# Pathogenic residue insertion in neuronal nicotinic receptor alters intra- and inter-subunit interactions that tune channel gating

## TL;DR

A mutation in a brain receptor linked to a sleep-related epilepsy causes prolonged channel activity by altering structural interactions.

## Contribution

Identifies a novel structural region in the α4β2 nicotinic receptor that tunes channel stability through interhelical interactions.

## Key findings

- A leucine insertion in the M2 domain stabilizes the open channel state and promotes reopening.
- Functional effects are independent of the receptor's stoichiometric form and mutant copy number.
- Burst duration depends linearly on the size and hydrophobicity of the inserted residue.

## Abstract

We describe molecular-level functional changes in the α4β2 nicotinic acetylcholine receptor by a leucine residue insertion in the M2 transmembrane domain of the α4 subunit associated with sleep-related hyperkinetic epilepsy. Measurements of agonist-elicited single-channel currents reveal the primary effect is to stabilize the open channel state, while the secondary effect is to promote reopening of the channel. These dual effects prolong the durations of bursts of channel openings equally for the two major stoichiometric forms of the receptor, (α4)2(β2)3 and (α4)3(β2)2, indicating the functional impact is independent of mutant copy number per receptor. Altering the location of the residue insertion within M2 shows that functionally pivotal structures are confined to a half turn of the M2 α-helix. Residue substitutions within M2 and surrounding α-helices reveal that both intrasubunit and intersubunit interactions mediate the increase in burst duration. These interactions impacting burst duration depend linearly on the size and hydrophobicity of the substituting residue. Together, the results reveal a novel structural region of the α4β2 nicotinic acetylcholine receptor in which interhelical interactions tune the stability of the open channel state.