How Does the 5-HT3 Receptor MX Helix Modulate Function?

Recent structures have allowed excellent characterisation of the extracellular and transmembrane domains of pentameric ligand-gated ion channels, which contain respectively the ligand-binding site and the channel pore itself. However, the intracellular domain (ICD) remains largely unresolved and poorly understood. While the ICD can be completely removed without ablating receptor function, one of the resolved sections, the MX helix, moves dramatically on channel opening. We examined the nature of this MX movement in the 5-HT3 receptor by analysing MD simulations of an open and a closed structure, and performed functional assays on mutant receptors (an MX Ala scan) expressed in HEK293 cells and loaded with a membrane-potential sensitive dye.Our analysis emphasises that in addition to twisting almost 180 degrees between the closed and open structures, the MX helix moves from the periphery of the lipid bilayer to being completely embedded within it.