Expression of fusion proteins of the nicotinic acetylcholine receptor from mammalian muscle identifies the membrane-spanning regions in the alpha and delta subunits

A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.

Structural basis for lipid modulation of nicotinic acetylcholine receptor function

The nicotinic acetylcholine receptor (AChR) is the archetype molecule in the superfamily of ligand-gated ion channels (LGIC). Members of this superfamily mediate fast intercellular communication in response to endogenous neurotransmitters. This review is focused on the structural and functional crosstalk between the AChR and lipids in the membrane microenvironment, and the modulation exerted by the latter on ligand binding and ion translocation. Experimental approaches using Laurdan extrinsic fluorescence and Förster-type resonance energy transfer (FRET) that led to the characterization of the polarity and molecular dynamics of the liquid-ordered phase AChR-vicinal lipids and the bulk membrane lipids, and the asymmetry of the AChR-rich membrane are reviewed first. The topological relationship between protein and lipid moieties and the changes in physical properties induced by exogenous lipids are discussed next. This background information lays the basis for understanding the occurrence of lipid sites in the AChR transmembrane region, and the selectivity of the protein-lipid interactions. Changes in FRET efficiency induced by fatty acids, phospholipid and cholesterol (Chol), led to the identification of discrete sites for these lipids on the AChR protein, and electron-spin resonance (ESR) spectroscopy has recently facilitated determination of the stoichiometry and selectivity for the AChR of the shell lipid. The influence of lipids on AChR function is discussed next. Combined single-channel and site-directed mutagenesis data fostered the recognition of lipid-sensitive residues in the transmembrane region, dissecting their contribution to ligand binding and channel gating, opening and closing. Experimental evidence supports the notion that the interface between the protein moiety and the adjacent lipid shell is the locus of a variety of pharmacologically relevant processes, including the action of steroids and other lipids.

Cooperativity and flexibility of cystic fibrosis transmembrane conductance regulator transmembrane segments participate in membrane localization of a charged residue

Polytopic protein topology is established in the endoplasmic reticulum (ER) by sequence determinants encoded throughout the nascent polypeptide. Here we characterize 12 topogenic determinants in the cystic fibrosis transmembrane conductance regulator, and identify a novel mechanism by which a charged residue is positioned within the plane of the lipid bilayer. During cystic fibrosis transmembrane conductance regulator biogenesis, topology of the C-terminal transmembrane domain (TMs 7-12) is directed by alternating signal (TMs 7, 9, and 11) and stop transfer (TMs 8, 10, and 12) sequences. Unlike conventional stop transfer sequences, however, TM8 is unable to independently terminate translocation due to the presence of a single charged residue, Asp(924), within the TM segment. Instead, TM8 stop transfer activity is specifically dependent on TM7, which functions both to initiate translocation and to compensate for the charged residue within TM8. Moreover, even in the presence of TM7, the N terminus of TM8 extends significantly into the ER lumen, suggesting a high degree of flexibility in establishing TM8 transmembrane boundaries. These studies demonstrate that signal sequences can markedly influence stop transfer behavior and indicate that ER translocation machinery simultaneously integrates information from multiple topogenic determinants as they are presented in rapid succession during polytopic protein biogenesis.

Immunological characterization of 5-HT3 receptor transmembrane topology

The 5-hydroxytryptamine3 (5-HT3) receptor is a member of the Cys-loop family of ligand-gated ion channels. These receptors are pentamers with the greatest homology to nicotinic acetylcholine (nACh) receptors. The proposed topological organization of a 5-HT3 receptor subunit is based largely on hydropathy profiles and by homology to nACh receptors, and indicates a large N-terminal extracellular domain and four transmembrane regions. There is, however, little direct evidence for this model. We therefore investigated the topology of the 5-HT3A receptor subunit using a panel of 5-HT3 receptor-specific antisera that interact with defined regions of the receptor. An antiserum generated against a short peptide from the N-terminal domain of the 5-HT3A receptor subunit, pAb120, was shown to bind to 5-HT3 receptor-expressing cells with intact cell membranes, indicating that the N-terminal end of the subunit is extracellular. Two antisera generated against regions of the loop between predicted transmembrane regions three and four did not bind to cells with intact membranes. However on membrane permeabilization these antibodies both bound to the receptor in intracellular areas, thus indicating that the loop between transmembrane domains three and four is intracellular. These data therefore provide direct evidence for an extracellular N-terminal domain and an intracellular loop between the third and fourth transmembrane domains, thus supporting the conventional ligand-gated ion channel subunit topological model.

Connexins/connexons. Cell-free expression

With a few exceptions, all secretory and plasma membrane proteins studied to date are synthesized in the endoplasmic reticulum (ER) membrane. Then, they are transported by successive vesicle budding and fusion from the ER through the Golgi stacks to the plasma membrane following the general intracellular transport route referred to as secretory pathway (originally reviewed in 1). Gap junction connexins have been shown to follow this pathway.

Topography of nicotinic acetylcholine receptor membrane-embedded domains

The topography of nicotinic acetylcholine receptor (AChR) membrane-embedded domains and the relative affinity of lipids for these protein regions were studied using fluorescence methods. Intact Torpedo californica AChR protein and transmembrane peptides were derivatized with N-(1-pyrenyl)maleimide (PM), purified, and reconstituted into asolectin liposomes. Fluorescence mapped to proteolytic fragments consistent with PM labeling of cysteine residues in alphaM1, alphaM4, gammaM1, and gammaM4. The topography of the pyrene-labeled Cys residues with respect to the membrane and the apparent affinity for representative lipids were determined by differential fluorescence quenching with spin-labeled derivatives of fatty acids, phosphatidylcholine, and the steroids cholestane and androstane. Different spin label lipid analogs exhibit different selectivity for the whole AChR protein and its transmembrane domains. In all cases labeled residues were found to lie in a shallow position. For M4 segments, this is compatible with a linear alpha-helical structure, but not so for M1, for which "classical" models locate Cys residues at the center of the hydrophobic stretch. The transmembrane topography of M1 can be rationalized on the basis of the presence of a substantial amount of non-helical structure, and/or of kinks attributable to the occurrence of the evolutionarily conserved proline residues. The latter is a striking feature of M1 in the AChR and all members of the rapid ligand-gated ion channel superfamily.

Reorientation of aquaporin-1 topology during maturation in the endoplasmic reticulum

The topology of most eukaryotic polytopic membrane proteins is established cotranslationally in the endoplasmic reticulum (ER) through a series of coordinated translocation and membrane integration events. For the human aquaporin water channel AQP1, however, the initial four-segment-spanning topology at the ER membrane differs from the mature six-segment-spanning topology at the plasma membrane. Here we use epitope-tagged AQP1 constructs to follow the transmembrane (TM) orientation of key internal peptide loops in Xenopus oocyte and cell-free systems. This analysis revealed that AQP1 maturation in the ER involves a novel topological reorientation of three internal TM segments and two peptide loops. After the synthesis of TMs 4-6, TM3 underwent a 180-degree rotation in which TM3 C-terminal flanking residues were translocated from their initial cytosolic location into the ER lumen and N-terminal flanking residues underwent retrograde translocation from the ER lumen to the cytosol. These events convert TM3 from a type I to a type II topology and reposition TM2 and TM4 into transmembrane conformations consistent with the predicted six-segment-spanning AQP1 topology. AQP1 topological reorientation was also associated with maturation from a protease-sensitive conformation to a protease-resistant structure with water channel function. These studies demonstrate that initial protein topology established via cotranslational translocation events in the ER is dynamic and may be modified by subsequent steps of folding and/or maturation.

A topological study of the human γ-glutamyl carboxylase

γ-Glutamyl carboxylase (GC), a polytopic membrane protein found in the endoplasmic reticulum (ER), catalyzes vitamin K–dependent posttranslational modification of glutamate to γ-carboxyl glutamate. In an attempt to delineate the structure of this important enzyme, in vitro translation and in vivo mapping were used to study its membrane topology. Using terminus-tagged full-length carboxylase, expressed in 293 cells, it was demonstrated that the amino-terminus of the GC is on the cytoplasmic side of the ER, while the carboxyl-terminus is on the lumenal side. In addition, a series of fusions were made to encode each predicted transmembrane domain (TMD) followed by a leader peptidase (Lep) reporter tag, as analyzed by the computer algorithm TOPPRED II. Following in vitro translation of each fusion in the presence of canine microsomes, the topological orientation of the Lep tag was determined by proteinase K digestion and endoglycosidase H (Endo H) cleavage. From the topological orientation of the Lep tag in each fusion, the GC spans the ER membrane at least 5 times, with its N-terminus in the cytoplasm and its C-terminus in the lumen.

A topological study of the human γ-glutamyl carboxylase

γ-Glutamyl carboxylase (GC), a polytopic membrane protein found in the endoplasmic reticulum (ER), catalyzes vitamin K–dependent posttranslational modification of glutamate to γ-carboxyl glutamate. In an attempt to delineate the structure of this important enzyme, in vitro translation and in vivo mapping were used to study its membrane topology. Using terminus-tagged full-length carboxylase, expressed in 293 cells, it was demonstrated that the amino-terminus of the GC is on the cytoplasmic side of the ER, while the carboxyl-terminus is on the lumenal side. In addition, a series of fusions were made to encode each predicted transmembrane domain (TMD) followed by a leader peptidase (Lep) reporter tag, as analyzed by the computer algorithm TOPPRED II. Following in vitro translation of each fusion in the presence of canine microsomes, the topological orientation of the Lep tag was determined by proteinase K digestion and endoglycosidase H (Endo H) cleavage. From the topological orientation of the Lep tag in each fusion, the GC spans the ER membrane at least 5 times, with its N-terminus in the cytoplasm and its C-terminus in the lumen.

Biotinylation of substituted cysteines in the nicotinic acetylcholine receptor reveals distinct binding modes for alpha-bungarotoxin and erabutoxin a

Although previous results indicate that alpha-subunit residues Trp(187), Val(188), Phe(189), Tyr(190), and Pro(194) of the mouse nicotinic acetylcholine receptor are solvent-accessible and are in a position to contribute to the alpha-bungarotoxin (alpha-Bgtx) binding site (Spura, A., Russin, T. S., Freedman, N. D., Grant, M., McLaughlin, J. T., and Hawrot, E. (1999) Biochemistry 38, 4912-4921), little is known about the accessibility of other residues within this region. By determining second-order rate constants for the reaction of cysteine mutants at alpha184-alpha197 with the thiol-specific biotin derivative (+)-biotinyl-3-maleimidopropionamidyl-3,6-dioxaoctanediamine , we now show that only very subtle differences in reactivity (approximately 10-fold) are detectable, arguing that the entire region is solvent-exposed. Importantly, biotinylation in the presence of saturating concentrations of the long neurotoxin alpha-Bgtx is significantly retarded for positions alphaW187C, alphaF189C, and reduced wild-type receptors (alphaCys(192) and alphaCys(193)), further emphasizing their major contribution to the alpha-Bgtx binding site. Interestingly, although biotinylation of position alphaV188C is not affected by the presence of alpha-Bgtx, erabutoxin a, which is a member of the short neurotoxin family, inhibits biotinylation at position alphaV188C, but not at alphaW187C or alphaF189C. Taken together, these results indicate that short and long neurotoxins establish interactions with distinct amino acids on the nicotinic acetylcholine receptor.

Altered glycosylation sites of the delta subunit of the acetylcholine receptor (AChR) reduce alpha delta association and receptor assembly

We have used mutagenesis to investigate the potential N-glycosylation sites in the delta subunit of the mouse muscle acetylcholine receptor (AChR). Of the three sites, Asn76, Asn143, and Asn169, only the first two were glycosylated when the delta subunit was expressed in COS cells. Because the heterologously expressed delta subunit was similar in its properties to that expressed in C2 muscle cells, the sites of glycosylation are likely to be the same in both cases. In COS cells, mutations of the delta subunit that prevented glycosylation at either of the sites did not change its metabolic stability nor its steady-state level. These results are in contrast to those found previously for the alpha subunit, in which glycosylation at a single site metabolically stabilized the polypeptide (Blount, P., and Merlie, J. P. (1990) J. Cell Biol. 111, 2613-2622). Mutations of the delta subunit that prevented glycosylation, however, decreased its ability to form an alpha delta heterodimer when the alpha and delta subunit were expressed together. When all four subunits of the AChR (alpha, beta, delta, and epsilon) were coexpressed, mutation of the delta subunit to prevent glycosylation resulted in a reduced amount of fully assembled AChR and reduced surface AChR levels, consistent with the role of the heterodimer in the assembly reaction. These results suggest that glycosylation of the delta subunit at both Asn76 and Asn143 is needed for its efficient folding and/or its subsequent interaction with the alpha subunit.

Disorders of neuromuscular junction ion channels

Ion channel defects produce a clinically diverse set of disorders that range from cystic fibrosis and some forms of migraine to renal tubular defects and episodic ataxias. This review discusses diseases related to impaired function of the skeletal muscle acetylcholine receptor and calcium channels of the motor nerve terminal. Myasthenia gravis is an autoimmune disease caused by antibodies directed toward the skeletal muscle acetylcholine receptor that compromise neuromuscular transmission. Congenital myasthenias are genetic disorders, a subset of which are caused by mutations of the acetylcholine receptor. Lambert-Eaton myasthenic syndrome is an immune disorder characterized by impaired synaptic vesicle release likely related to a defect of calcium influx. The disorders will illustrate new insights into synaptic transmission and ion channel structure that are relevant for all ion channel disorders.

Anatomy of the antigenic structure of a large membrane autoantigen, the muscle-type nicotinic acetylcholine receptor

The neuromuscular junction nicotinic acetylcholine receptor (AChR), a pentameric membrane glycoprotein, is the autoantigen involved in the autoimmune disease myasthenia gravis (MG). In animals immunized with intact AChR and in human MG, the anti-AChR antibody response is polyclonal. However, a small extracellular region of the AChR alpha-subunit, the main immunogenic region (MIR), seems to be a major target for anti-AChR antibodies. A major loop containing overlapping epitopes for several anti-MIR monoclonal antibodies (mAbs) lies within residues alpha 67-76 at the extreme synaptic end of each alpha-subunit: however, anti-MIR mAbs are functionally and structurally quite heterogeneous. Anti-MIR mAbs do not affect channel gating, but are very effective in the passive transfer of MG to animals; in contrast, their Fab or Fv fragments protect the AChR from the pathogenic effects of the intact antibodies. Antibodies against the cytoplasmic region of the AChR can be elicited by immunization with denatured AChR and the precise epitopes of many such mAbs have been identified; however, it is unlikely that such antibodies are present in significant amounts in human MG. Antibodies to other extracellular epitopes on all AChR subunits are present in both experimental and human MG; these include antibodies to the acetylcholine-binding site which affect AChR function in various ways and also induce acute experimental MG. Finally, anti-AChR antibodies cross-reactive with non-AChR antigens exist, suggesting that MG may result from molecular mimicry. Despite extensive studies, many gaps remain in our understanding of the antigenic structure of the AChR; especially in relation to human MG. A thorough understanding of the antigenic structure of the AChR is required for an in-depth understanding, and for possible specific immunotherapy, of MG.

Connexin membrane protein biosynthesis is influenced by polypeptide positioning within the translocon and signal peptidase access

We reported previously (Falk, M. M., Kumar, N. M., and Gilula, N. B. (1994) J. Cell Biol. 127, 343-355) that the membrane integration of polytopic connexin polypeptides can be accompanied by an inappropriate cleavage that generates amino-terminal truncated connexins. While this cleavage was not detected in vivo, translation in standard cell-free translation/translocation systems resulted in the complete cleavage of all newly integrated connexins. Partial cleavage occurred in heterologous expression systems that correlated with the expression level. Here we report that the transmembrane topology of connexins generated in microsomal membranes was identical to the topology of functional connexins in plasma membranes. Characterization of the cleavage site and reaction showed that the connexins were processed by signal peptidase immediately downstream of their first transmembrane domain in a reaction similar to the removal of signal peptides from pre-proteins. Increasing the length and hydrophobic character of the signal anchor sequence of connexins completely prevented the aberrant cleavage. This result indicates that their signal anchor sequence was falsely recognized and positioned as a cleavable signal peptide within the endoplasmic reticulum translocon, and that this mispositioning enabled signal peptidase to access the cleavage sites. The results provide direct evidence for the involvement of unknown cellular factors in the membrane integration process of connexins.

Water-soluble nicotinic acetylcholine receptor formed by alpha7 subunit extracellular domains

Water-soluble models of ligand-gated ion channels would be advantageous for structural studies. We investigated the suitability of three versions of the N-terminal extracellular domain (ECD) of the alpha7 subunit of the nicotinic acetylcholine receptor (AChR) family for this purpose by examining their ligand-binding and assembly properties. Two versions included the first transmembrane domain and were solubilized with detergent after expression in Xenopus oocytes. The third was truncated before the first transmembrane domain and was soluble without detergent. For all three, their equilibrium binding affinities for alpha-bungarotoxin, nicotine, and acetylcholine, combined with their velocity sedimentation profiles, were consistent with the formation of native-like AChRs. These characteristics imply that the alpha7 ECD can form a water-soluble AChR that is a model of the ECD of the full-length alpha7 AChR.

Co- and posttranslational translocation mechanisms direct cystic fibrosis transmembrane conductance regulator N terminus transmembrane assembly

Transmembrane topology of most eukaryotic polytopic proteins is established cotranslationally at the endoplasmic reticulum membrane through the action of alternating signal and stop transfer sequences. Here we demonstrate that the cystic fibrosis transmembrane conductance regulator (CFTR) achieves its N terminus topology through a variation of this mechanism that involves both co- and posttranslational translocation events. Using a series of defined chimeric and truncated proteins expressed in a reticulocyte lysate system, we have identified two topogenic determinants encoded within the first (TM1) and second (TM2) membrane-spanning segments of CFTR. Each sequence independently (i) directed endoplasmic reticulum targeting, (ii) translocated appropriate flanking residues, and (iii) achieved its proper membrane-spanning orientation. Signal sequence activity of TM1, however, was inefficient due to the presence of two charged residues, Glu92 and Lys95, located within its hydrophobic core. As a result, TM1 was able to direct correct topology for less than half of nascent CFTR chains. In contrast to TM1, TM2 signal sequence activity was both efficient and specific. Even in the absence of a functional TM1 signal sequence, TM2 was able to direct CFTR N terminus topology through a ribosome-dependent posttranslational mechanism. Mutating charged residues Glu92 and Lys95 to alanine improved TM1 signal sequence activity as well as the ability of TM1 to independently direct CFTR N terminus topology. Thus, a single functional signal sequence in either the first or second TM segment was sufficient for directing proper CFTR topology. These results identify two distinct and redundant translocation pathways for CFTR N terminus transmembrane assembly and support a model in which TM2 functions to ensure correct topology of CFTR chains that fail to translocate via TM1. This novel arrangement of topogenic information provides an alternative to conventional cotranslational pathways of polytopic protein biogenesis.

Site-specific incorporation of biotinylated amino acids to identify surface-exposed residues in integral membrane proteins

BACKGROUND

A key structural issue for all integral membrane proteins is the exposure of individual residues to the intracellular or extracellular media. This issue involves the basic transmembrane topology as well as more subtle variations in surface accessibility. Direct methods to evaluate the degree of exposure for residues in functional proteins expressed in living cells would be highly valuable. We sought to develop a new experimental method to determine highly surface-exposed residues, and thus transmembrane topology of membrane proteins expressed in Xenopus oocytes.

RESULTS

We have used the in vivo nonsense suppression technique to incorporate biotinylated unnatural amino acids into functional ion channels expressed in Xenopus oocytes. Binding of 125I-streptavidin to biotinylated receptors was used to determine the surface exposure of individual amino acids. In particular, we studied the main immunogenic region of the nicotinic acetylcholine receptor. The biotin-containing amino acid biocytin was efficiently incorporated into five sites in the main immunogenic region and extracellular streptavidin bound to one residue in particular, alpha 70. The position of alpha 70 as highly exposed on the receptor surface was thus established.

CONCLUSIONS

The in vivo nonsense suppression technique has been extended to provide the first in a potential series of methods to identify exposed residues and to assess their relative exposure in functional proteins expressed in Xenopus oocytes.

Tyrosine phosphorylation of nicotinic acetylcholine receptor mediates Grb2 binding

Tyrosine phosphorylation of the nicotinic acetylcholine receptor (AChR) is associated with an altered rate of receptor desensitization and also may play a role in agrin-induced receptor clustering. We have demonstrated a previously unsuspected interaction between Torpedo AChR and the adaptor protein Grb2. The binding is mediated by the Src homology 2 (SH2) domain of Grb2 and the tyrosine-phosphorylated delta subunit of the AChR. Dephosphorylation of the delta subunit abolishes Grb2 binding. A cytoplasmic domain of the delta subunit contains a binding motif (pYXNX) for the SH2 domain of Grb2. Indeed, a phosphopeptide corresponding to this region of the delta subunit binds to Grb2 SH2 fusion proteins with relatively high affinity, whereas a peptide lacking phosphorylation on tyrosine exhibits no binding. Grb2 is colocalized with the AChR on the innervated face of Torpedo electrocytes. Furthermore, Grb2 specifically copurifies with AChR solubilized from postsynaptic membranes. These data suggest a novel role for tyrosine phosphorylation of the AChR in the initiation of a Grb2-mediated signaling cascade at the postsynaptic membrane.

Transmembrane topology of a CLC chloride channel

CLC chloride channels form a large and conserved gene family unrelated to other channel proteins. Knowledge of the transmembrane topology of these channels is important for understanding the effects of mutations found in human myotonia and inherited hypercalciuric kidney stone diseases and for the interpretation of structure-function studies. We now systematically study the topology of human ClC-1, a prototype CLC channel that is defective in human myotonia. Using a combination of in vitro glycosylation scanning and protease protection assays, we show that both N and C termini face the cytoplasm and demonstrate the presence of 10 (or less likely 12) transmembrane spans. Difficult regions were additionally tested by inserting cysteines and probing the effect of cysteine-modifying reagents on ClC-1 currents. The results show that D3 crosses the membrane and D4 does not, and that L549 between D11 and D12 is accessible from the outside. Further, since the modification of cysteines introduced between D11 and D12 and at the extracellular end of D3 strongly affect ClC-1 currents, these regions are suggested to be important for ion permeation.

Tyrosine phosphorylation of nicotinic acetylcholine receptor mediates Grb2 binding

Tyrosine phosphorylation of the nicotinic acetylcholine receptor (AChR) is associated with an altered rate of receptor desensitization and also may play a role in agrin-induced receptor clustering. We have demonstrated a previously unsuspected interaction between Torpedo AChR and the adaptor protein Grb2. The binding is mediated by the Src homology 2 (SH2) domain of Grb2 and the tyrosine-phosphorylated delta subunit of the AChR. Dephosphorylation of the delta subunit abolishes Grb2 binding. A cytoplasmic domain of the delta subunit contains a binding motif (pYXNX) for the SH2 domain of Grb2. Indeed, a phosphopeptide corresponding to this region of the delta subunit binds to Grb2 SH2 fusion proteins with relatively high affinity, whereas a peptide lacking phosphorylation on tyrosine exhibits no binding. Grb2 is colocalized with the AChR on the innervated face of Torpedo electrocytes. Furthermore, Grb2 specifically copurifies with AChR solubilized from postsynaptic membranes. These data suggest a novel role for tyrosine phosphorylation of the AChR in the initiation of a Grb2-mediated signaling cascade at the postsynaptic membrane.

Analysis of the transmembrane topology and membrane assembly of the GAT-1 gamma-aminobutyric acid transporter

The transmembrane topology of the Na+- and Cl--dependent gamma-aminobutyric acid transporter GAT-1 has been studied using protein chimeras in Xenopus oocytes. A series of COOH-terminal truncations was generated to which a prolactin epitope was fused. Following expression of transporter-prolactin chimeras in Xenopus oocytes, the transmembrane orientation of each chimera was determined by testing for protease sensitivity in an oocyte membrane preparation. Data from protease protection assays with GAT-1-prolactin chimeras has shown that residues in the loops connecting hydrophobic domain (HD)3 and HD4 and HD7 and HD8 are accessible to protease in the cytoplasm and suggest the presence of pore loop structures which extend into the membrane from the extracellular face. Such pore loop structures may be involved in the formation of the substrate-binding pocket. Studies presented herein confirm that the NH2 and COOH termini are cytosolic and hydrophobic domains span the membrane in a manner consistent with the predicted hydropathy model for Na+- and Cl--dependent transporters. These data also provide insight into GAT-1 transmembrane assembly and suggest that a complex series of topogenic sequences directs this process. A potential pause-transfer sequence has been identified and may be responsible for the translocational pausing observed in this study.

Evidence for a six-transmembrane domain structure of presenilin 1

Mutations in genes encoding presenilin 1 and presenilin 2 account for the majority of cases of early-onset familial Alzheimer's disease. The presenilins have been localized to the endoplasmic reticulum and Golgi, but which of the multiple hydrophobic segments of the polypeptide chain span the lipid bilayer is unclear. To address this question, we have constructed a series of chimeric molecules in which a topologically neutral reporter protein (a C-terminal fragment of prolactin) containing three artificial glycosylation sites is fused to presenilin 1 following each of the 10 potential transmembrane domains identified in hydrophobicity plots. We have expressed these chimeras by translation in reticulocyte lysate containing canine pancreatic microsomes and by synthesis in transfected COS cells. Based on utilization of the glycosylation sites and sensitivity of the reporter to protease digestion, we provide evidence that presenilin 1 has six transmembrane segments with the N and C termini in the cytoplasm. This model provides important clues to the potential functions of different parts of the presenilin molecule and how these might relate to the pathogenesis of Alzheimer's disease.

Analysis of the transmembrane topology of the glycine transporter GLYT1

A theoretical 12-transmembrane segment model based on the hydrophobic moment has been proposed for the transmembrane topology of the glycine transporter GLYT1 and all other members of the sodium- and chloride-dependent transporter family. We tested this model by introducing N-glycosylation sites along the GLYT1 sequence as reporter for an extracellular localization and by an in vitro transcription/translation assay that allows the analysis of the topogenic properties of different segments of the protein. The data reported herein are compatible with the existence of 12 transmembrane segments, but support a rearrangement of the first third of the protein. Contrary to prediction, hydrophobic domain 1 seems not to span the membrane, and the loop connecting hydrophobic domains 2 and 3, formerly believed to be intracellular, appears to be extracellularly located. In agreement with the theoretical model, we provide evidence for the extracellular localization of loops between hydrophobic segments 5 and 6, 7 and 8, 9 and 10, and 11 and 12.

Functional interaction of Src family kinases with the acetylcholine receptor in C2 myotubes

Tyrosine phosphorylation of the beta subunit of the acetylcholine receptor (AChR) has been postulated to play a role in AChR clustering during development of the neuromuscular junction. We have investigated the mechanism of this phosphorylation in mammalian C2 myotubes and report that the tyrosine kinase Src binds and phosphorylates glutathione S-transferase fusion proteins containing the N-terminal half of the cytoplasmic loop of the beta subunit. No binding occurs to the related kinases Fyn or Yes or to the corresponding regions from the gamma and delta subunits. Furthermore, AChRs affinity-isolated from C2 myotubes using alpha-bungarotoxin-Sepharose were specifically associated with Src and Fyn and had tyrosine-phosphorylated beta subunits. We suggest that AChRs are initially phosphorylated by Src and subsequently bind Fyn in a phosphotyrosine-dependent manner. These interactions are likely to play an important role in construction of the specialized postsynaptic membrane during synaptogenesis.

The emerging three-dimensional structure of a receptor. The nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is the neurotransmitter receptor with the most-characterized protein structure. The amino acid sequences of its five subunits have been elucidated by cDNA cloning and sequencing. Its shape and dimensions (approximately 12.5 nm x 8 nm) were deduced from electron-microscopy studies. Its subunits are arranged around a five-fold axis of pseudosymmetry in the order (clockwise) alpha H gamma alpha L delta beta. Its two agonist/competitive-antagonist-binding sites have been localized by photolabelling studies to a deep gorge between the subunits near the membrane surface. Its ion channel is formed by five membrane-spanning (M2) helices that are contributed by the five subunits. This finding has been generalized as the Helix M2 model for the superfamily of ligand-gated ion channels. The binding site for regulatory non-competitive antagonists has been localized by photolabelling and site-directed-mutagenesis studies within this ion channel. Therefore a three-dimensional image of the nicotinic acetylcholine receptor is emerging, the most prominent feature of which is an active site that combines the agonist/ competitive-antagonist-binding sites, the regulatory site and the ion channel within a relatively narrow space close to and within the bilayer membrane.

Structure and function of glutamate and nicotinic acetylcholine receptors

The past year has seen remarkable progress in defining the structure of various ligand-gated ion channels. Images of opened and closed nicotinic acetylcholine receptors at 9 A resolution have now made it easier to identify the conformational changes underlying gating. In addition, recent studies on glutamate receptors have led to a radical revision of their postulated transmembrane topology: models for agonist-binding and allosteric domains now use sites previously thought to lie in cytoplasmic loops. Other areas that are being actively pursued include identification of the amino acids lining the ion channels, accurate measurements of Ca2+ fluxes, and tests of transmembrane topology in kainate receptor subunits.

Structural conservation of ion conduction pathways in K channels and glutamate receptors

Single channel recordings demonstrate that ion channels switch stochastically between an open and a closed pore conformation. In search of a structural explanation for this universal open/close behavior, we have uncovered a striking degree of amino acid homology across the pore-forming regions of voltage-gated K channels and glutamate receptors. This suggested that the pores of these otherwise unrelated classes of channels could be structurally conserved. Strong experimental evidence supports a hairpin structure for the pore-forming region of K channels. Consequently, we hypothesized the existence of a similar structure for the pore of glutamate receptors. In ligand-gated channels, the pore is formed by M2, the second of four putative transmembrane segments. A hairpin structure for M2 would affect the subsequent membrane topology, inverting the proposed orientation of the next segments, M3. We have tested this idea for the NR1 subunit of the N-methyl-D-aspartate receptor. Mutations that affected the glycosylation pattern of the NR1 subunit localize both extremes of the M3-M4 linker to the extracellular space. Whole cell currents and apparent agonist affinities were not affected by these mutations. Therefore it can be assumed that they represent the native transmembrane topology. The extracellular assignment of the M3-M4 linker challenged the current topology model by inverting M3. Taken together, the amino acid homology and the new topology suggest that the pore-forming M2 segment of glutamate receptors does not transverse the membrane but, rather, forms a hairpin structure, similar to that found in K channels.

Binding of monoclonal antibodies against the carboxyl terminal segment of the nicotinic receptor delta subunit suggests an unusual transmembrane disposition of this sequence region

Monoclonal antibodies (mAbs) specific for the carboxyl terminal region of the delta subunit of Torpedo nicotinic acetylcholine receptor (AChR), derived from mice immunized with AChR or a synthetic carboxyl terminal sequence of the delta subunit (C delta-mAbs), were used to determine the transmembrane disposition of their epitope(s) by immunoelectron microscopy, using AChR-rich postsynaptic membrane fragments from Torpedo electroplax. Some C delta-mAbs recognized only the cytoplasmic side of the membranes, some both sides to a similar extent, and others bound mostly, but not exclusively, to the cytoplasmic side. Binding of C delta-mAbs to the membranes was specifically blocked by synthetic peptides containing the carboxyl terminal region of the delta subunit. Control anti-AChR mAbs specific for the alpha or the delta subunits, whose epitopes have known transmembrane topology, uniquely recognized the expected side of the postsynaptic membrane. Residues involved in C delta-mAb binding were identified using single residue substituted peptide analogues of the sequence delta 481-501. All C delta-mAbs recognized epitopes within the same sequence segment, delta 485-493, at the carboxyl terminal of the AChR delta subunit. These results suggest that the delta subunit of the AChR might have alternative conformations, leading to exposure of the same sequence region on the extracellular or the cytoplasmic surface. Several Pro residues are present in this region. The alternative cis or trans conformation of one or more of them might result in different folding patterns of the carboxyl terminal sequence of the delta subunit, as described for a viral protein [Liddington, R. C., Yan, Y., Moulai, J., Sahli, R., Benjamin, T. L., & Harrison, S. C. (1991) Nature 354, 278-284.

A topological analysis of goldfish kainate receptors predicts three transmembrane segments

Glutamate receptors are the most abundant excitatory neurotransmitter receptors in vertebrate brain. We have previously cloned cDNAs encoding two homologous kainate receptors (GFKAR alpha, 45 kDa, and GFKAR beta, 41 kDa) from goldfish brain and proposed a topology with three transmembrane domains (Wo, Z. G., and Oswald, R. E. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 7154-7158). These studies have been extended using an in vitro translation/translocation system in conjunction with site-specific antibodies and point and deletion mutations. We report here that the entire region between the previously proposed third and fourth transmembrane segments is translocated and likely to be extracellular in mature receptors. This was based on the following results. 1) The entire segment was protected from Proteinase K and trypsin digestion and could be immunoprecipitated by a site-specific antibody. 2) Functional sites for N-glycosylation are present in the C-terminal half of the segment, and 3) a mutation, constructed with an additional consensus site for N-glycosylation in the N-terminal half of the segment, was found to be glycosylated at that site. Given the fact that the N terminus of the protein is likely to be extracellular, this would place an even number of transmembrane segments between the extracellular N terminus and the glycosylated segment. In addition, results of N-glycosylation and proteolysis protection assays of GFKAR alpha mutations indicated that the previously proposed second transmembrane segment is not a true transmembrane domain. These results provide further evidence in support of a topology with three transmembrane domains that has important implications for the relationship of structure to function in ionotropic glutamate receptors.

Topology profile for a glutamate receptor: three transmembrane domains and a channel-lining reentrant membrane loop

We investigated the transmembrane topology of the GluR3 subunit that was translated in rabbit reticulocytes supplemented with microsomal membranes. A prolactin reporter epitope was fused to GluR3 at six locations, bracketing each of the proposed transmembrane domains. The sidedness of the epitope in the microsomal membrane was then assessed by proteinase K sensitivity. The N terminus and the entire region between M3 and M4 was extracellular, and the C terminus was intracellular by this method. Four native N-linked glycosylation sites in the amino terminus and one introduced site between M3 and M4 were utilized, confirming the extracellular location of these regions. Epitopes inserted upstream and downstream of M2 were protease sensitive and thus intracellular. Our results support a topological model for glutamate receptor subunits that consists of three transmembrane domains, M1, M3, and M4, and another domain, the proposed channel-lining M2, which forms a reentrant membrane segment with both ends facing the cytoplasm.

Membrane insertion of gap junction connexins: polytopic channel forming membrane proteins

Connexins, the proteins that form gap junction channels, are polytopic plasma membrane (PM) proteins that traverse the plasma membrane bilayer four times. The insertion of five different connexins into the membrane of the ER was studied by synthesizing connexins in translation-competent cell lysates supplemented with pancreatic ER-derived microsomes, and by expressing connexins in vivo in several eucaryotic cell types. In addition, the subcellular distribution of the connexins was determined. In vitro-synthesis in the presence of microsomes resulted in the signal recognition particle-dependent membrane insertion of the connexins. The membrane insertion of all connexins was accompanied by an efficient proteolytic processing that was dependent on the microsome concentration. Endogenous unprocessed connexins were detectable in the microsomes used, indicating that the pancreatic microsomes serve as a competent recipient in vivo for unprocessed full length connexins. Although oriented with their amino terminus in the cytoplasm, the analysis of the cleavage reaction indicated that an unprecedented processing by signal peptidase resulted in the removal of an amino-terminal portion of the connexins. Variable amounts of similar connexin cleavage products were also identified in the ER membranes of connexin overexpressing cells. The amount generated correlated with the level of protein expression. These results demonstrate that the connexins contain a cryptic signal peptidase cleavage site that can be processed by this enzyme in vitro and in vivo in association with their membrane insertion. Consequently, a specific factor or condition must be required to prevent this aberrant processing of connexins under normal conditions in the cell.

Beta-structure in the membrane-spanning part of the nicotinic acetylcholine receptor (or how helical are transmembrane helices?)

The 'four-transmembrane-helix receptors' transmit their signals from the extracellular space to the cytoplasm via an intramembrane domain. In the case of the nicotinic acetylcholine receptor this domain comprises an ion channel formed by homologous secondary structure elements in the receptor subunits. It was believed to be exclusively alpha-helical, but recent experimental evidence questions the widely accepted model: beta-strands seem to be part of the membrane-spanning domain.

A sequence in the main cytoplasmic loop of the alpha subunit is required for assembly of mouse muscle nicotinic acetylcholine receptor

We have investigated the role of intracellular cytoplasmic sequences in the assembly of the mouse muscle nicotinic acetylcholine receptor (AChR) transiently expressed in COS cells. A chimeric protein in which the region from M1 to M4 of the alpha subunit was replaced by the corresponding region in the beta subunit was unable to support AChR assembly when substituted for the alpha subunit; a chimeric alpha subunit containing only the long cytoplasmic loop from the beta subunit was likewise inactive. Systematic mutation of short segments of the loop identified a sequence of 17 amino acids near the C-terminal end of the loop for which the beta sequence could not be substituted. Each of the inactive chimeric and mutated alpha subunits bound alpha-bungarotoxin when expressed alone and formed a heterodimer when expressed with the delta subunit. An alpha subunit truncated after M1 formed both an alpha delta heterodimer and an alpha delta beta heterotrimer, demonstrating that the cytoplasmic loop is dispensable for the early steps of assembly. A sequence in the long cytoplasmic loop of the alpha subunit thus appears to play a role in a late step of AChR assembly.

Extracytoplasmic disposition of lysine beta 165 of acetylcholine receptor

The location, with respect to the membrane, of Lys 165 in the folded beta polypeptide of native nicotinic acetylcholine receptor has been determined by site-directed immunochemistry. Sealed, right-side-out vesicles rich in acetylcholine receptor were modified with pyridoxal phosphate and sodium [3H]-borohydride. Saponin was added to one portion of the vesicles to make them permeable to the pyridoxal phosphate and sodium borohydride; the other portion was modified in the absence of saponin. Both samples were then exhaustively succinylated and digested with trypsin and thermolysin to produce the peptide LDAKGER, which contains Lys beta 165. The digests were passed over an immunoadsorbent specific for peptides with the sequence LDAXGER, where X represents any modified or unmodified amino acid, and specifically bound peptides were eluted with 0.1 M sodium phosphate, pH 2.5. The eluates were submitted to high-pressure liquid chromatography, and two peptides, N epsilon-phospho[3H]pyridoxalLDAKGER and N epsilon-succinylLDAKGER, modified at the epsilon amino group of lysine with pyridoxal phosphate and sodium [3H]-borohydride or succinic anhydride, respectively, were identified by comparison to standards. The relative specific radioactivity of N epsilon-phospho[3H]pyridoxalLDAKGER modified in the presence or absence of saponin, respectively, was 0.9 +/- 0.4. The incorporation of phospho[3H]pyridoxyl groups into Lys alpha 380, a residue located on the cytoplasmic surface of acetylcholine receptor, was also monitored. The relative specific radioactivity of the peptide that contains the modified Lys alpha 380, N epsilon-phospho[3H]pyridoxalGVKYIAE, increased 3.6-fold when the modification was performed in the presence of saponin. This result verifies that the vesicles used in these experiments were sealed and right-side-out. Because the incorporation of [3H]pyridoxyl groups into Lys beta 165 is the same in the presence or absence of saponin, Lys beta 165 must have been located on the outside surface of the sealed, right-side-out vesicles, and therefore on the extracytoplasmic surface of native acetylcholine receptor.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

The segregation and expression of glutamate receptor subunits in cultured hippocampal neurons

The distribution and expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-selective glutamate receptor subunits (GluR1-4) were studied in cultured hippocampal neurons using antibodies generated against peptides corresponding to the C-termini of GluR1, GluR2/3 and GluR4, and with a set of oligonucleotide probes designed complementary to specific pan, flip and flop GluR1-4 messenger RNA sequences. GluR1-4 subunit proteins were localized in fixed hippocampal neurons (2 h to three weeks after plating) by immunocytochemistry with light and electron microscopy. At early stages in culture, moderate staining with antibodies to GluR1 and GluR2/3 and very light staining with antibody to GluR4 was observed in cell bodies and proximal portions of all neurites of some neurons. Upon establishment of identified axons and dendrites by seven days in culture, staining was intense with specific antibodies to GluR1 and GluR2/3 and light with anti-GluR4 antibody in cell bodies and dendrites. Little or no staining was observed in axons. Cells at seven days in culture exhibited a variety of morphologies. However, we could not assign a pattern of staining to a particular type. As the cultures matured over two and three weeks, staining was limited to the somatodendritic compartment. The intensity of glutamate receptor subunit staining increased and the extent of staining proceeded to the distal extreme of many dendrites. Moreover, antibodies to GluR1-4 subunits were co-localized in neurons. Immunocytochemistry on living neurons did not result in any significant labeling, suggesting that the epitope is either not expressed on the surface of the neurons, or is present, but inaccessible to the antibody. Electron microscopy demonstrated receptor localization similar to that found in brain, with staining of postsynaptic membrane and density, dendritic cytoplasm and cell body, but not within the synaptic cleft. We examined the possible role of "cellular compartmentation" in the pattern of glutamate receptor expression in hippocampal neurons. Compartmentalization studies of the subcellular distribution of messenger RNAs encoding GluR1-4 subunits was determined in mature cultures by in situ hybridization. Significant silver grain appearance was restricted to the cell body, indicating that the synthesis of glutamate receptor subunits is limited largely to the neuronal cell body. The expression of microtubule-associated protein 2 was studied in parallel. Microtubule-associated protein 2 expression appeared 6 h after plating, while glutamate receptor subunit expression was present at 2 h. This indicates that microtubule-associated protein 2 does not regulate the initial distribution of glutamate receptor subunits into neurites.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple structural determinants of voltage-dependent magnesium block in recombinant NMDA receptors

The voltage-dependent block of NMDA channels by Mg2+ is an important functional element of NMDA receptors, since relief of block by depolarization plays a key role in some forms of ischemic neurodegeneration and synaptic plasticity. To identify the relevant structural domains responsible for block by Mg2+ and TCP, we used site-directed mutagenesis to change individual amino acids of the rat NR1A subunit in a transmembrane region (599-DALTLSSAMWFSWGVLLNSGIGE-621, mutated residues underlined) previously shown to donate residues that influence ionic selectivity. Ten mutant NR1A subunits were co-expressed in Xenopus oocytes with either the epsilon 1 or NR2A subunits, and receptor properties were analyzed under two-electrode voltage clamp. The mutation N616R virtually abolished voltage-dependent Mg2+ block, reduced Zn2+ block 5-fold and greatly reduced Ba2+ permeability in confirmation of previous reports. This mutation also reduced the potency of TCP as a use-dependent blocker by 200-fold. The remaining low-affinity TCP block did not appear to be use-dependent, suggesting two blocking sites for TCP. None of the other mutations differed significantly from NR1A itself except S617N, which displayed a 6-fold reduction in Mg2+ block. A well-barrier model of permeation through the NMDA receptor channel is presented that quantitatively reproduces voltage-dependent Mg2+ block. This model demonstrates that only minimum changes energy profiles experienced by permeating ions, equivalent to the energy of a single hydrogen or ionic bond, are required to abolish Mg2+ block. These findings indicate that only small structural changes are needed to convert a Mg(2+)-insensitive ion channel to a channel with pronounced voltage-dependent Mg2+ block.

A splice variant of the N-methyl-D-aspartate (NMDAR1) receptor

A splice variant of the NMDA receptor (NMDAR1) was discovered containing a deletion of 37 amino acids near the carboxyl tail and has been designated NMDAR1b. The 111 nucleotides corresponding to the deleted amino acid sequence were found in a separate exon bounded by consensus intron/exon junction sequences in rat genomic DNA. A partial restriction map of genomic DNA bounding this region placed the deleted exon approximately 600 base pairs (bp) downstream of the upstream exon. RT/PCR analysis of RNA from different brain regions showed that the deletion variant is more abundantly expressed in the brain stem and cerebellum while the full-length form is expressed more abundantly in the olfactory bulb, striatum, hippocampus, and cortex. Northern analysis of poly(A)+ RNA from different brain regions with probes specific for the deleted exon (i.e., full-length form) and for the splice junction (deletion form) indicated approximately 4.4 kb transcripts. The probe for the deleted exon hybridized to transcripts in olfactory bulb, cortex, striatum, and hippocampus while the splice junction probe hybridized most strongly to transcripts in cerebellum. The results suggest an interesting rostral to caudal shift in the expression of splice variants of the NMDAR1 which may signify important functional differences in native forms of NMDA receptors.

Reporter epitopes: a novel approach to examine transmembrane topology of integral membrane proteins applied to the alpha 1 subunit of the nicotinic acetylcholine receptor

The development of a novel immunological method called the "reporter epitope" technique to probe the transmembrane topology of integral membrane proteins is described. Using this method, synthetic oligonucleotides encoding epitopes (reporter epitopes) for well characterized monoclonal antibodies (reporter mAbs) were inserted at various locations within the human acetylcholine receptor (AChR) alpha 1 subunit cDNA. The engineered subunits were then expressed along with Torpedo beta 1, gamma, and delta subunits in Xenopus oocytes, and the transmembrane location of the site of insertion was determined by the binding of the 125I-labeled reporter mAbs to whole oocytes. Control reporter epitope insertions at alpha 347 exhibited the expected cytoplasmic location. Reporter epitopes inserted at alpha 429 are located on the extracellular surface. Reporter epitopes that are 16-48 amino acids long do not disrupt assembly or function of hybrid AChRs when inserted near the carboxy terminus (at alpha 429) or in the large cytoplasmic domain (at alpha 347). However, because two reporter epitopes inserted at alpha 157 obliterated subunit assembly and a third reporter epitope when tolerated at this position was inaccessible from the extracellular surface and only marginally accessible after detergent solubilization of the AChRs, a definitive transmembrane location for this region was not possible. Nonetheless, the use of this approach has been successfully demonstrated, and it may be generally applicable to the study of other integral membrane proteins.

T helper cell recognition of muscle acetylcholine receptor in myasthenia gravis. Epitopes on the gamma and delta subunits

We tested the response of CD4+ cells and/or total lymphocytes from the blood of 22 myasthenic patients and 10 healthy controls to overlapping synthetic peptides, 20 residues long, to screen the sequence of the gamma and delta subunits of human muscle acetylcholine receptor (AChR). The gamma subunit is part of the AChR expressed in embryonic muscle and is substituted in the AChRs of most adult muscles by an epsilon subunit. The delta subunit is present in both embryonic and adult AChRs. Adult extrinsic ocular muscles, which are preferentially and sometimes uniquely affected by myasthenic symptoms, and thymus, which has a still obscure but important role in the pathogenesis of myasthenia gravis, express the embryonic gamma subunit. Anti-AChR CD4+ responses were more easily detected after CD8+ depletion. All responders recognized epitopes on both the gamma and delta subunits and had severe symptoms. In four patients the CD4+ cell response was tested twice, when the symptoms were severe and during a period of remission. Consistently, the response was only detectable, or larger, when the patients were severely affected.

Structure of nicotinic acetylcholine receptors

Nicotinic acetylcholine (ACh) receptors convert the binding of ACh into the opening of a cation-conducting channel. New information about the regions of the receptor most immediately involved in its function, namely the ACh-binding sites, the gate and the channel, has come from two approaches. One is the identification by labelling and by mutagenesis of residues contributing to these regions. Another is the determination of the three-dimensional structure of the receptor by electron microscopy. Although the identification of functionally relevant residues is incomplete and residues cannot yet be resolved in the three-dimensional structure, the two approaches are converging. There is still room in the gap for speculation.

Predicting the topology of eukaryotic membrane proteins

We show that the so-called 'positive inside' rule, i.e. the observation that positively charged amino acids tend to be more prevalent in cytoplasmic than in extra-cytoplasmic segments in transmembrane proteins [von Heijne, G. (1986) EMBO J. 5, 3021-3027], seems to hold for all polar segments in multi-spanning eukaryotic membrane proteins irrespective of their position in the sequence and hence can be used in conjunction with hydrophobicity analysis to predict their transmembrane topology. Further, as suggested by others, we confirm that the net charge difference across the first transmembrane segment correlates well with its orientation [Hartmann, E., Rapoport, T. A. and Lodish H. F. (1989) Proc. Natl Acad. Sci. USA 86, 5786-5790], and that the overall amino-acid composition of long polar segments can also be used to predict their cytoplasmic or extra-cytoplasmic location [Nakashima, H. and Nishikawa, K. (1992) FEBS Lett. 303, 141-146]. We present an approach to the topology prediction problem for eukaryotic membrane proteins based on a combination of these methods.

Myasthenia gravis: an autoimmune response against the acetylcholine receptor

Myasthenia gravis (MG) is an organ-specific autoimmune disease caused by an antibody-mediated assault on the muscle nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Binding of antibodies to the AChR leads to loss of functional AChRs and impairs the neuromuscular signal transmission, resulting in muscular weakness. Although a great deal of information on the immunopathological mechanisms involved in AChR destruction exists due to well-characterized animal models, it is not known which etiological factors determine the susceptibility for the disease. This review gives an overview of the literature on the AChR, MG and experimental models for this autoimmune disease.