The cell membrane constitutes the main barrier for ion movement, and specialized proteins have been selected to catalyze the transport of various ions across the membrane: ion channels. Ion channels are integral membrane proteins (they cross the entire length of the membrane), and carry a hydrophilic water-filled pore that permit the passive flow of ions down their electrochemical gradient.
Our group is interested in a particular class of channels, which carry an internal gate undergoing opening/closing motions in response to ligands: Ligand-gated ion channel (LGICs). Among them, the pentameric LGICs (pLGICs) compose a large superfamily of phylogeneticaly-related membrane proteins, for which more than 40 genes are found in the human, accomplish a wide range of function in the cellular communication, particularly in neuronal communication. This superfamily includes nicotinic acetylcholine receptors (nAChRs), which mediate important pathways of cholinergic neuromodulation in the brain, GABA-A and glycine receptors, which mediate the majority of the inhibitory transmission in the brain and the spinal cord, as well as GABA-C and 5HT3 receptors. These proteins are involved in many human pathologies, and are the target of important therapeutic drugs including nicotinic derivatives, anxiolytics and anesthetics.
We aim at understanding, at an atomic resolution, the molecular mechanism governing the function of these proteins. They are known to fold as symmetrical pentamers in the membrane, and to undergo global allosteric transitions in the course of activation and desensitization. To this aim, we combine structural (X-ray crystallography and other biophysical techniques) and functional (electrophysiology coupled to site-directed mutagenesis) experiments.
In the search of pLGIC prototypes suitable for X-ray crystallographic approaches, our group was the first one to identify a functional bacterial homolog from the archaic cyanobacteriumGloeobacter violaceus. We overexpressed and crystallized this proton-gated ion channel at low pH, in collaboration with Marc Delarue (Pasteur Institute), yielding a 2.9 Å structure of an open conformation of the channel, that gives an atomic resolution picture of both the mechanisms of ion translocation within the channel and of the activation process. We further solved the structure of a locally closed conformation, and identified the binding site for general anesthetics and related allosteric modulators acting within the transmembrane domain.