Trans-Cis Proline Switches in a Pentameric Ligand-Gated Ion Channel: How They Are Affected by and How They Affect the Biomolecular Environment.

Pentameric ligand-gated ion channels (pLGICs) are important neuroreceptors, embedded in neuronal membranes, that mediate fast synaptic transmission. The molecular details of their working mechanisms have still to be fully unravelled due to their complexity and limited structural information available. Here we focus on a potential molecular switch in a prototypical pLGIC, the serotonin-activated 5-HT3 receptor, consisting of the trans-cis isomerization of a proline at the interface between the extracellular and transmembrane domain. Mutagenesis electrophysiology experiments previously showed that if such isomerization could not take place, the channel would not open, but the hypothetical role of this mechanism as key to channel gating is still debated. We investigate this switch within the receptor with molecular dynamics and enhanced sampling simulations. We analyze how the isomerization free energy landscape is affected by the receptor environment in comparison to simplified models. Moreover, we reveal how the isomerization, in turn, affects the structural and electrostatic properties of the receptor at the extracellular-transmembrane domain interface, e.g., by tuning the ion selectivity filter.