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Ion Channels and Fast Synaptic Transmission

N. Unwin, B. Sheehan

Ion channels play a central role in the rapid transmission of electrical signals throughout the nervous system. To determine how these membrane proteins work, we are using electron microscopy to analyze their structures trapped in different physiologic states. Current studies center on the nicotinic acetylcholine receptor, the neurotransmitter-gated ion channel at the nerve-muscle synapse. We wish to find out how this ion channel achieves its ion selectivity and high transport rate and how it opens and desensitizes in response to acetylcholine released into the synaptic cleft.

The nicotinic acetylcholine receptor is a pentameric complex of approximately 300 kD with large water-filled vestibules extending from the membrane that shape the acetylcholine-binding pockets and facilitate selective transport of cations across a narrow membrane-spanning pore. The structure of the receptor in Torpedo postsynaptic membranes has been determined by electron microscopy at nearly 4-Å resolution. The gate of the channel is made by side chains extending from a ring of a-helical segments, creating a hydrophobic girdle around the middle part of the pore. Tunnels, framed by twisted b-sheet strands, are resolved in the extracellular wall of the channel, connecting the vestibule to the acetylcholine-binding regions in the 2 a subunits. Sets of narrow openings through which ions can flow are resolved between a-helical segments, forming part of the cytoplasmic wall of the channel. The extracellular tunnels may be access routes to the binding pockets for acetylcholine, and the cytoplasmic openings appear to act as filters to exclude anions and other impermeant species from the vicinity of the pore.

Freeze-trapping experiments indicated that when acetylcholine enters the binding pockets, it triggers a concerted conformational change that opens the pore by destabilizing the gate in the middle of the membrane. The alternative "open" configuration of pore-lining a-helical segments reshapes the lumen and creates new surfaces, allowing the ions to pass through. The conformational change that triggers opening of the channel involves changes in the configuration of the b-sheet cores that compose the extracellular domains of the 2 ligand-binding a subunits.

PUBLICATIONS

Unwin, N., Miyazawa, A., Li, J., Fujiyoshi, Y. Activation of the nicotinic acetylcholine receptor involves a switch in conformation of the a subunits. J. Mol. Biol. 319:1165, 2002.