Abstract
The muscle nicotinic acetylcholine receptor is a large, allosteric, ligand-gated ion channel with the subunit composition alpha2betagammadelta. Although much is now known about the structure of the binding site, relatively little is understood about how the binding event is communicated to the channel gate, causing the pore to open. Here we identify a key hydrogen bond near the binding site that is involved in the gating pathway. Using mutant cycle analysis with the novel unnatural residue alpha-hydroxyserine, we find that the backbone N-H of alphaSer-191 in loop C makes a hydrogen bond to an anionic side chain of the complementary subunit upon agonist binding. However, the anionic partner is not the glutamate predicted by the crystal structures of the homologous acetylcholine-binding protein. Instead, the hydrogen-bonding partner is the extensively researched aspartate gammaAsp-174/deltaAsp-180, which had originally been identified as a key binding residue for cationic agonists.
Highlights
The Cys loop family of ligand-gated ion channels is involved in mediating fast synaptic transmission throughout the central and peripheral nervous systems [1,2,3]
It is widely appreciated that a tryptophan residue (␣Trp149) plays a key role in neurotransmitter binding by forming a cation- interaction with the quaternary ammonium group of acetylcholine [4], a result supported by structural data
In a classic experiment on the Torpedo nicotinic acetylcholine receptor (nAChR), Czajkowski and Karlin [5] concluded that a key aspartate (␥Asp-174/␦Asp-180) from the complementary binding subunit could come within 9 Å of the agonist binding site
Summary
We evaluate several anionic residues in loop F and potential interactions with loop C. Using a combination of natural and unnatural mutagenesis, we find that, the backbone N-H of ␣Ser-191 does make a hydrogen bond to a loop F residue. The partner is not the glutamate seen in the AChBP crystal structure. The aspartate (␥Asp-174/␦Asp-180) originally identified by Czajkowski and Karlin [5] is the hydrogen-bonding partner
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
