Structural transitions of proton-gated ion channels : Involving pH sensing, heterogeneity and lipid interactions

Sammanfattning: Studying protein structure and function involves analyzing the relationship between a protein's three-dimensional assembly and its dynamic biochemical activity. The superfamily of pentameric ligand-gated ion channels (pLGICs) constitutes a classic yet illuminating experimental system for structure-function studies. Atomistic details of pLGIC structure can reveal molecular determinants of gating, ion selection, and permeation. In parallel, functional studies can quantify dynamic changes in activation state and ion flow. Structure-function relationships in pLGICs may critically inform our understanding of electrochemical signal transduction from bacteria to the human brain, and the development of drugs from antiparasitics to neurotherapeutics.My research, detailed in this thesis, has focused on pLGIC structure determination by cryo-electron microscopy (cryoEM). Complementary insights have been drawn from small-angle neutron scattering (SANS), which provides lowerresolution average structures under room-temperature, solution-phase conditions. These structural studies have been supported by comparison to previous electrophysiology data, and to new molecular dynamics (MD) simulations. In the latter approach, the motions of individual atoms in a three-dimensional protein model are computed over time, allowing us to predict its functional behavior and interactions.Although pLGICs play crucial roles in human physiology, our biophysical understanding of these proteins has been greatly supported by bacterial family members, which can be readily produced and characterized in the laboratory. This thesis revolves around studies of two pentameric proton-gated ion channels, one from the cyanobacterium Gloeobacter violaceus, called GLIC, and one from a Desulfofustis deltaproteobacterium, called DeCLIC. A combination of structural and functional methods have been applied to reveal mechanisms of activation, ion interaction, and domain rearrangement in these systems, including their relevance to human homologs.