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Energetics of gating at the apo-acetylcholine receptor transmitter binding site.

ABSTRACT: Acetylcholine receptor channels switch between conformations that have a low versus high affinity for the transmitter and conductance for ions (R<-->R*; gating). The forward isomerization, which begins at the transmitter binding sites and propagates approximately 50 A to the narrow region of the pore, occurs by approximately the same sequence of molecular events with or without agonists present at the binding sites. To pinpoint the forces that govern the R versus R* agonist affinity ratio, we measured single-channel activation parameters for apo-receptors having combinations of mutations of 10 transmitter binding site residues in the alpha (Y93, G147, W149, G153, Y190, C192, and Y198), epsilon (W55 and P121), or delta (W57) subunit. Gating energy changes were largest for the tryptophan residues. The alphaW149 energy changes were coupled with those of the other aromatic amino acids. Mutating the aromatic residues to Phe reduces the R/R* equilibrium dissociation constant ratio, with alphaY190 and alphaW149 being the most sensitive positions. Most of the mutations eliminated long-lived spontaneous openings. The results provide a foundation for understanding how ligands trigger protein conformational change.

SUBMITTER: Purohit P

PROVIDER: S-EPMC2847916 | biostudies-literature | 2010 Apr

REPOSITORIES: biostudies-literature

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Energetics of gating at the apo-acetylcholine receptor transmitter binding site.

Purohit Prasad P Auerbach Anthony A

The Journal of general physiology 20100401 4

Acetylcholine receptor channels switch between conformations that have a low versus high affinity for the transmitter and conductance for ions (R<-->R*; gating). The forward isomerization, which begins at the transmitter binding sites and propagates approximately 50 A to the narrow region of the pore, occurs by approximately the same sequence of molecular events with or without agonists present at the binding sites. To pinpoint the forces that govern the R versus R* agonist affinity ratio, we me ...[more]

PMID: 20351060

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Project description:Background and purposeThe binding of transmitter to specialized binding pockets leads to rearrangements in the structure of the receptor eventually resulting in channel opening. We used voltage-clamp fluorometry to investigate the pharmacological basis and biophysical processes that underlie structural changes at the transmitter binding site of the rat ?1?2?2L GABA(A) receptor.Experimental approachSimultaneous electrophysiological and site-specific fluorescence measurements were conducted on receptors expressed in Xenopus oocytes and labelled with an environmentally-sensitive fluorophore, Alexa 546 maleimide, at the ?1L127C site.Key resultsReceptors activated by GABA demonstrate a concentration-dependent increase in fluorescence intensity, indicating that the environment surrounding the fluorophore becomes less polar upon activation. Qualitatively similar responses were observed with other GABA site ligands such as piperidine-4-sulphonic acid, muscimol, ?-alanine and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol. Fluorescence changes were not affected by the direction of current flow. During long applications of GABA significant desensitization developed, which was not accompanied by additional changes in fluorescence. Pentobarbital was an efficacious agonist of the labelled mutant receptor but did not cause changes in fluorescence. Direct activation by etomidate or the steroid allopregnanolone also did not result in fluorescence changes. Functional potentiation of GABA-activated receptors by allopregnanolone or etomidate enhanced both the GABA-elicited functional response and the fluorescence change. In contrast, potentiation by pentobarbital was not accompanied by an enhanced fluorescence response.Conclusions and implicationsThe data indicate that there is no direct correlation between current flow or position of the activation gate and the structural changes as detected by Alexa 546-labelled ?1L127C?2?2L GABA(A) receptors. Channel potentiation by pentobarbital qualitatively differs from potentiation by etomidate or allopregnanolone.

Project description:The nicotinic acetylcholine receptor (nAChR) is a key molecule involved in the propagation of signals in the central nervous system and peripheral synapses. Although numerous computational and experimental studies have been performed on this receptor, the structural dynamics of the receptor underlying the gating mechanism is still unclear. To address the mechanical fundamentals of nAChR gating, both conventional molecular dynamics (CMD) and steered rotation molecular dynamics (SRMD) simulations have been conducted on the cryo-electron microscopy (cryo-EM) structure of nAChR embedded in a dipalmitoylphosphatidylcholine (DPPC) bilayer and water molecules. A 30-ns CMD simulation revealed a collective motion amongst C-loops, M1, and M2 helices. The inward movement of C-loops accompanying the shrinking of acetylcholine (ACh) binding pockets induced an inward and upward motion of the outer beta-sheet composed of beta9 and beta10 strands, which in turn causes M1 and M2 to undergo anticlockwise motions around the pore axis. Rotational motion of the entire receptor around the pore axis and twisting motions among extracellular (EC), transmembrane (TM), and intracellular MA domains were also detected by the CMD simulation. Moreover, M2 helices undergo a local twisting motion synthesized by their bending vibration and rotation. The hinge of either twisting motion or bending vibration is located at the middle of M2, possibly the gate of the receptor. A complementary twisting-to-open motion throughout the receptor was detected by a normal mode analysis (NMA). To mimic the pulsive action of ACh binding, nonequilibrium MD simulations were performed by using the SRMD method developed in one of our laboratories. The result confirmed all the motions derived from the CMD simulation and NMA. In addition, the SRMD simulation indicated that the channel may undergo an open-close (O <--> C) motion. The present MD simulations explore the structural dynamics of the receptor under its gating process and provide a new insight into the gating mechanism of nAChR at the atomic level.

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Energetics of gating at the apo-acetylcholine receptor transmitter binding site.

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Energetics of gating at the apo-acetylcholine receptor transmitter binding site.

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