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Ion Channels and Fast Synaptic Transmission

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 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. For our studies, we use postsynaptic membranes isolated from the (muscle-derived) electric organ of the Torpedo ray, which form tubular crystals of acetylcholine receptors.

The acetylcholine receptor is a member of a superfamily of transmitter-gated ion channels, which includes the serotonin 5-HT₃, γ-aminobutyric acid (GABA_A and GABA_C), and glycine receptors. It has a cation-selective pore, delineated by a ring of 5 similar subunits, that opens upon binding of acetylcholine to distant sites in the 2 ligand-binding (α) subunits at or near the subunit interfaces. In earlier studies, we obtained a description of the N-terminal ligand-binding domain of the receptor by fitting the ß-sheet core structure from a homologous pentameric acetylcholine-binding protein to the 3-dimensional densities determined from electron images.

More recently, we extended the structural analysis to derive an atomic model of the closed membrane-spanning pore. We showed that the pore is shaped by an inner ring of 5 α-helices, which curve radially to create a tapering path for the ions, and an outer ring of 15 α-helices, which coil around each other and shield the inner ring from the lipids. The gate, near the middle of the lipid bilayer, is a constricting hydrophobic girdle formed by weak interactions between neighboring inner helices.

The details of this structure, together with those obtained from the receptor trapped in the open-channel form, have enabled us to understand in outline the mechanism by which acetylcholine opens the pore. When acetylcholine enters the ligand-binding domain, it triggers rotations of the protein chains on opposite sides of the entrance to the pore. These rotations are communicated through the inner helices and open the pore by breaking the hydrophobic girdle.

The information now revealed about the 3-dimensional fold of the acetylcholine receptor pore, and about the movement of the inner helices during gating, has additional significance for other members of the ion channel superfamily. For example, the inhibitory glycine and GABA_A receptors have specific binding sites for alcohols and anesthetics between the inner and outer sets of helices. We can now begin to understand how alcohols and anesthetics affect the relative movements of these helices and hence modify the function of the inhibitory receptors.

Publications

Miyazawa, A., Fujiyoshi, Y., Unwin, N. Structure and gating mechanism of the acetylcholine receptor pore. Nature 424:949, 2003.