Transducing Agonist Binding to Channel Gating Involves Different Interactions in 5-HT3 and GABAC Receptors

Abstract

5-Hydroxytryptamine (5-HT) 3 and ␥-aminobutyric acid, type C (GABA C ) receptors are members of the Cys-loop superfamily of neurotransmitter receptors, which also includes nicotinic acetylcholine, GABA A , and glycine receptors. The details of how agonist binding to these receptors results in channel opening is not fully understood but is known to involve charged residues at the extracellular/transmembrane interface. Here we have examined the roles of such residues in 5-HT 3 and GABA C receptors. Charge reversal experiments combined with data from activation by the partial agonist ␤-alanine show that in GABA C receptors there is a salt bridge between Glu-92 (in loop 2) and Arg-258 (in the pre-M1 region), which is involved in receptor gating. The equivalent residues in the 5-HT 3 receptor are important for receptor expression, but charge reversal experiments do not restore function, indicating that there is not a salt bridge here. There is, however, an interaction between Glu-215 (loop 9) and Arg-246 (pre-M1) in the 5-HT 3 receptor, although the coupling energy determined from mutant cycle analysis is lower than might be expected for a salt bridge. Overall the data show that charged residues at the extracellular/transmembrane domain interfaces in 5-HT 3 and GABA C receptors are important and that specific, but not equivalent, molecular interactions between them are involved in the gating process. Thus, we propose that the molecular details of interactions in the transduction pathway between the binding site and the pore can differ between different Cys-loop receptors.There have been many biochemical and functional studies on different members of the Cys-loop family of ligand-gated ion channels, but the prototypic member of this family, the neuromuscular nicotinic acetylcholine (nACh) 2 receptor, is still the best understood. This is the only receptor for which reasonably high resolution structural information (4 Å) is available from cryo-electron microscopy studies (1), and further knowledge has come from x-ray crystal structures of acetylcholine binding proteins (AChBP), which are homologous to the extracellular domain of this receptor (2, 3). Cys-loop receptors are composed of five pseudo-symmetrically arranged subunits surrounding a central ion-conducting pore. Each subunit is composed of an extracellular, a transmembrane, and an intracellular domain. The extracellular domain (ECD) contains the ligand binding site, which is formed at the interface of two adjacent subunits by the convergence of three amino acid loops (A-C) from one (the principal) subunit and three ␤-strands (D-F) from the adjacent (or complementary) subunit. The transmembrane domain (TMD) contains four membrane spanning ␣-helices (M1-M4) and a short C terminus. M2 from each subunit lines the pore and contains regions responsible for channel gating and ion selectivity. A large loop between M3-M4 forms the intracellular domain and is involved in channel conductance and modulation.Structural details extrapolated from AChBP have greatly enhanced o...

Roles for Loop 2 Residues of α1 Glycine Receptors in Agonist Activation

“…This pattern reversal suggests that there are small differences in the structure of Loop 2 between GlyRs and nAChRs. These subtle structural differences might underlie differences in the role of certain Loop 2 residues in agonist activation observed previously in Cys-loop receptors (4,9,10,12,(42)(43)(44)(45) and provide further evidence for heterogeneity in the gating mechanism among receptors in this superfamily.…”

A State-Dependent Salt-Bridge Interaction Exists across the β/α Intersubunit Interface of the GABAAReceptor

“…A tool that can be used to this end is the method of double-mutant cycle analysis, which quantifies the coupling energy between two mutated residues and clarifies the like- (Horovitz, 1996). One parameter that has been commonly used for double-mutant cycle analysis in the study of LGICs is the apparent affinity for ligand, or EC 50 (Kash et al, 2003;Price et al, 2007;Gleitsman et al, 2008). Therefore, the effects of the D163A and R120A mutations were initially characterized by determining the peak current EC 50-GABA value through concentration-response experiments.…”

“…⌬⌬GЈ o was calculated as R ⅐ T ln(k mutant /k wild-type ), where R is the ideal gas constant (1.987 calories/mol) and T is the absolute temperature (296 K). Although EC 50 and deactivation rates do not provide true kinetic rate constants, comparison of macroscopic parameters have been previously used to support side-chain interactions and establish coupling coefficients (Kash et al, 2003;Price et al, 2007;Gleitsman et al, 2008) Nonstationary Variance Analysis. Nonstationary variance analysis (Sigworth, 1980) was performed on responses to repeated 3-ms pulses of saturating GABA.…”

Nicotinic pharmacophore: The pyridine N of nicotine and carbonyl of acetylcholine hydrogen bond across a subunit interface to a backbone NH

“…4) is the standard way to determine whether pairs of mutations are independent or are coupled. EC 50 -based mutant cycle analyses have been performed by our lab and others to investigate multiple interactions in Cys-loop receptors and related structures (28,(34)(35)(36). For several different agonist pairs, coupling coefficients (Ω) and coupling energies (ΔΔG) were calculated ( Table 2).…”