postjunctional membrane at the neuromuscular junction. However, the underlying mechanism is unclear. We show that α-actinin, a protein known to cross-link F-actin, interacts with rapsyn, a scaffold protein essential for neuromuscular junction formation. α-Actinin, rapsyn, and surface AChR form a ternary complex. Moreover, the rapsyn-α-actinin interaction is increased by agrin, a factor known to stimulate AChR clustering. Downregulation of α-actinin expression inhibits agrin-mediated AChR clustering. Furthermore, the rapsyn-α-actinin interaction can be disrupted by inhibiting Abl and by cholinergic stimulation. Together these results indicate a role for α-actinin in AChR clustering.

Studies of acetylcholine receptor subunit gene expression: chromatin structural changes during myogenesis

Myogenesis proceeds stepwise from pluripotential stem cell to differentiated myotube. The precise number of transitions that occur along the developmental pathway remains to be determined. We examined the myogenic pathway as modelled by mouse mesodermal stem cell and muscle cell lines for stage-specific alterations in the chromatin structure of the acetylcholine receptor delta and gamma subunit genes. We reasoned that such an analysis would allow us to observe either the primary events in the activation of these muscle-specific genes or processes secondary to the binding of muscle-specific regulatory proteins. We probed chromatin structure with DNase I (deoxyribonuclease I) and precisely mapped to the 5' ends of the delta and gamma genes DNase I hypersensitive (DH) sites whose induction is unique to each myogenic stage. Putative mesodermal stem cells have the simplest pattern of DH sites with no sites near the 5' ends of the delta and gamma genes, whereas differentiated myotubes express the most complex pattern; the myoblast pattern is intermediate and of two types. In muscle cell lines where differentiation must be induced the myoblasts have a simple pattern (one more site than stem cells); in muscle lines where differentiation is spontaneous the myoblasts express a complex pattern of DH sites (one less site than myotubes). Inducible myoblasts seem to be arrested in an earlier step in the myogenic pathway than spontaneously differentiating myoblasts. Thus, myogenic activation of acetylcholine receptor subunit genes appears to be a stepwise process that can be detected by chromatin structural changes specific to four distinct stages of muscle development: stem cell, early myoblast, late myoblast, and differentiated myotube.

Rapsyn mutations in humans cause endplate acetylcholine-receptor deficiency and myasthenic syndrome

Congenital myasthenic syndromes (CMSs) stem from genetic defects in endplate (EP)-specific presynaptic, synaptic, and postsynaptic proteins. The postsynaptic CMSs identified to date stem from a deficiency or kinetic abnormality of the acetylcholine receptor (AChR). All CMSs with a kinetic abnormality of AChR, as well as many CMSs with a deficiency of AChR, have been traced to mutations in AChR-subunit genes. However, in a subset of patients with EP AChR deficiency, the genetic defect has remained elusive. Rapsyn, a 43-kDa postsynaptic protein, plays an essential role in the clustering of AChR at the EP. Seven tetratricopeptide repeats (TPRs) of rapsyn subserve self-association, a coiled-coil domain binds to AChR, and a RING-H2 domain associates with beta-dystroglycan and links rapsyn to the subsynaptic cytoskeleton. Rapsyn self-association precedes recruitment of AChR to rapsyn clusters. In four patients with EP AChR deficiency but with no mutations in AChR subunits, we identify three recessive rapsyn mutations: one patient carries L14P in TPR1 and N88K in TPR3; two are homozygous for N88K; and one carries N88K and 553ins5, which frameshifts in TPR5. EP studies in each case show decreased staining for rapsyn and AChR, as well as impaired postsynaptic morphological development. Expression studies in HEK cells indicate that none of the mutations hinders rapsyn self-association but that all three diminish coclustering of AChR with rapsyn.

The chaperone protein 14-3-3eta interacts with the nicotinic acetylcholine receptor alpha 4 subunit. Evidence for a dynamic role in subunit stabilization

By using the large cytoplasmic domain of the nicotinic acetylcholine receptor (AChR) alpha4 subunit as a bait in the yeast two-hybrid system, we isolated the first cytosolic protein, 14-3-3eta, known to interact directly with neuronal AChRs. 14-3-3eta is a member of a family of proteins that function as regulatory or chaperone/ scaffolding/adaptor proteins. 14-3-3eta interacted with the recombinant alpha4 subunit alone in tsA 201 cells following activation of cAMP-dependent protein kinase by forskolin. The interaction of 14-3-3eta with recombinant alpha4 subunits was abolished when serine 441 of the alpha4 subunit was mutated to alanine (alpha4(S441A)). The surface levels of recombinant wild-type alpha4beta2 AChRs were approximately 2-fold higher than those of mutant alpha4(S441A)beta2 AChRs. The interaction significantly increased the steady state levels of the alpha4 subunit and alpha4beta2 AChRs but not that of the mutant alpha4(S441A) subunit or mutant alpha4(S441A)beta2 AChRs. The EC50 values for activation by acetylcholine were not significantly different for alpha4beta2 AChRs and alpha4(S441A)beta2 AChRs coexpressed with 14-3-3eta in oocytes following treatment with forskolin. 14-3-3 coimmunopurified with native alpha4 AChRs from brain. These results support a role for 14-3-3 in dynamically regulating the expression levels of alpha4beta2 AChRs through its interaction with the alpha4 subunit.

Specific phosphorylation of Torpedo 43K rapsyn by endogenous kinase(s) with thiamine triphosphate as the phosphate donor

43K rapsyn is a peripheral protein specifically associated with the nicotinic acetylcholine receptor (nAChR) present in the postsynaptic membrane of the neuromuscular junction and of the electrocyte, and is essential for its clustering. Here, we demonstrate a novel specific phosphorylation of 43K rapsyn by endogenous protein kinase(s) present in Torpedo electrocyte nAChR-rich membranes and identify thiamine triphosphate (TTP) as the phosphate donor. In the presence of Mg(2+) and [gamma-(32)P]-TTP, 43K rapsyn is specifically phosphorylated with a (32)P-half-maximal incorporation at approximately 5-25 microM TTP. The presence of TTP in the cytosol and of 43K rapsyn at the cytoplasmic face of the postsynaptic membrane, together with TTP-dependent phosphorylation of 43K rapsyn without added exokinases, suggests that TTP-dependent-43K-rapsyn phosphorylation may occur in vivo. In addition, phosphoamino acid and chemical stability analysis suggests that the residues phosphorylated are predominantly histidines. Inhibition of phosphorylation by Zn(2+) suggests a possible control of 43K rapsyn phosphorylation state by its zinc finger domain. Endogenous kinase(s) present in rodent brain membranes can also use [gamma-(32)P]-TTP as a phosphodonor. The use of a phosphodonor (TTP) belonging to the thiamine family but not to the classical (ATP, GTP) purine triphosphate family represents a novel phosphorylation pathway possibly important for synaptic proteins.

Phosphorylation and cytoskeletal anchoring of the acetylcholine receptor by Src class protein-tyrosine kinases. Activation by rapsyn

Src class protein-tyrosine kinases bind to and phosphorylate the nicotinic acetylcholine receptor of skeletal muscle. This study provided evidence for the functional importance of Src kinases in regulating the nicotinic acetylcholine receptor at the neuromuscular junction. Three Src class kinases, Fyn, Fyk, and Src, each formed a complex with the endplate-specific cytoskeletal protein rapsyn. In addition, cellular phosphorylation by each kinase was stimulated by rapsyn in heterologous transfected cells. Several lines of evidence supported rapsyn as a substrate for Src kinases. Most importantly, rapsyn regulation of Fyn, Fyk, and Src resulted in phosphorylation of the nicotinic acetylcholine receptor beta and delta subunits and anchoring of the receptor to the cytoskeleton. Both nicotinic acetylcholine receptor phosphorylation and cytoskeletal anchoring were blocked by the Src kinase-selective inhibitor herbimycin A. Rapsyn alone also induced a modest increase in nicotinic acetylcholine receptor phosphorylation and cytoskeletal translocation. However, inhibition by herbimycin A and a catalytically inactive dominant negative Src demonstrated that the effects of rapsyn were mediated by endogenous Src kinases. These data support the importance of Src class kinases for stabilization of the nicotinic acetylcholine receptor at the endplate during synaptic differentiation at the neuromuscular junction.

Development of the vertebrate neuromuscular junction

We describe the formation, maturation, elimination, maintenance, and regeneration of vertebrate neuromuscular junctions (NMJs), the best studied of all synapses. The NMJ forms in a series of steps that involve the exchange of signals among its three cellular components--nerve terminal, muscle fiber, and Schwann cell. Although essentially any motor axon can form NMJs with any muscle fiber, an additional set of cues biases synapse formation in favor of appropriate partners. The NMJ is functional at birth but undergoes numerous alterations postnatally. One step in maturation is the elimination of excess inputs, a competitive process in which the muscle is an intermediary. Once elimination is complete, the NMJ is maintained stably in a dynamic equilibrium that can be perturbed to initiate remodeling. NMJs regenerate following damage to nerve or muscle, but this process differs in fundamental ways from embryonic synaptogenesis. Finally, we consider the extent to which the NMJ is a suitable model for development of neuron-neuron synapses.

The zinc finger domain of the 43-kDa receptor-associated protein, rapsyn: role in acetylcholine receptor clustering

We injected rat myotubes with proteins and antibodies to assess the importance of the zinc finger (ZnF) domain of the 43-kDa receptor-associated protein, rapsyn, in clustering acetylcholine receptors (AChR). Injection of rat myotubes with a fusion protein containing the ZnF domain of rapsyn disrupted AChR clusters. Clusters were unaffected by a fusion protein containing a double mutant that does not bind zinc. Similar results were obtained with the purified wild type and mutant ZnF domains. The ZnF of HIV-1 nucleocapsid protein had no effect. AChR clusters were also disrupted in myotubes injected with antibodies to the ZnF domain, followed by injection of anti-antibodies. Injection of antibodies directed against a different rapsyn epitope or against the cytoplasmic domain of the AChR had no effect. In transfection experiments with HEK 293 cells, the ZnF domain failed to associate with membrane aggregates containing full-length rapsyn, AChR, or rapsyn and AChR together. We conclude that the ZnF domain of rapsyn provides a binding site essential for AChR clustering, but that this site is unlikely to be involved in high affinity binding of rapsyn to itself or to AChR.

Chick ciliary ganglion neurons contain transcripts coding for acetylcholine receptor-associated protein at synapses (rapsyn)

A peripheral membrane protein of approximately 43 kDa (rapsyn) clusters muscle nicotinic acetylcholine receptors (AChRs), but molecules relevant to clustering neuronal AChRs have not been identified. Here, we have detected rapsyn transcripts in the chick nervous system, localized rapsyn mRNA in ciliary ganglion (CG) neurons, which are known to cluster AChRs, and identified three rapsyn cDNAs derived from the ganglion. Our initial Northern blots, performed using a mouse probe, revealed rapsyn-like transcripts in chick muscle and brain. To develop species-specific probes, we prepared a chick rapsyn cDNA construct, Ch43K.1, that encodes a protein having extensive homology to mouse rapsyn. Using primers designed to anneal near the 5' and 3' boundaries of Ch43K.1, three prominent cDNAs were amplified from chick muscle templates by reverse transcriptase based-PCR. Products of similar size were also amplified using cDNA prepared from neuronal tissues expected to contain clustered AChRs (CG and brain), whereas none were detected using templates from tissues not displaying clustered AChRs (sensory ganglia and liver). In situ hybridization confirmed that rapsyn mRNA is expressed both in chick muscle fibers and in CG neurons. Sequencing the three cDNAs amplified from CG templates revealed the largest to be Ch43K.1, whereas the smaller two may represent splice variants. These findings suggest that multiple rapsyn-like molecules are involved in clustering the distinct AChRs expressed by muscle fibers and neurons.

Chick ciliary ganglion neurons contain transcripts coding for acetylcholine receptor-associated protein at synapses (rapsyn)

A peripheral membrane protein of approximately 43 kDa (rapsyn) clusters muscle nicotinic acetylcholine receptors (AChRs), but molecules relevant to clustering neuronal AChRs have not been identified. Here, we have detected rapsyn transcripts in the chick nervous system, localized rapsyn mRNA in ciliary ganglion (CG) neurons, which are known to cluster AChRs, and identified three rapsyn cDNAs derived from the ganglion. Our initial Northern blots, performed using a mouse probe, revealed rapsyn-like transcripts in chick muscle and brain. To develop species-specific probes, we prepared a chick rapsyn cDNA construct, Ch43K.1, that encodes a protein having extensive homology to mouse rapsyn. Using primers designed to anneal near the 5' and 3' boundaries of Ch43K.1, three prominent cDNAs were amplified from chick muscle templates by reverse transcriptase based-PCR. Products of similar size were also amplified using cDNA prepared from neuronal tissues expected to contain clustered AChRs (CG and brain), whereas none were detected using templates from tissues not displaying clustered AChRs (sensory ganglia and liver). In situ hybridization confirmed that rapsyn mRNA is expressed both in chick muscle fibers and in CG neurons. Sequencing the three cDNAs amplified from CG templates revealed the largest to be Ch43K.1, whereas the smaller two may represent splice variants. These findings suggest that multiple rapsyn-like molecules are involved in clustering the distinct AChRs expressed by muscle fibers and neurons.

Nicotinic receptor-associated 43K protein and progressive stabilization of the postsynaptic membrane

An extrinsic membrane protein of apparent molecular mass 43 kDa is specifically localized in postsynaptic membranes closely associated with the nicotinic acetylcholine receptor (AChR). Since its discovery in 1977, biochemical and morphological studies have combined to provide relatively clear pictures of 43K protein structure and subcellular compartmentalization. Nevertheless, despite these advances, the precise function of this synapse-specific protein remains unclear. Data gathered in recent years indicate that the postsynaptic apparatus develops through the incremental agglomeration of receptor microaggregates; evidence derived from a number of sources points to a role for 43K protein in certain underlying reactions. In this paper, I review 43K protein structural and anatomical data and analyze evidence for its role in the organization and maintenance of the postsynaptic membrane. Finally, I offer a model presenting a view of the role of 43K protein in the ontogeny of the motor endplate.

Mutagenesis of the 43-kD postsynaptic protein defines domains involved in plasma membrane targeting and AChR clustering

The postsynaptic membrane of the neuromuscular junction contains a myristoylated 43-kD protein (43k) that is closely associated with the cytoplasmic face of the nicotinic acetylcholine receptor (AChR)-rich plasma membrane. Previously, we described fibroblast cell lines expressing recombinant AChRs. Transfection of these cell lines with 43k was necessary and sufficient for reorganization of AChR into discrete 43k-rich plasma membrane domains (Phillips, W. D., C. Kopta, P. Blount, P. D. Gardner, J. H. Steinbach, and J. P. Merlie. 1991. Science (Wash. DC). 251:568-570). Here we demonstrate the utility of this expression system for the study of 43k function by site-directed mutagenesis. Substitution of a termination codon for Asp254 produced a truncated (28-kD) protein that associated poorly with the cell membrane. The conversion of Gly2 to Ala2, to preclude NH2-terminal myristoylation, reduced the frequency with which 43k formed plasma membrane domains by threefold, but did not eliminate the aggregation of AChRs at these domains. Since both NH2 and COOH-termini seemed important for association of 43k with the plasma membrane, a deletion mutant was constructed in which the codon Gln15 was fused in-frame to Ile255 to create a 19-kD protein. This mutated protein formed 43k-rich plasma membrane domains at wild-type frequency, but the domains failed to aggregate AChRs, suggesting that the central part of the 43k polypeptide may be involved in AChR aggregation. Our results suggest that membrane association and AChR interactions are separable functions of the 43k molecule.

ACh receptor-rich membrane domains organized in fibroblasts by recombinant 43-kildalton protein

Neurotransmitter receptors are generally clustered in the postsynaptic membrane. The mechanism of clustering was analyzed with fibroblast cell lines that were stably transfected with the four subunits for fetal (alpha, beta, gamma, delta) or adult (alpha, beta, epsilon, delta) type mouse muscle nicotinic acetylcholine receptors (AChRs). Immunofluorescent staining indicated that AChRs were dispersed on the surface of these cells. When transiently transfected with an expression construct encoding a 43-kilodalton protein that is normally concentrated under the postsynaptic membrane, AChRs expressed in these cells became aggregated in large cell-surface clusters, colocalized with the 43-kilodalton protein. This suggests that 43-kilodalton protein can induce AChR clustering and that cluster induction involves direct contact between AChR and 43-kilodalton protein.

A novel 87,000-Mr protein associated with acetylcholine receptors in Torpedo electric organ and vertebrate skeletal muscle

To identify proteins associated with nicotinic postsynaptic membranes, mAbs have been prepared to proteins extracted by alkaline pH or lithium diiodosalicylate from acetylcholine receptor-rich (AChR) membranes of Torpedo electric organ. Antibodies were obtained that recognized two novel proteins of 87,000 Mr and a 210,000:220,000 doublet as well as previously described proteins of 43,000 Mr, 58,000 (51,000 in our gel system), 270,000, and 37,000 (calelectrin). The 87-kD protein copurified with acetylcholine receptors and with 43- and 51-kD proteins during equilibrium centrifugation on continuous sucrose gradients, whereas a large fraction of the 210/220-kD protein was separated from AChRs. The 87-kD protein remained associated with receptors and 43-kD protein during velocity sedimentation through shallow sucrose gradients, a procedure that separated a significant amount of 51-kD protein from AChRs. The 87- and 270-kD proteins were cleaved by Ca++-activated proteases present in crude preparations and also in highly purified postsynaptic membranes. With the exception of anti-37-kD antibodies, some of the monoclonals raised against Torpedo proteins also recognized determinants in frozen sections of chick and/or rat skeletal muscle fibers and in permeabilized chick myotubes grown in vitro. Anti-87-kD sites were concentrated at chick and rat endplates, but the antibodies also recognized determinants present at lower site density in the extrasynaptic membrane. Anti-210:220-kD labeled chick endplates, but studies of neuron-myotube cocultures showed that this antigen was located on neurites rather than the postsynaptic membrane. As reported in other species, 43-kD determinants were restricted to chick endplates and anti-51-kD and anti-270-kD labeled extrasynaptic as well as synaptic membranes. None of the cross reacting antibodies recognized determinants on intact (unpermeabilized) myotubes, so the antigens must be located on the cytoplasmic aspect of the surface membrane. The role that each intracellular determinant plays in AChR immobilization at developing and mature endplates remains to be investigated.

Three-dimensional structure of the nicotinic acetylcholine receptor and location of the major associated 43-kD cytoskeletal protein, determined at 22 A by low dose electron microscopy and x-ray diffraction to 12.5 A

The three-dimensional structure of the nicotinic acetylcholine receptor (AChR) from Torpedo californica, crystallized both before and after removal of associated proteins, most notably the main 43-kD cytoskeletal protein that interacts both with AChR and actin, is determined to a resolution of 22 A. This is the first structural analysis where the 43-kD protein has been removed from the sample before crystallization. Thus, it provides the most reliable assessment of what constitutes the structure of the minimal five subunit AChR complex, and, by comparison with the native membrane, of the location of the 43-kD cytoskeletal protein. Image reconstruction of two-dimensional crystals includes information from electron images of up to +/- 52 degrees tilted specimens of latticed AChR. Hybrid density maps that include x-ray diffraction perpendicular to the membrane to 12.5 A resolution were used and eliminate some of the distortions introduced in maps based only on electron microscopic analyses. Comparison of the difference Fourier density maps between AChR with its normal complement of associated proteins, and without them shows that the main density, assigned to the actin-binding 43-kD component is closely associated with the lipid bilayer as well as with the cytoplasmic domain of the AChR. It binds beside the AChR, not beneath it as suggested by others (C. Toyoshima and N. Unwin 1988. Nature [Lond.]. 336:237-240). There is good agreement between the volumes of density for structural components and expected volumes based on their molecular weight. Acetylcholine receptors aggregate in the absence of any cytoskeletal proteins, suggesting that the AChR alone is sufficient to encode and stabilize clustering, and perhaps to do so during synaptogenesis. The main 43-kD component may play a role in location and rate of association of AChR. We show that the disulfide bond that cross-links delta-delta chains of adjacent pentamers in about 80% of AChR, is not required to stabilize the lattice of AChR. Latticed tube structures are stable indefinitely. The lattices described here have 20% less volume of lipid than those originally obtained and characterized by J. Kistler and R. M. Stroud (1981. Proc. Natl. Acad. Sci. USA. 78:3678-3682), or those subsequently characterized by A. Brisson and P. N. T. Unwin (1984. J. Cell Biol. 99:1202-1211) and A. Brisson and P. N. T. Unwin (1985. Nature (Lond.). 315:474-477).

Primary structure of a novel 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS)-binding membrane protein highly expressed in Torpedo californica electroplax

Polyclonal rabbit antibodies were raised against 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS), an inhibitor of a variety of anion transport proteins. These antibodies specifically recognize SITS-reacted erythrocyte band 3 in immunoprecipitations and Western blots. In Western blots of SITS-reacted membrane proteins derived from vesicles of the electric organ of Torpedo californica (known to express a SITS-sensitive Cl- channel) the antibodies recognized two major species of approximately 93 kDa and approximately 105 kDa. The approximately 93 kDa protein was identified as the alpha-subunit of the Na,K-ATPase. The approximately 105 kDa protein (designated sp105) is a glycoprotein which binds to wheat-germ agglutinin and concanavalin A and is present as a disulphide-linked homodimer under non-reducing conditions. A partial amino acid sequence and a polyclonal antibody were used to clone the corresponding cDNA. sp105 is encoded in electroplax by two abundant mRNAs of approximately 6 and approximately 6.8 kb. A hybridizing mRNA of approximately 5 kb was over 200-fold and over 500-fold less abundant in brain and heart respectively. Sequence analysis of the cDNA predicted a novel protein of 697 amino acids containing eight potential N-linked glycosylation sites. Analysis of hydrophobicity indicated the presence of at least one, and possibly three, putative membrane-spanning domains. When expressed from the Sp6 message in Xenopus laevis oocytes, the protein was inserted into membranes, glycosylated and processed to form a dimer. However, no increase in 36Cl uptake or in membrane conductance could be detected. We found no effect of hybrid depleting the specific message on expression of the Torpedo electroplax Cl- channel in oocytes. Thus we conclude that this novel electroplax membrane protein is probably not a functional part of the chloride channel.

Expression of RNA transcripts for the postsynaptic 43 kDa protein in innervated and denervated rat skeletal muscle

A cDNA clone encoding the mouse muscle postsynaptic 43 kDa protein was isolated and sequenced. The amino acid sequence of this protein, which is closely associated with nicotinic acetylcholine receptors at Torpedo electrocyte and vertebrate skeletal muscle synapses, is very similar in different species. A cysteine-rich region homologous to part of the regulatory domain of protein kinase C may be important in interactions of this protein with the lipid bilayer. RNA transcripts for the 43 kDa protein increase only 2-3 fold after denervation of rat skeletal muscle, in sharp contrast to the alpha-subunit of the muscle nicotinic receptor which increases more than 30-fold. Thus, the expression of these two proteins is regulated by different mechanisms.

Activity-dependent regulation of gene expression in muscle and neuronal cells

In both the central and the peripheral nervous systems, impulse activity regulates the expression of a vast number of genes that code for synaptic proteins, including neuropeptides, enzymes involved in neurotransmitter biosynthesis and degradation, and membrane receptors. In recent years, the mechanisms involved in these regulations became amenable to investigation by the methods of recombinant DNA technology. The first part of this review focuses on the activity-dependent control of nicotinic acetylcholine receptor biosynthesis in vertebrate muscle, a model case for the regulation of synaptic protein biosynthesis at the postsynaptic level. The second part summarizes some examples of neuronal proteins whose biosynthesis is under the control of transsynaptic impulse activity. The first, second, and third intracellular messengers involved in membrane-to-gene signaling are discussed, as are possible posttranscriptional control mechanisms. Finally, models are proposed for a role of neuronal activity in the genesis and stabilization of the synapse.

Myristic acid is the NH2-terminal blocking group of the 43-kDa protein of Torpedo nicotinic post-synaptic membranes

The NH2-terminal blocking group of the 43-kDa peripheral membrane protein (43-kDa protein) of Torpedo post-synaptic membranes has been identified as myristic acid. To identify that blocking group pure 43-kDa protein was digested with trypsin and the blocked tryptic peptide was isolated by reverse phase HPLC. That peptide coeluted with and had the same amino acid composition as a synthetic peptide, myristoyl-Gly-Gln-Asp-Gln-Thr-Lys, the structure of the amino terminus predicted from the protein sequence deduced from a cDNA clone. The presence of myristate was confirmed by the precise molecular mass of the peptide, 886.5266, determined by fast atom bombardment mass spectroscopy.

Protein phosphorylation of nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (nAcChR) is a ligand-gated ion channel found in the postsynaptic membranes of electric organs, at the neuromuscular junction, and at nicotinic cholinergic synapses of the mammalian central and peripheral nervous system. The nAcChR from Torpedo electric organ and mammalian muscle is the most well-characterized neurotransmitter receptor in biology. It has been shown to be comprised of five homologous (two identicle) protein subunits (alpha 2 beta gamma delta) that form both the ion channel and the neurotransmitter receptor. The nAcChR has been purified and reconstituted into lipid vesicles with retention of ion channel function and the primary structure of all four protein subunits has been determined. Protein phosphorylation is a major posttranslational modification known to regulate protein function. The Torpedo nAcChR was first shown to be regulated by phosphorylation by the discovery that postsynaptic membranes contain protein kinases that phosphorylate the nAcChR. Phosphorylation of the nAcChR has since been shown to be regulated by the cAMP-dependent protein kinase, protein kinase C, and a tyrosine-specific protein kinase. Phosphorylation of the nAcChR by cAMP-dependent protein kinase has been shown to increase the rate of nAcChR desensitization, the process by which the nAcChR becomes inactivated in the continued presence of agonist. In cultured muscle cells, phosphorylation of the nAcChR has been shown to be regulated by cAMP-dependent protein kinase, a Ca2+-sensitive protein kinase, and a tyrosine-specific protein kinase. Stimulation of the cAMP-dependent protein kinase in muscle also increases the rate of nAcChR desensitization and correlates well with the increase in nAcChR phosphorylation. The AcChR represents a model system for how receptors and ion channels are regulated by second messengers and protein phosphorylation.

The lipid environment of the nicotinic acetylcholine receptor in native and reconstituted membranes

Detailed knowledge of the membrane framework surrounding the nicotinic acetylcholine receptor (AChR) is key to an understanding of its structure, dynamics, and function. Recent theoretical models discuss the structural relationship between the AChR and the lipid bilayer. Independent experimental data on the composition, metabolism, and dynamics of the AChR lipid environment are analyzed in the first part of the review. The composition of the lipids in which the transmembrane AChR chains are inserted bears considerable resemblance among species, perhaps providing this evolutionarily conserved protein with an adequate milieu for its optimal functioning. The effects of lipids on the latter are discussed in the second part of the review. The third part focuses on the information gained on the dynamics of AChR and lipids in the membrane, a section that also covers the physical properties and interactions between the protein, its immediate annulus, and the bulk lipid bilayer.

Asynchronous assembly of the acetylcholine receptor and of the 43-kD nu1 protein in the postsynaptic membrane of developing Torpedo marmorata electrocyte

The assembly of the nicotinic acetylcholine receptor (AchR) and the 43-kD protein (v1), the two major components of the post synaptic membrane of the electromotor synapse, was followed in Torpedo marmorata electrocyte during embryonic development by immunocytochemical methods. At the first developmental stage investigated (45-mm embryos), accumulation of AchR at the ventral pole of the newly formed electrocyte was observed within columns before innervation could be detected. No concomitant accumulation of 43-kD immunoreactivity in AchR-rich membrane domains was observed at this stage, but a transient asymmetric distribution of the extracellular protein, laminin, which paralleled that of the AchR, was noticed. At the subsequent stage studied (80-mm embryos), codistribution of the two proteins was noticed on the ventral face of the cell. Intracellular pools of AchR and 43-kD protein were followed at the EM level in 80-mm electrocytes. AchR immunoreactivity was detected within membrane compartments, which include the perinuclear cisternae of the endoplasmic reticulum and the plasma membrane. On the other hand, 43-kD immunoreactivity was not found associated with the AchR in the intracellular compartments of the cell, but codistributed with the AchR at the level of the plasma membrane. The data reported in this study suggest that AchR clustering in vivo is not initially determined by the association of the AchR with the 43-kD protein, but rather relies on AchR interaction with extracellular components, for instance from the basement membrane, laid down in the tissue before the entry of the electromotor nerve endings.

Acetylcholine receptor-associated 43K protein contains covalently bound myristate

Torpedo electroplaque and vertebrate neuromuscular junctions contain high levels of a nonactin, 43,000-Mr peripheral membrane protein referred to as the 43K protein. 43K protein is associated with the cytoplasmic face of postsynaptic membranes at areas of high acetylcholine receptor density and has been implicated in the establishment and/or maintenance of these receptor clusters. Cloning of cDNAs encoding Torpedo 43K protein revealed that its amino terminus contains a consensus sequence sufficient for the covalent attachment of the rare fatty acid myristate. To examine whether 43K protein is, in fact, myristoylated, mouse muscle BC3H1 cells were metabolically labeled with either [35S]cysteine or [3H]myristate and immunoprecipitated with a monospecific antiserum raised against isolated Torpedo 43K protein. In cells incubated with either precursor, a single labeled species was specifically recovered that comigrated on SDS-PAGE with 43K protein purified from Torpedo electric organ. Approximately 95% of the 3H labeled material released from [3H]myristate-43K protein by acid methanolysis was extractable in organic solvents and eluted from a C18 reverse-phase HPLC column exclusively at the position of the methyl myristate internal standard. Thus, 43K protein contains authentic myristic acid rather than an amino or fatty acid metabolite of [3H]myristate. Myristate appears to be added to 43K protein cotranslationally and cannot be released from it by prolonged incubation in SDS, 2-mercaptoethanol, or hydroxylamine (pH 7.0 or 10.0), characteristics consistent with amino terminal myristoylation. Covalently linked myristate may be responsible for the high affinity of purified 43K protein for lipid bilayers despite the absence of a notably hydrophobic amino acid sequence.

Regulation of nicotinic acetylcholine receptors by protein phosphorylation

Neurotransmitter receptors and ion channels play a critical role in the transduction of signals at chemical synapses. The modulation of neurotransmitter receptor and ion channel function by protein phosphorylation is one of the major regulatory mechanisms in the control of synaptic transmission. The nicotinic acetylcholine receptor (nAcChR) has provided an excellent model system in which to study the modulation of neurotransmitter receptors and ion channels by protein phosphorylation since the structure and function of this receptor have been so extensively characterized. In this article, the structure of the nAcChR from the electric organ of electric fish, skeletal muscle, and the central and peripheral nervous system will be briefly reviewed. Emphasis will be placed on the regulation of the phosphorylation of nAcChR by second messengers and by neurotransmitters and hormones. In addition, recent studies on the functional modulation of nicotinic receptors by protein phosphorylation will be reviewed.