Closed-state cross-linking of adjacent beta1 subunits in alpha1beta1 GABAa receptors via introduced 6' cysteines

Metal bridges to probe membrane ion channel structure and function

Ion channels are integral membrane proteins that undergo important conformational changes as they open and close to control transmembrane flux of different ions. The molecular underpinnings of these dynamic conformational rearrangements are difficult to ascertain using current structural methods. Several functional approaches have been used to understand two- and three-dimensional dynamic structures of ion channels, based on the reactivity of the cysteine side-chain. Two-dimensional structural rearrangements, such as changes in the accessibility of different parts of the channel protein to the bulk solution on either side of the membrane, are used to define movements within the permeation pathway, such as those that open and close ion channel gates. Three-dimensional rearrangements – in which two different parts of the channel protein change their proximity during conformational changes – are probed by cross-linking or bridging together two cysteine side-chains. Particularly useful in this regard are so-called metal bridges formed when two or more cysteine side-chains form a high-affinity binding site for metal ions such as Cd2+ or Zn2+. This review describes the use of these different techniques for the study of ion channel dynamic structure and function, including a comprehensive review of the different kinds of conformational rearrangements that have been studied in different channel types via the identification of intra-molecular metal bridges. Factors that influence the affinities and conformational sensitivities of these metal bridges, as well as the kinds of structural inferences that can be drawn from these studies, are also discussed.

Experimental determination of the vertical alignment between the second and third transmembrane segments of muscle nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChR) are members of the Cys-loop ligand-gated ion channel superfamily. Muscle nAChR are heteropentamers that assemble from two α, and one each of β, γ, and δ subunits. Each subunit is composed of three domains, extracellular, transmembrane and intracellular. The transmembrane domain consists of four α-helical segments (M1-M4). Pioneering structural information was obtained using electronmicroscopy of Torpedo nAChR. The recently solved X-ray structure of the first eukaryotic Cys-loop receptor, a truncated (intracellular domain missing) glutamate-gated chloride channel α (GluClα) showed the same overall architecture. However, a significant difference with regard to the vertical alignment between the channel-lining segment M2 and segment M3 was observed. Here, we used functional studies utilizing disulfide trapping experiments in muscle nAChR to determine the spatial orientation between M2 and M3. Our results are in agreement with the vertical alignment as obtained when using the GluClα structure as a template to homology model muscle nAChR, however, they cannot be reconciled with the current Torpedo nAChR model. The vertical M2-M3 alignments as observed in X-ray structures of prokaryotic Gloeobacter violaceus ligand-gated ion channel and GluClα are in agreement. Our results further confirm that this alignment in Cys-loop receptors is conserved between prokaryotes and eukaryotes.

The TM2 6' position of GABA(A) receptors mediates alcohol inhibition

Ionotropic GABA(A) receptors (GABA(A)Rs), which mediate inhibitory neurotransmission in the central nervous system, are implicated in the behavioral effects of alcohol and alcoholism. Site-directed mutagenesis studies support the presence of discrete molecular sites involved in alcohol enhancement and, more recently, inhibition of GABA(A)Rs. We used Xenopus laevis oocytes to investigate the 6' position in the second transmembrane region of GABA(A)Rs as a site influencing alcohol inhibition. We asked whether modification of the 6' position by substitution with larger residues or methanethiol labeling [using methyl methanethiosulfonate (MMTS)] of a substituted cysteine, reduced GABA action and/or blocked further inhibition by alcohols. Labeling of the 6' position in either α2 or β2 subunits reduced responses to GABA. In addition, methanol and ethanol potentiation increased after MMTS labeling or substitution with tryptophan or methionine, consistent with elimination of an inhibitory site for these alcohols. Specific alcohols, but not the anesthetic etomidate, competed with MMTS labeling at the 6' position. We verified a role for the 6' position in previously tested α2β2 as well as more physiologically relevant α2β2γ2s GABA(A)Rs. Finally, we built a novel molecular model based on the invertebrate glutamate-gated chloride channel receptor, a GABA(A)R homolog, revealing that the 6' position residue faces the channel pore, and modification of this residue alters volume and polarity of the pore-facing cavity in this region. These results indicate that the 6' positions in both α2 and β2 GABA(A)R subunits mediate inhibition by short-chain alcohols, which is consistent with the presence of multiple counteracting sites of action for alcohols on ligand-gated ion channels.

Disulphide trapping of the GABA(A) receptor reveals the importance of the coupling interface in the action of benzodiazepines

BACKGROUND AND SIGNIFICANCE

Although the functional effects of benzodiazepines (BZDs) on GABA(A) receptors have been well characterized, the structural mechanism by which these modulators alter activation of the receptor by GABA is still undefined.

EXPERIMENTAL APPROACH

We used disulphide trapping between engineered cysteines to probe BZD-induced conformational changes within the γ₂ subunit and at the α₁/γ₂ coupling interface (Loops 2, 7 and 9) of α₁β₂γ₂ GABA(A) receptors.

KEY RESULTS

Crosslinking γ₂ Loop 9 to γ₂β-strand 9 (via γ₂ S195C/F203C and γ₂ S187C/L206C) significantly decreased maximum potentiation by flurazepam, suggesting that modulation of GABA-induced current (I(GABA)) by flurazepam involves movements of γ₂ Loop 9 relative to γ₂β-strand 9. In contrast, tethering γ₂β-strand 9 to the γ₂ pre-M1 region (via γ₂S202C/S230C) significantly enhanced potentiation by both flurazepam and zolpidem, indicating γ₂S202C/S230C trapped the receptor in a more favourable conformation for positive modulation by BZDs. Intersubunit disulphide bonds formed at the α/γ coupling interface between α₁ Loop 2 and γ₂Loop 9 (α₁D56C/γ₂L198C) prevented flurazepam and zolpidem from efficiently modulating I(GABA) . Disulphide trapping α₁ Loop 2 (α₁D56C) to γ₂β-strand 1 (γ₂P64C) decreased maximal I(GABA) as well as flurazepam potentiation. None of the disulphide bonds affected the ability of the negative modulator, 3-carbomethoxy-4-ethyl-6,7-dimethoxy-β-carboline (DMCM), to inhibit I(GABA) .

CONCLUSIONS AND IMPLICATIONS

Positive modulation of GABA(A) receptors by BZDs requires reorganization of the loops in the α₁/γ₂ coupling interface. BZD-induced movements at the α/γ coupling interface likely synergize with rearrangements induced by GABA binding at the β/α subunit interfaces to enhance channel activation by GABA.

Ligand- and subunit-specific conformational changes in the ligand-binding domain and the TM2-TM3 linker of {alpha}1 {beta}2 {gamma}2 GABAA receptors

Cys-loop receptor ligand binding sites are located at subunit interfaces where they are lined by loops A-C from one subunit and loops D-F from the adjacent subunit. Agonist binding induces large conformational changes in loops C and F. However, it is controversial as to whether these conformational changes are essential for gating. Here we used voltage clamp fluorometry to investigate the roles of loops C and F in gating the α1 β2 γ2 GABA(A) receptor. Voltage clamp fluorometry involves labeling introduced cysteines with environmentally sensitive fluorophores and inferring structural rearrangements from ligand-induced fluorescence changes. Previous attempts to define the roles of loops C and F using this technique have focused on homomeric Cys-loop receptors. However, the problem with studying homomeric receptors is that it is difficult to eliminate the possibility of bound ligands interacting directly with attached fluorophores at the same site. Here we show that ligands binding to the β2-α1 interface GABA binding site produce conformational changes at the adjacent subunit interface. This is most likely due to agonist-induced loop C closure directly altering loop F conformation at the adjacent α1-β2 subunit interface. However, as antagonists and agonists produce identical α1 subunit loop F conformational changes, these conformational changes appear unimportant for gating. Finally, we demonstrate that TM2-TM3 loops from adjacent β2 subunits in α1 β2 receptors can dimerize via K24'C disulfides in the closed state. This result implies unexpected conformational mobility in this crucial part of the gating machinery. Together, this information provides new insights into the activation mechanisms of Cys-loop receptors.

Cross-linking of sites involved with alcohol action between transmembrane segments 1 and 3 of the glycine receptor following activation

The glycine receptor is a member of the Cys-loop, ligand-gated ion channel family and is responsible for inhibition in the CNS. We examined the orientation of amino acids I229 in transmembrane 1 (TM1) and A288 in TM3, which are both critical for alcohol and volatile anesthetic action. We mutated these two amino acids to cysteines either singly or in double mutants and expressed the receptors in Xenopus laevis oocytes. We tested whether disulfide bonds could form between A288C in TM3 paired with M227C, Y228C, I229C, or S231C in TM1. Application of cross-linking (mercuric chloride) or oxidizing (iodine) agents had no significant effect on the glycine response of wild-type receptors or the single mutants. In contrast, the glycine response of the I229C/A288C double mutant was diminished after application of either mercuric chloride or iodine only in the presence of glycine, indicating that channel gating causes I229C and A288C to fluctuate to be within 6 A apart and form a disulfide bond. Molecular modeling was used to thread the glycine receptor sequence onto a nicotinic acetylcholine receptor template, further demonstrating that I229 and A288 are near-neighbors that can cross-link and providing evidence that these residues contribute to a single binding cavity.