|Channel Receptors - CNRS URA 2182|
|HEAD||Dr Pierre-Jean Corringer / firstname.lastname@example.org|
|MEMBERS||Chantal Le Poupon / Dr Hugues Nury / Dr Nicolas Bocquet / Dr Catherine van Renterghem / Virginie Dufresne / Marie Prevost
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 receptors, which mediate the majority of the inhibitory transmission in the brain, as well as GABA-C and 5HT3receptors. 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 cyanobacterium Gloeobacter violaceus. We overexpressed and crystallized this proton-gated ion channel at low pH, in collaboration with the Unit of Structural dynamics of macromolecules headed by 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. The structure is arranged as a funnel-shaped transmembrane pore widely open on the outer side and lined by hydrophobic residues. On the inner side, a 5 Å constriction matches with rings of hydrophilic residues likely to contribute to the ionic selectivity. Structural comparison with ELIC (Hilf and Dutzler, Nature, 452:375 (2008)), a bacterial homolog from Erwinia chrysanthemi solved in a presumptively closed conformation, shows a wider pore where the narrow hydrophobic constriction found in ELIC is removed. Comparative analysis of GLIC and ELIC reveals, in concert, a rotation of each extracellular β-sandwich domain as a rigid-body, interface rearrangements, and a reorganization of the transmembrane domain, involving a tilt of the M2 and M3 α-helices away from the pore axis. These data are consistent with a model of pore opening based on both quaternary twist and tertiary deformation.
Keywords: membrane protein, ion channel, allostery, electrophysiology, nicotinic receptor
Taly, A., Delarue, M., Grutter, T., Nilges, M., Le Novère, N., Corringer, P.J., Changeux, J.P. A quaternary twist model for nicotinic receptor gating, Biophys. J. 88(6):3954-65 (2005)
Sallette, J.,Pons, S., Devillers-Thiery A., Soudant, M., Prado de Carvalho, L., Changeux J.P. and Corringer, P.J., Nicotine up-regulates its own receptors through enhanced intracellular maturation, Neuron, 46:595-607 (2005)
Taly, A., Corringer, P.J., Grutter, T., Prado de Carvalho L., Karplus, M., Changeux, J.P. Implications of the quaternary twist allosteric model for the physiology and pathology of nicotinic acetylcholine receptors. Proc Natl Acad Sci U S A. 103:16965-16970 (2006)
Bocquet, N., Prado de Carvalho, L., Cartaud, J., Neyton, J., Le Poupon, C., Taly, A., Grutter, T., Changeux, J.P., Corringer, P.J. A prokaryotic proton-gated ion channel from the nicotinic acetylcholine receptor family, Nature, 445: 116-119 (2007)
Even, N., Cardona, A., Soudant, M., Corringer, P.J., Changeux, J.P., Cloëz-Tayarani, I. Regional differential effects of chronic nicotine on brain alpha4-containing and alpha6-containing receptors. Neuroreport. 19(15):1545-50 (2008)
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