Gating Movement of Acetylcholine Receptor Caught by Plunge-Freezing

https://doi.org/10.1016/j.jmb.2012.07.010Get rights and content
Under a Creative Commons license
open access

Abstract

The nicotinic acetylcholine (ACh) receptor converts transiently to an open-channel form when activated by ACh released into the synaptic cleft. We describe here the conformational change underlying this event, determined by electron microscopy of ACh-sprayed and freeze-trapped postsynaptic membranes. ACh binding to the α subunits triggers a concerted rearrangement in the ligand-binding domain, involving an ~ 1‐Å outward displacement of the extracellular portion of the β subunit where it interacts with the juxtaposed ends of α-helices shaping the narrow membrane-spanning pore. The β-subunit helices tilt outward to accommodate this displacement, destabilising the arrangement of pore-lining helices, which in the closed channel bend inward symmetrically to form a central hydrophobic gate. Straightening and tangential motion of the pore-lining helices effect channel opening by widening the pore asymmetrically and increasing its polarity in the region of the gate. The pore-lining helices of the αγ and δ subunits, by flexing between alternative bent and straight conformations, undergo the greatest movements. This coupled allosteric transition shifts the structure from a tense (closed) state toward a more relaxed (open) state.

Graphical Abstract

Research Highlights

► Effect of acetylcholine at binding site is communicated to the gate ~50Å away. ► The structural change has been analysed by spray-freeze-trapping electron microscopy. ► A small rearrangement in the extracellular domain pushes the β subunit outward. ► This destabilises the closed-channel configuration of membrane helices. ► Channel opens mainly by flexure of pore-lining helices of the αγ and δ subunits.

Abbreviations

ACh
acetylcholine
AChBP
acetylcholine-binding protein
FSC
Fourier shell correlation
CMS
congenital myasthenic syndrome
PDB
Protein Data Bank

Keywords

acetylcholine receptor
freeze-trapping
asymmetric gating
allosteric mechanism
electron microscopy

Cited by (0)

1

Present address: Y. Fujiyoshi, Cellular and Structural Physiology Unit (CeSPI), Nagoya University, Furo‐cho, Chikusa, Nagoya 464‐8601, Japan.