Blue native PAGE as a useful method for the analysis of the assembly of distinct combinations of nicotinic acetylcholine receptor subunits

In vitro assembly of the trehalose bi-enzyme complex with artificial scaffold protein

Introduction: Trehalose is a significant rare sugar known for its stable properties and ability to protect biomolecules from environmental factors. Methods: In this study, we present a novel approach utilizing a scaffold protein-mediated assembly method for the formation of a trehalose bi-enzyme complex. This complex consists of maltooligosyltrehalose synthase (MTSase) and maltooligosyltrehalose trehalohydrolase (MTHase), which work in tandem to catalyze the substrate and enhance the overall catalytic efficiency. Utilizing the specific interaction between cohesin and dockerin, this study presents the implementation of an assembly, an analysis of its efficiency, and an exploration of strategies to enhance enzyme utilization through the construction of a bi-enzyme complex under optimal conditions in vitro. Results and Discussion: The bi-enzyme complex demonstrated a trehalose production level 1.5 times higher than that of the free enzyme mixture at 40 h, with a sustained upward trend. Compared to free enzyme mixtures, the adoption of a scaffold protein-mediated bi-enzyme complex may improve cascade reactions and catalytic effects, thus presenting promising prospects.

Functional Characterization of CLCN4 Variants Associated With X-Linked Intellectual Disability and Epilepsy

Early/late endosomes, recycling endosomes, and lysosomes together form the endo-lysosomal recycling pathway. This system plays a crucial role in cell differentiation and survival, and dysregulation of the endo-lysosomal system appears to be important in the pathogenesis of neurodevelopmental and neurodegenerative diseases. Each endo-lysosomal compartment fulfils a specific function, which is supported by ion transporters and channels that modify ion concentrations and electrical gradients across endo-lysosomal membranes. CLC-type Cl–/H+ exchangers are a group of endo-lysosomal transporters that are assumed to regulate luminal acidification and chloride concentration in multiple endosomal compartments. Heterodimers of ClC-3 and ClC-4 localize to various internal membranes, from the endoplasmic reticulum and Golgi to recycling endosomes and late endosomes/lysosomes. The importance of ClC-4-mediated ion transport is illustrated by the association of naturally occurring CLCN4 mutations with epileptic encephalopathy, intellectual disability, and behavioral disorders in human patients. However, how these mutations affect the expression, subcellular localization, and function of ClC-4 is insufficiently understood. We here studied 12 CLCN4 variants that were identified in patients with X-linked intellectual disability and epilepsy and were already characterized to some extent in earlier work. We analyzed the consequences of these mutations on ClC-4 ion transport, subcellular trafficking, and heterodimerization with ClC-3 using heterologous expression in mammalian cells, biochemistry, confocal imaging, and whole-cell patch-clamp recordings. The mutations led to a variety of changes in ClC-4 function, ranging from gain/loss of function and impaired heterodimerization with ClC-3 to subtle impairments in transport functions. Our results suggest that even slight functional changes to the endosomal Cl–/H+ exchangers can cause serious neurological symptoms.

Fluorescence-based techniques for the detection of the oligomeric status of proteins: implication in amyloidogenic diseases

Intrinsically disordered proteins (IDPs) have captured attention in the last couple of decades due to their functional roles despite a lack of specific structure. Moreover, these proteins are found to be highly aggregation prone depending on the mutational and environmental changes to which they are subjected. The aggregation of such proteins either in the intracellular context or extracellular matrix is associated with several adverse pathophysiological conditions such as Alzheimer's, Parkinson's, and Huntington's diseases, Spinocerebellar ataxia, and Type-II diabetes. Interestingly, it has been noted that the smaller oligomers formed by IDPs are more toxic to cells than their larger aggregates. This necessitates the development of techniques that can detect the smaller oligomers formed by IDPs for diagnosis of such diseases during their early onset. Fluorescence-based spectroscopic and microscopic techniques are highly effective as compared to other techniques for the evaluation of protein oligomerization, organization, and dynamics. In this review, we discuss several fluorescence-based techniques including fluorescence/Förster resonance energy transfer (FRET), homo-FRET, fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS), fluorescence lifetime imaging (FLIM), and photobleaching image correlation spectroscopy (pbICS) that are routinely used to identify protein oligomers in extracellular and intracellular matrices.

Zinc modulation of proton currents in a new voltage-gated proton channel suggests a mechanism of inhibition

The H_V1 voltage‐gated proton (H_V1) channel is a key component of the cellular proton extrusion machinery and is pivotal for charge compensation during the respiratory burst of phagocytes. The best‐described physiological inhibitor of H_V1 is Zn²⁺. Externally applied ZnCl₂ drastically reduces proton currents reportedly recorded in Homo sapiens, Rattus norvegicus, Mus musculus, Oryctolagus cuniculus, Rana esculenta, Helix aspersa, Ciona intestinalis, Coccolithus pelagicus, Emiliania huxleyi, Danio rerio, Helisoma trivolvis, and Lingulodinium polyedrum, but with considerable species variability. Here, we report the effects of Zn²⁺ and Cd²⁺ on H_V1 from Nicoletia phytophila, NpH_V1. We introduced mutations at potential Zn²⁺ coordination sites and measured Zn²⁺ inhibition in different extracellular pH, with Zn²⁺ concentrations up to 1000 μm. Zn²⁺ inhibition in NpH_V1 was quantified by the slowing of the activation time constant and a positive shift of the conductance–voltage curve. Replacing aspartate in the S3‐S4 loop with histidine (D145H) enhanced both the slowing of activation kinetics and the shift in the voltage–conductance curve, such that Zn²⁺ inhibition closely resembled that of the human channel. Histidine is much more effective than aspartate in coordinating Zn²⁺ in the S3‐S4 linker. A simple Hodgkin Huxley model of NpH_V1 suggests a decrease in the opening rate if it is inhibited by zinc or cadmium. Limiting slope measurements and high‐resolution clear native gel electrophoresis (hrCNE) confirmed that NpH_V1 functions as a dimer. The data support the hypothesis that zinc is coordinated in between the dimer instead of the monomer. Zinc coordination sites may be potential targets for drug development.

Human importin α3 and its N-terminal truncated form, without the importin-β-binding domain, are oligomeric species with a low conformational stability in solution

BACKGROUND

Eukaryotic cells have a continuous transit of macromolecules between the cytoplasm and the nucleus. Several carrier proteins are involved in this transport. One of them is importin α, which must form a complex with importin β to accomplish its function, by domain-swapping its 60-residue-long N terminus. There are several human isoforms of importin α; among them, importin α3 has a particularly high flexibility.

METHODS

We studied the conformational stability of intact importin α3 (Impα3) and its truncated form, where the 64-residue-long, N-terminal importin-β-binding domain (IBB) has been removed (ΔImpα3), in a wide pH range, with several spectroscopic, biophysical, biochemical methods and with molecular dynamics (MD).

RESULTS

Both species acquired native-like structure between pH 7 and 10.0, where Impα3 was a dimer (with an apparent self-association constant of ~10 μM) and ΔImpα3 had a higher tendency to self-associate than the intact species. The acquisition of secondary, tertiary and quaternary structure, and the burial of hydrophobic patches, occurred concomitantly. Both proteins unfolded irreversibly at physiological pH, by using either temperature or chemical denaturants, through several partially folded intermediates. The MD simulations support the presence of these intermediates.

CONCLUSIONS

The thermal stability of Impα3 at physiological pH was very low, but was higher than that of ΔImpα3. Both proteins were stable in a narrow pH range, and they unfolded at physiological pH populating several intermediate species.

GENERAL SIGNIFICANCE

The low conformational stability explains the flexibility of Impα3, which is needed to carry out its recognition of complex cargo sequences.

Preferential association with ClC-3 permits sorting of ClC-4 into endosomal compartments

ClC-4 is an intracellular Cl^-/H⁺ exchanger that is highly expressed in the brain and whose dysfunction has been linked to intellectual disability and epilepsy. Here we studied the subcellular localization of human ClC-4 in heterologous expression systems. ClC-4 is retained in the endoplasmic reticulum (ER) upon overexpression in HEK293T cells. Co-expression with distinct ClC-3 splice variants targets ClC-4 to late endosome/lysosomes (ClC-3a and ClC-3b) or recycling endosome (ClC-3c). When expressed in cultured astrocytes, ClC-4 sorted to endocytic compartments in WT cells but was retained in the ER in Clcn3^-/- cells. To understand the virtual absence of ER-localized ClC-4 in WT astrocytes, we performed association studies by high-resolution clear native gel electrophoresis. Although other CLC channels and transporters form stable dimers, ClC-4 was mostly observed as monomer, with ClC-3-ClC-4 heterodimers being more stable than ClC-4 homodimers. We conclude that unique oligomerization properties of ClC-4 permit regulated targeting of ClC-4 to various endosomal compartment systems via expression of different ClC-3 splice variants.

Key sites for P2X receptor function and multimerization: overview of mutagenesis studies on a structural basis

P2X receptors constitute a seven-member family (P2X1-7) of extracellular ATP-gated cation channels of widespread expression. Because P2X receptors have been implicated in neurological, inflammatory and cardiovascular diseases, they constitute promising drug targets. Since the first P2X cDNA sequences became available in 1994, numerous site-directed mutagenesis studies have been conducted to disclose key sites of P2X receptor function and oligomerization. The publication of the 3-Å crystal structures of the zebrafish P2X4 (zfP2X4) receptor in the homotrimeric apo-closed and ATP-bound open states in 2009 and 2012, respectively, has ushered a new era by allowing for the interpretation of the wealth of molecular data in terms of specific three-dimensional models and by paving the way for designing more-decisive experiments. Thanks to these structures, the last five years have provided invaluable insight into our understanding of the structure and function of the P2X receptor class of ligandgated ion channels. In this review, we provide an overview of mutagenesis studies of the pre- and post-crystal structure eras that identified amino acid residues of key importance for ligand binding, channel gating, ion flow, formation of the pore and the channel gate, and desensitization. In addition, the sites that are involved in the trimerization of P2X receptors are reviewed based on mutagenesis studies and interface contacts that were predicted by the zfP2X4 crystal structures.

Competing Lipid-Protein and Protein-Protein Interactions Determine Clustering and Gating Patterns in the Potassium Channel from Streptomyces lividans (KcsA)

There is increasing evidence to support the notion that membrane proteins, instead of being isolated components floating in a fluid lipid environment, can be assembled into supramolecular complexes that take part in a variety of cooperative cellular functions. The interplay between lipid-protein and protein-protein interactions is expected to be a determinant factor in the assembly and dynamics of such membrane complexes. Here we report on a role of anionic phospholipids in determining the extent of clustering of KcsA, a model potassium channel. Assembly/disassembly of channel clusters occurs, at least partly, as a consequence of competing lipid-protein and protein-protein interactions at nonannular lipid binding sites on the channel surface and brings about profound changes in the gating properties of the channel. Our results suggest that these latter effects of anionic lipids are mediated via the Trp(67)-Glu(71)-Asp(80) inactivation triad within the channel structure and its bearing on the selectivity filter.

The monomeric state of the proton-coupled folate transporter represents the functional unit in the plasma membrane

Folic acid is an essential vitamin required for de novo biosynthesis of nucleotides and amino acids. The proton-coupled folate transporter (PCFT; SLC46A1) has been identified as the major contributor for intestinal folate uptake. It is also involved in folate transport across the blood-brain barrier and into solid tumors. PCFT belongs to the major facilitator superfamily. Major facilitator superfamily members can exist in either monomeric or homo-oligomeric form. Here, we utilized blue native polyacrylamide gel electrophoresis (BN/PAGE) and crosslinking with bi-functional chemicals to investigate the quaternary structure of human PCFT after heterologous expression in Xenopus laevis oocytes and CHO cells. PCFT was expressed in the plasma membrane in both expression systems. The functionality of the utilized PCFT construct was confirmed in oocytes by folic acid induced currents at acidic pH. For both the oocyte and CHO expression system [(3)H]folic acid uptake studies indicated that PCFT was functional. To analyze the oligomeric state of PCFT in the plasma membrane, plasma membranes were isolated by polymerization with colloidal silica and polyacrylic acid and subsequent centrifugation. The digitonin-solubilized non-denatured PCFT migrated during BN/PAGE as a monomer, as judged by comparison with a membrane protein (5-HT(3A) receptor) of known pentameric assembly that was used to create a molecular sizing ladder. The chemical crosslinkers glutaraldehyde and dimethyl adipimidate were not able to covalently link potential higher order PCFT structures to form oligomers that were stable following SDS treatment. Together, our results demonstrate that plasma-membrane PCFT functions as a monomeric protein.

Mass spectrometry-based proteomics of endoscopically collected pancreatic fluid in chronic pancreatitis research

MS-based investigation of pancreatic fluid enables the high-throughput identification of proteins present in the pancreatic secretome. Pancreatic fluid is a complex admixture of digestive, inflammatory, and other proteins secreted by the pancreas into the duodenum, and thus is amenable to MS-based proteomic analysis. Recent advances in endoscopic techniques, in particular the endoscopic pancreatic function test (ePFT), have improved the collection methodology of pancreatic fluid for proteomic analysis. Here, we provide an overview of MS-based proteomic techniques as applied to the study of pancreatic fluid. We address sample collection, protein extraction, MS sample preparation and analysis, and bioinformatic approaches, and summarize current MS-based investigations of pancreatic fluid. We then examine the limitations and the future potential of such technologies in the investigation of pancreatic disease. We conclude that pancreatic fluid represents a rich reservoir of potential biomarkers and that the study of the molecular mechanisms of chronic pancreatitis may benefit substantially from MS-based proteomics.

TMEM16A(a)/anoctamin-1 shares a homodimeric architecture with CLC chloride channels

TMEM16A/anoctamin-1 has been identified as a protein with the classic properties of a Ca(2+)-activated chloride channel. Here, we used blue native polyacrylamide gel electrophoresis (BN-PAGE) and chemical cross-linking to assess the quaternary structure of the mouse TMEM16A(a) and TMEM16A(ac) splice variants as well as a genetically concatenated TMEM16A(a) homodimer. The constructs carried hexahistidyl (His) tags to allow for their purification using a nondenaturing metal affinity resin. Neither His-tagging nor head-to-tail concatenation of two copies of TMEM16A(a) noticeably affected Ca(2+)-induced measured macroscopic Cl(-) currents compared with the wild-type TMEM16A(a) channel. The digitonin-solubilized, nondenatured TMEM16A(a) protein migrated in the BN-PAGE gel as a homodimer, as judged by comparison with the concatenated TMEM16A(a) homodimer and channel proteins of known oligomeric structures (e.g. the voltage-gated Cl(-) channel CLC-1). Cross-linking with glutaraldehyde corroborated the homodimeric structure of TMEM16A(a). The TMEM16A(a) homodimer detected in Xenopus laevis oocytes and HEK 293 cells dissociated into monomers following denaturation with SDS, and reducing versus nonreducing SDS-PAGE provided no evidence for the presence of intersubunit disulfide bonds. Together, our data demonstrate that the Ca(2+)-activated chloride channel member TMEM16A shares an obligate homodimeric architecture with the hCLC-1 channel.

An intramembrane aromatic network determines pentameric assembly of Cys-loop receptors

Cys-loop receptors are pentameric ligand-gated ion channels (pLGICs) that mediate fast synaptic transmission. Here functional pentameric assembly of truncated fragments comprising the ligand-binding N-terminal ectodomains and the first three transmembrane helices, M1-M3, of both the inhibitory glycine receptor (GlyR) alpha1 and the 5HT(3)A receptor subunits was found to be rescued by coexpressing the complementary fourth transmembrane helix, M4. Alanine scanning identified multiple aromatic residues in M1, M3 and M4 as key determinants of GlyR assembly. Homology modeling and molecular dynamics simulations revealed that these residues define an interhelical aromatic network, which we propose determines the geometry of M1-M4 tetrahelical packing such that nascent pLGIC subunits must adopt a closed fivefold symmetry. Because pLGIC ectodomains form random nonstoichiometric oligomers, proper pentameric assembly apparently depends on intersubunit interactions between extracellular domains and intrasubunit interactions between transmembrane segments.

LUMA interacts with emerin and influences its distribution at the inner nuclear membrane

We present here a first characterization of LUMA, an unique integral inner nuclear membrane (INM) protein. LUMA is a highly conserved protein even in some bacteria and shares a PFAM domain of unknown function with orthologs from many species. Assessing LUMA topology by using protease protection of membrane-inserted LUMA and antibody epitope accessibility assays reveals that LUMA contains four transmembrane domains and a large hydrophilic domain located between membrane spans 1 and 2. The large hydrophilic domain is exposed to the perinuclear space whereas both LUMA termini reside cyto- or nucleoplasmically. Nuclear envelope targeting of LUMA mainly depends on the membrane spans. LUMA's transmembrane domains also promote homooligomerization. LUMA binds A- and B-type lamins and depends on A-type lamins for its INM localization. Furthermore, it interacts with emerin. Both downregulation of LUMA and overexpression of dominant-negative acting LUMA fragments causes redistribution of emerin. We propose that LUMA functions as a tetraspanin-like membrane organizer and has the potential to contribute to the pathomechanism of dystrophic diseases, such as Emery-Dreifuss muscular dystrophy.

Conserved dimeric subunit stoichiometry of SLC26 multifunctional anion exchangers

The SLC26 gene family encodes multifunctional transport proteins in numerous tissues and organs. Some paralogs function as anion exchangers, others as anion channels, and one, prestin (SLC26A5), represents a membrane-bound motor protein in outer hair cells of the inner ear. At present, little is known about the molecular basis of this functional diversity. We studied the subunit stoichiometry of one bacterial, one teleost, and two mammalian SLC26 isoforms expressed in Xenopus laevis oocytes or in mammalian cells using blue native PAGE and chemical cross-linking. All tested SLC26s are assembled as dimers composed of two identical subunits. Co-expression of two mutant prestins with distinct voltage-dependent capacitances results in motor proteins with novel electrical properties, indicating that the two subunits do not function independently. Our results indicate that an evolutionarily conserved dimeric quaternary structure represents the native and functional state of SLC26 transporters.

A model for the assembly of nicotinic receptors based on subunit-subunit interactions

Neuronal ion-channels are complex multimeric proteins. Within a given family, the variability of their pharmacological responses depends on subunit composition and subunit arrangement. We report here that protein assembly in the pentameric nicotinic acetylcholine receptor family, the best characterized of all neuronal receptors, can be predicted using information derived from homology modeled surface to surface subunit interactions based on the atomic structure of a snail acetylcholine-binding protein. An empirical assembly model is able to establish both subunit stoichiometry and subunit arrangement of known neuronal and muscle nicotinic receptors. This contribution to the understanding of nicotinic receptor assembly and variability might be extended to other types of ion-channels.

P2X5 Subunit Assembly Requires Scaffolding by the Second Transmembrane Domain and a Conserved Aspartate

Functional homomeric and heteromeric ATP-gated P2X receptor channels have been shown to display a characteristic trimeric architecture. Of the seven different isoforms (designated P2X(1)-P2X(7)), P2X(5) occurs in humans primarily as a non-functional variant lacking the C-terminal end of the ectodomain and the outer half of the second transmembrane domain. We show that this truncated variant, which results from the splice-skipping of exon 10, is prone to subunit aggregation because the residual transmembrane domain 2 is too short to insert into the membrane. Alleviation of the negative hydrophobic mismatch by the addition of a stretch of moderately hydrophobic residues enabled formation of a second membrane-spanning domain and strictly parallel homotrimerization. Systematic mutagenesis identified only one transmembrane domain 2 residue, Asp(355), which supported homotrimerization in a side chain-specific manner. Our results indicate that transmembrane domain 2 formation contributes 2-fold to hP2X(5) homotrimerization by tethering the end of the ectodomain to the membrane, thereby topologically restricting conformational mobility, and by intramembrane positioning of Asp(355). While transmembrane domain 2 appears to favor assembly by enabling productive subunit interactions in the ectodomain, Asp(355) seems to assist by simultaneously driving intramembrane helix interactions. Overall, these results indicate a complex interplay between topology, helix-helix interactions, and oligomerization to achieve a correctly folded structure.

Assembly of nicotinic α7 subunits inXenopusoocytes is partially blocked at the tetramer level

The assembly of nicotinic alpha1beta1gammadelta, alpha3beta4, and alpha7 receptors and 5-hydroxytryptamine 3A (5HT3A) receptors was comparatively evaluated in Xenopus oocytes by blue native PAGE analysis. While alpha1betagammadelta subunits, alpha3beta4 subunits, and 5HT3A subunits combined efficiently to pentamers, alpha7 subunits existed in various assembly states including trimers, tetramers, pentamers, and aggregates. Only alpha7 subunits that completed the assembly process to homopentamers acquired complex-type carbohydrates and appeared at the cell surface. We conclude that Xenopus oocytes have a limited capacity to guide the assembly of alpha7 subunits, but not 5HT3A subunits to homopentamers. Accordingly, ER retention of imperfectly assembled alpha7 subunits rather than inefficient routing of fully assembled alpha7 receptors to the cell surface limits surface expression levels of alpha7 nicotinic acetylcholine receptors.

Regulation of nicotinic acetylcholine receptor assembly

The four muscle-type nicotinic acetylcholine receptor (AChR) subunits, alpha, beta, gamma, and delta, assemble into functional alpha(2)betagammadelta pentamers in the endoplasmic reticulum (ER) through a series of interdependent folding and oligomerization events. The first stable assembly intermediate is a trimer composed of alpha, beta, and gamma subunits. The formation of alphabetagamma trimers initiates a series of subunit folding and processing events that allow addition of delta subunits to form alphabetagammadelta tetramers. Subunit folding and processing continue with formation of the ligand-binding sites on the alpha subunit of alphabetagammadelta tetramers and the second alpha subunit added to assemble alpha(2)betagammadelta pentamers. AChR assembly is inefficient. Only 20-30% of synthesized subunits assemble into mature receptors in the ER, while the remaining unassembled subunits are degraded. However, the efficiency of subunit assembly can be regulated under certain conditions leading to higher AChR expression. Increased intracellular cAMP levels cause a 2- to 3-fold increase in AChR assembly efficiency and a comparable increase in surface expression. Additionally, block of ubiquitin-proteasome degradation appears to enhance AChR assembly and expression. Thus, the regulation of AChR assembly through posttranslational mechanisms is a potential therapeutic target for increasing AChR expression in diseases in which expression is compromised.

A basic cluster determines topology of the cytoplasmic M3-M4 loop of the glycine receptor alpha1 subunit

The inhibitory glycine receptor is a member of the ligand-gated ion channel superfamily of neurotransmitter receptors, which are composed of homologous subunits with four transmembrane segments (M1-M4), each. Here, we demonstrate that the correct topology of the glycine receptor alpha1 subunit depends critically on six positively charged residues within a basic cluster, RFRRKRR, located in the large cytoplasmic loop (designated M3-M4 loop) following the C-terminal end of M3. Neutralization of one or more charges of this cluster, but not of other charged residues in the M3-M4 loop, led to an aberrant translocation into the endoplasmic reticulum lumen of the M3-M4 loop. However, when two of the three basic charges located in the ectodomain linking M2 and M3 were neutralized, in addition to two charges of the basic cluster, endoplasmic reticulum disposition of the M3-M4 loop was prevented. We conclude that a high density of basic residues C-terminal to M3 is required to compensate for the presence of positively charged residues in the M2-M3 ectodomain, which otherwise impair correct membrane integration of the M3 segment.

Activation and desensitization of the recombinant P2X1 receptor at nanomolar ATP concentrations

Activation and desensitization kinetics of the rat P2X1 receptor at nanomolar ATP concentrations were studied in Xenopus oocytes using two-electrode voltage-clamp recording. The solution exchange system used allowed complete and reproducible solution exchange in <0.5 s. Sustained exposure to 1-100 nM ATP led to a profound desensitization of P2X1 receptors. At steady-state, desensitization could be described by the Hill equation with a K1/2 value of 3.2 +/- 0.1 nM. Also, the ATP dependence of peak currents could be described by a Hill equation with an EC50 value of 0.7 microM. Accordingly, ATP dose-effect relationships of activation and desensitization practically do not overlap. Recovery from desensitization could be described by a monoexponential function with the time-constant tau = 11.6 +/-1.0 min. Current transients at 10-100 nM ATP, which elicited 0.1-8.5% of the maximum response, were compatible with a linear three-state model, C-O-D (closed-open-desensitized), with an ATP concentration-dependent activation rate and an ATP concentration-independent (constant) desensitization rate. In the range of 18-300 nM ATP, the total areas under the elicited current transients were equal, suggesting that P2X1 receptor desensitization occurs exclusively via the open conformation. Hence, our results are compatible with a model, according to which P2X1 receptor activation and desensitization follow the same reaction pathway, i.e., without significant C to D transition. We assume that the K1/2 of 3.2 nM for receptor desensitization reflects the nanomolar ATP affinity of the receptor found by others in agonist binding experiments. The high EC50 value of 0.7 microM for receptor activation is a consequence of fast desensitization combined with nonsteady-state conditions during recording of peak currents, which are the basis of the dose-response curve. Our results imply that nanomolar extracellular ATP concentrations can obscure P2X1 receptor responses by driving a significant fraction of the receptor pool into a long-lasting refractory closed state.

Monomeric and dimeric byproducts are the principal functional elements of higher order P2X1 concatamers

Heteromultimeric assembly of ion channel subunits generates high diversity in ion channel subtypes with distinct pharmacological and functional properties. To determine the subunit stoichiometry and order of ion channels, constructs with several concatenated subunits have been widely used in electrophysiological studies. Here we used primarily biochemical techniques to analyze the synthesis, assembly, and surface expression of P2X1 concatamers. We found that full-length concatamers consisting of two to six contiguous copies of the P2X1 subunit, although readily synthesized in Xenopus laevis oocytes, were entirely retained as aggregates in the endoplasmic reticulum. In contrast, minute levels of lower order byproducts, such as monomers and dimers, that were inherently formed with all the concatamers combined to form defined protein complexes equal in mass to the homotrimeric P2X1 receptor assembled from P2X1 monomers. Besides these complexes consisting of three monomers or one monomer plus one concatenated dimer, only small amounts of concatenated P2X1 trimers reached the plasma membrane. Complexes comprising more than three subunits were not observed in the plasma membrane. The byproduct complexes can account fully for the ATP-gated currents arising from expression of concatenated P2X1 subunits. These results strongly corroborate a trimeric architecture for P2X receptors yet indicate that formation of lower order by-products can be a pitfall of the concatamer approach.

A transmembrane motif governs the surface trafficking of nicotinic acetylcholine receptors

Surface expression of the nicotinic acetylcholine receptor (AChR) requires the assembly of multiple subunits in the endoplasmic reticulum (ER). Little is known, however, about the mechanism by which assembled receptor pentamers are transported to the cell membrane while unassembled subunits are retained in the ER. Here we report that a motif conserved in the transmembrane domain of AChR subunits is critically involved in this process. In COS cells, mutation within this signal allowed surface expression of unassembled subunits. Conversely, insertion of the sequence to unrelated proteins that are normally transported to the surface resulted in ER retention. The signal is buried in AChR pentamers, but is exposed on unassembled subunits in the ER, where it promotes protein degradation. We therefore conclude that this signal ensures surface trafficking of only functional AChRs.

Oligomeric state of membrane transport proteins analyzed with blue native electrophoresis and analytical ultracentrifugation

Blue native electrophoresis is used widely for the analysis of non-dissociated protein complexes with respect to composition, oligomeric state and molecular mass. However, the effects of detergent or dye binding on the mass and stability of the integral membrane proteins have not been studied. By comparison with analytical ultracentrifugation, we have evaluated whether the oligomeric state of membrane transport proteins is reflected reliably with blue native electrophoresis. For the analysis we have used two well-characterized transporters, that is, the major facilitator superfamily protein LacS and the phosphotransferase system EII(Mtl). For another member of the major facilitator superfamily, the xyloside transporter XylP from Lactobacillus pentosus, the complete analysis of the quaternary structure determined by analytical ultracentrifugation and freeze-fracture electron microscopy is presented. Our experiments show that during blue native electrophoresis the detergent bound to the proteins is replaced by the amphipathic Coomassie brilliant blue (CBB) dye. The mass of the bound CBB dye was quantified. Provided this additional mass of bound CBB dye is accounted for and care is taken in the choice and concentration of the detergent used, the mass of LacS, XylP and EII(Mtl) and four other membrane (transport) proteins could be deduced within 10 % error. Our data underscore the fact that the oligomeric state of many membrane transport proteins is dimeric.

Identification of molecular determinants that are important in the assembly of N-methyl-D-aspartate receptors

To determine which domains of the N-methyl-d-aspartate (NMDA) receptor are important for the assembly of functional receptors, a number of N- and C-terminal truncations of the NR1a subunit have been produced. Truncations containing a complete ligand binding domain bound glycine antagonist and gave binding constants similar to those of the native subunit, suggesting they were folding to form antagonist binding sites. Since NR2A is not transported to the cell surface unless it is associated with NR1 (McIlhinney, R. A. J., Le Bourdellès, B., Tricuad, N., Molnar, E., Streit, P., and Whiting, P. J. (1998) Neuropharmacology 37, 1355-1367), surface expression of NR2A can be used to monitor the association of the subunits. There was progressive loss of NR2A cell surface expression as the N terminus of NR1a was shortened, with complete loss when truncated beyond residue 380. Removal of the C terminus and/or the last transmembrane domain did not affect NR2A surface expression. Similar results were obtained in co-immunoprecipitation experiments. The oligomerization status of the co-expressed NR1a constructs and NR2A subunits was investigated using a non-denaturing gel electrophoresis system (blue native-polyacrylamide gel electrophoresis) and sucrose density gradient centrifugation. The blue native-polyacrylamide gel electrophoresis system also showed that the NR1a subunits could form a homodimer, which was confirmed using soluble constructs of the NR1a subunit. Together these results suggest the residues N-terminal of residue 380 are important for the association of NR2A with NR1a and that the complete N-terminal domain of the NR1a subunit is required for oligomerization with NR2A.

Roles of individual N-glycans for ATP potency and expression of the rat P2X1 receptor

P2X(1) receptor subunits assemble in the ER of Xenopus oocytes to homotrimers that appear as ATP-gated cation channels at the cell surface. Here we address the extent to which N-glycosylation contributes to assembly, surface appearance, and ligand recognition of P2X(1) receptors. SDS-polyacrylamide gel electrophoresis (PAGE) analysis of glycan minus mutants carrying Gln instead of Asn at five individual NXT/S sequons reveals that Asn(284) remains unused because of a proline in the +4 position. The four other sites (Asn(153), Asn(184), Asn(210), and Asn(300)) carry N-glycans, but solely Asn(300) located only eight residues upstream of the predicted reentry loop of P2X(1) acquires complex-type carbohydrates. Like parent P2X(1), glycan minus mutants migrate as homotrimers when resolved by blue native PAGE. Recording of ATP-gated currents reveals that elimination of Asn(153) or Asn(210) diminishes or increases functional expression levels, respectively. In addition, elimination of Asn(210) causes a 3-fold reduction of the potency for ATP. If three or all four N-glycosylation sites are simultaneously eliminated, formation of P2X(1) receptors is severely impaired or abolished, respectively. We conclude that at least one N-glycan per subunit of either position is absolutely required for the formation of P2X(1) receptors and that individual N-glycans possess marked positional effects on expression levels (Asn(154), Asn(210)) and ATP potency (Asn(210)).

Electron microscopic evidence for the assembly of soluble pentameric extracellular domains of the nicotinic acetylcholine receptor

Exploitation of soluble extracellular domains (ECDs) of the nicotinic acetylcholine receptor may provide a route to crystallographic studies aimed at exploring the structure and function of the intact receptor. The first step towards this goal is to manufacture and isolate soluble fragments that fold and assemble to form a functionally relevant complex. The baculovirus insect cell expression system was used to co-express soluble ECDs of all four muscle-type nicotinic acetylcholine receptor subunits (alpha, beta, gamma & delta-ECD) from Torpedo. Protein complexes were purified using either the conformationally sensitive monoclonal antibody mAb35, specific for a folded alpha subunit, or a NiNTA affinity resin, specific for a polyhistidine tag engineered on the delta-ECD. Western blotting with subunit specific antibodies confirmed the co-expression of each ECD and furthermore, indicated that the alpha, beta and gamma-ECDs were being co-purified with the polyhistidine-tagged delta-ECD. Chemical cross-linking was used to show that these co-purified proteins had indeed interacted specifically to form soluble oligomeric complexes. A low-resolution, three-dimensional image of these purified complexes, composed only of ECDs, was obtained by electron microscopy. They were shown to resemble the extracellular vestibule of the native receptor, having the same pseudo-pentameric symmetry, size and shape. Expression of incomplete sets of the four nicotinic acetylcholine receptor ECDs did not yield detectable complexes.

Amino acid determinants of alpha 7 nicotinic acetylcholine receptor surface expression

Transient transfection has not been a successful method to express the alpha7 nicotinic acetylcholine receptor such that these receptors are detected on the cell surface. This is not the case for all ligand-gated ion channels. Transient transfection with the 5-hydroxytryptamine type 3 subunit cDNA results in detectable surface receptor expression. Cell lines stably expressing the alpha7 nicotinic acetylcholine receptor produce detectable, albeit variable, levels of surface receptor expression. alpha7 nicotinic acetylcholine receptor surface expression is dependent, at least in part, on cell-specific factors. In addition to factors provided by the cells used for receptor expression, we hypothesize that the surface expression level in transfected cells is an intrinsic property of the receptor protein under study. Employing a set of alpha7-5-hydroxytryptamine type 3 chimeric receptor subunit cDNAs, we expressed these constructs in a transient transfection system and quantified surface receptor expression. We have identified amino acids that control receptor distribution between surface and intracellular pools; surface receptor expression can be manipulated without affecting the total number of receptors. These determinants function independently of the cell line used for expression and the transfection method employed. How these surface expression determinants in the alpha7 nicotinic acetylcholine receptor might influence synaptic efficacy is discussed.