Pentameric ligand-gated ion channels exhibit distinct transmembrane domain archetypes for folding/expression and function

J P Daniel Therien,John E Baenziger

doi:10.1038/s41598-017-00573-2

J P Daniel Therien, John E Baenziger

Open Access

https://doi.org/10.1038/s41598-017-00573-2

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Journal: Scientific Reports	Publication Date: Mar 27, 2017
Citations: 19	License type: open-access

Affiliation: University of Ottawa

Abstract

Although transmembrane helix-helix interactions must be strong enough to drive folding, they must still permit the inter-helix movements associated with conformational change. Interactions between the outermost M4 and adjacent M1 and M3 α-helices of pentameric ligand-gated ion channels have been implicated in folding and function. Here, we evaluate the role of different physical interactions at this interface in the function of two prokaryotic homologs, GLIC and ELIC. Strikingly, disruption of most interactions in GLIC lead to either a reduction or a complete loss of expression and/or function, while analogous disruptions in ELIC often lead to gains in function. Structural comparisons suggest that GLIC and ELIC represent distinct transmembrane domain archetypes. One archetype, exemplified by GLIC, the glycine and GABA receptors and the glutamate activated chloride channel, has extensive aromatic contacts that govern M4-M1/M3 interactions and that are essential for expression and function. The other archetype, exemplified by ELIC and both the nicotinic acetylcholine and serotonin receptors, has relatively few aromatic contacts that are detrimental to function. These archetypes likely have evolved different mechanisms to balance the need for strong M4 “binding” to M1/M3 to promote folding/expression, and the need for weaker interactions that allow for greater conformational flexibility.

Highlights

The folding of transmembrane proteins occurs via two stages[1]
Both GLIC and ELIC are cation-selective ion channels that transiently gate open in response to agonist binding, GLIC responds to protons while ELIC responds to primary amines, such as cysteamine[33, 34]
It appears that the M4-M1/M3 interface in GLIC is highly optimized for both folding/expression and function, while in ELIC it is not

Summary

Introduction

The folding of transmembrane proteins occurs via two stages[1]. First, independently stable transmembrane α-helices are released into the membrane bilayer by the translocon machinery. In the absence of effective helix-helix associations, the immature form remains in the endoplasmic reticulum and is targeted for degradation Transmembrane proteins, such as the nAChR and other pentameric ligand-gated ion channels (pLGICs), must strike a balance between strong helix-helix associations that drive folding of the transmembrane domain (TMD) and weaker associations that facilitate inter-helix movements associated with protein conformational change[4, 5]. M3 associations and different sensitivities to allosteric effectors that act by influencing M4-M1/M3 associations[23, 29] This hypothesis, ignores the possibility that polar and/or van der Waals interactions may compensate for the absence of aromatic residues in some pLGICs to enhance helix-helix associations. M4-M1/M3 interactions appear to be optimized and essential for both expression and function in GLIC, but not in ELIC

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Conclusion