Delineation of a membrane-proximal beta-rich domain in the GABAA receptor by progressive deletions

Topology characterization of a benzodiazepine-binding beta-rich domain of the GABAA receptor alpha1 subunit

Structural investigation of GABAA receptors has been limited by difficulties imposed by its trans-membrane-complex nature. In the present study, the topology of a membrane-proximal beta-rich (MPB) domain in the C139-L269 segment of the receptor alpha1 subunit was probed by mapping the benzodiazepine (BZ)-binding and epitopic sites, as well as fluorescence resonance energy transfer (FRET) analysis. Ala-scanning and semiconservative substitutions within this segment revealed the contribution of the phenyl rings of Y160 and Y210, the hydroxy group of S186 and the positive charge on R187 to BZ-binding. FRET with the bound BZ ligand indicated the proximity of Y160, S186, R187, and S206 to the BZ-binding site. On the other hand, epitope-mapping using the monoclonal antibodies (mAbs) against the MPB domain established a clustering of T172, R173, E174, Q196, and T197. Based on the lack of FRET between Trp substitutionally placed at R173 or V198 and bound BZ, this epitope-mapped cluster is located on a separate end of the folded protein from the BZ-binding site. Mutations of the five conserved Cys and Trp residues in the MPB domain gave rise to synergistic and rescuing effects on protein secondary structures and unfolding stability that point to a CCWCW-pentad, reminiscent to the CWC-triad "pin" of immunoglobulin (Ig)-like domains, important for the structural maintenance. These findings, together with secondary structure and fold predictions suggest an anti-parallel beta-strand topology with resemblance to Ig-like fold, having the BZ-binding and the epitopic residues being clustered at two different ends of the fold.

Surface activity of thymol: implications for an eventual pharmacological activity

In the present work, we studied the ability of thymol to affect the organization of model membranes and the activity of an intrinsic membrane protein, the GABA(A) receptor (GABA(A)-R). In this last aspect, we tried to elucidate if the action mechanism of this terpene at the molecular level, involves its binding to the receptor protein, changes in the organization of the receptor molecular environment, or both. The self-aggregation of thymol in water with a critical micellar concentration approximately = 4 microM and its ability to penetrate in monomolecular layers of soybean phosphatidylcholine (sPC) at the air-water interface, even at surface pressures above the equilibrium, lateral pressure of natural bilayers were demonstrated. Thymol affected the self-aggregation of Triton X-100 and the topology of sPC vesicles. It also increased the polarity of the membrane environment sensed by the electrochromic dye merocyanine. A dipolar moment of 1.341 Debye was calculated from its energy-minimized structure. Its effect on the binding of [3H]-flunitrazepam ([3H]-FNZ) to chick brain synaptosomal membranes changed qualitatively from a tendency to the inhibition to a clear activatory regime, up on changing the phase state of the terpene (from a monomeric to a self-aggregated state). Above its CMC, thymol increased the affinity of the binding of [3H]-FNZ (K(d-control)= 2.9, K(d-thymol)= 1.7 nM) without changing the receptor density (B(max-control)= 910, B(max-thymol)= 895 fmol/mg protein). The activatory effect of thymol on the binding of [ [3H]-FNZ was observed even in the presence of the allosteric activator gamma-aminobutyric acid (GABA) at a concentration of maximal activity, and was blocked by the GABA antagonist bicuculline. Changes in the dipolar arrangement and in the molecular packing of GABA(A)-R environment are discussed as possible mediators of the action mechanism of thymol.

Glycine receptors: lessons on topology and structural effects of the lipid bilayer

The members of the superfamily of nicotinicoid receptors, sometimes referred to as the ligand-gated ion channel superfamily (LGICS), are essential mediators in the propagation of electrical signals between cells at neuronal and neuromuscular synapses. Given the significant sequence and proposed topological similarities between family members, the structural architecture of any one of these neuroreceptors is believed to be archetypic for the family of ligand-gated channels. We have focused our biophysical studies on the glycine receptor (GlyR) since homomeric expression of just the alpha1 chain of the receptor is sufficient to reconstitute native-like activity when expressed in heterologous cells, and we have successfully overexpressed and purified relatively large quantities of this receptor. Our CD data suggests that the historical four transmembrane helix topology model for nicotinicoid receptors may be erroneous. Proteolytic studies as well as chemical modification studies coupled with mass spectroscopy (MS) have provided additional evidence that this model may be inappropriate. While we suggest a novel topological model for the superfamily of nicotinicoid receptors, the absence of high resolution data for the transmembrane regions of these ion channels precludes further refinement of this model. In addition, we observe structural changes in the recombinant alpha1 GlyR as a function of bilayer composition, suggesting that these receptors may be dynamically modulated by cellular control over the properties of the plasma membrane.

Two beta-rich structural domains in GABA(A) receptor alpha(1) subunit with different physical properties: Evidence for multidomain nature of the receptor

The type A gamma-aminobutyric acid (GABA(A)) receptor is a major inhibitory neurotransmitter-gated ion channel. Previously, we identified a membrane-proximal beta-rich (MPBR) domain in fragment C166-L296 of GABA(A) receptor alpha(1) subunit, forming nativelike pentamers. In the present study, another structural domain, the amino-terminal domain, was shown to exist in the fragment Q28-E165. The secondary structures of both fragments were beta-rich as measured using FTIR spectroscopy and estimated from the CD spectra to be 42% and 51% beta-strand for Q28-E165 and C166-L296, respectively. The CD spectrum of the combined fragment Q28-L296 was additive of the spectra of the two fragments. In addition, denaturation curves of both fragments were characteristic of cooperative transitions, supporting their domainlike nature. C166-L296 required 6.5 M of guanidine chloride for total denaturation, therefore it is extraordinarily stable, more so than Q28-E165. Moreover, effects of detergent on the molecular masses of Q28-E165 and C166-L296, as monitored using laser-scattering spectroscopy, indicated that intermolecular interactions were much more significant in C166-L296 than in Q28-E165. Effects of pH on their molecular masses suggested that ionic forces were involved in these interactions. Together the results show that the two adjacent fragments form independent folding units, MPBR and amino-terminal domains, different in secondary structure content, denaturation profile, and polymerization status, and suggest that the former may play a more important role in receptor assembly and that the extraordinary stability may underlie its intrinsic tendency to form oligomers. More significantly, the present study has provided direct evidence for the long-postulated multidomain nature of this family of receptors.

Mercury interaction with the GABA(A) receptor modulates the benzodiazepine binding site in primary cultures of mouse cerebellar granule cells

Mercury compounds are neurotoxic compounds with a great specificity for cerebellar granule cells. The interaction of mercury compounds with proteins in the central nervous system may underlie some of their effects on neurotransmission. In this work we study the interaction of mercuric chloride (HgCl2) and methylmercury (MeHg) with the GABA(A) receptor in primary cultures of cerebellar granule cells. Both compounds increased, dose dependently, the binding of [3H]flunitrazepam to the benzodiazepine recognition site. EC50 values for this effect were 3.56 and 15.24 microM for HgCl2 and MeHg, respectively, after 30 min exposure of intact cultured cerebellar granule cells. The increase of [3H]flunitrazepam binding by mercury compounds was completely inhibited by the GABA(A) receptor antagonists bicuculline and picrotoxinin, and by the organochlorine pesticide alpha-endosulfan. It was also partially inhibited by the anion transporter blocker DIDS, however this effect could be due to a possible chelation of mercury by DIDS. Intracellular events, like intracellular calcium, kinase activation/inactivation or antioxidant conditions did not affect [3H]flunitrazepam binding or its increase induced by mercury compounds. The sulfhydryl alkylating agent N-ethylmaleimide mimicked the effect of mercury compounds on [3H]flunitrazepam binding suggesting a common mechanism. We conclude that mercury compounds interact with the GABA(A) receptor by the way of alkylation of SH groups of cysteinyl residues found in GABA(A) receptor subunit sequences.

Structure of ligand-gated ion channels: critical assessment of biochemical data supports novel topology

Rapid signaling across the synaptic junction is partially mediated by the ligand-gated ion channel superfamily (LGICS), which includes inhibitory glycine and GABA receptors and excitatory acetylcholine and serotonin receptors. The glycine receptor (GlyR) can assemble as homopentamers of alpha subunits, and baculovirus expression systems are capable of overexpressing large quantities of active receptors. Limited proteolysis coupled to mass spectrometry on reconstituted alpha1 GlyR homopentamers identified proteolytic cleavages within proposed transmembrane domains postulated to fold as bilayer-spanning alpha helices in the "classical" model and identified unexpected membrane-associated regions in the N-terminal domain (J. F. Leite et al., 2000, J. Biol. Chem. 275, 13683-13689). In this review, optimized sequence alignments were used to integrate these proteolysis data with biochemical information determined in studies of all the LGICS members in order to construct a novel topological model.

Nicotinic receptors at the amino acid level

nAChRs are pentameric transmembrane proteins into the superfamily of ligand-gated ion channels that includes the 5HT3, glycine, GABAA, and GABAC receptors. Electron microscopy, affinity labeling, and mutagenesis experiments, together with secondary structure predictions and measurements, suggest an all-beta folding of the N-terminal extracellular domain, with the connecting loops contributing to the ACh binding pocket and to the subunit interfaces that mediate the allosteric transitions between conformational states. The ion channel consists of two distinct elements symmetrically organized along the fivefold axis of the molecule: a barrel of five M2 helices, and on the cytoplasmic side five loops contributing to the selectivity filter. The allosteric transitions of the protein underlying the physiological ACh-evoked activation and desensitization possibly involve rigid body motion of the extracellular domain of each subunit, linked to a global reorganization of the transmembrane domain responsible for channel gating.

Ligand binding and structural properties of segments of GABAA receptor alpha 1 subunit overexpressed in Escherichia coli

The gamma-aminobutyric acid, type A (GABA(A)), receptor is the target for numerous therapeutic compounds. In the present study, the Gln(28)-Leu(296), Gln(28)-Arg(276), Gln(28)-Arg(248), and Gln(28)-Glu(165) (numbering of bovine precursor protein) segments of its alpha(1) subunit were overexpressed in Escherichia coli, along with Cys(166)-Leu(296) produced previously, for structural analysis by circular dichroism and ligand binding studies by fluorescence spectroscopy. Results showed that the protein segments were rich in beta-sheet structures. Binding of the fluorescent benzodiazepine Bodipy-FL Ro-1986 was evident from fluorescence resonance energy transfer and fluorescence anisotropy measurements. The binding affinity was in the micromolar range. The binding was attributable more to Cys(166)-Leu(296) than to Gln(28)-Glu(165) and was inhibited by known central benzodiazepine site ligands. Three point mutations, Y187A, T234A, and Y237A, were found to perturb protein secondary structures. Studies with the single Trp mutants W198Y and W273Y indicated that Trp(273) was closer to the binding site than Trp(198).

A fragment of recombinant GABA(A) receptor alpha1 subunit forming rosette-like homo-oligomers

The type A gamma-aminobutyric acid (GABA(A)) receptor plays a major role in inhibitory synaptic transmission in the central nervous system. A fragment consisting of residues Cys166 to Leu296 of the alpha1 subunit of the GABA(A) receptor was overexpressed in Escherichia coli and was found to have stable beta-rich structures. Here, results from laser scattering, gel electrophoresis and electron microscopy demonstrated that this recombinant protein formed rosette-like homo-oligomers, mainly pentamers in solution. Therefore, the fragment apparently provides a valuable model system for studying the pentameric holoreceptor assembly. Non-reducing sodium dodecyl sulfate polyacrylamide gel electrophoresis of the fragment showed that disulfide bonds formed between monomers contributed to the oligomerization of the fragment. The fact that this fragment alone could form pentamers in vitro strongly suggests that amino acid residues located within the Cys166-Leu296 region of the alpha1 subunit may contribute to the oligomerization of GABA(A) receptor in vivo.