Gating of the proton-gated ion channel from Gloeobacter violaceus at pH 4 as revealed by X-ray crystallography.Gonzalez-Gutierrez, G., Cuello, L.G., Nair, S.K., Grosman, C.
(2013) Proc.Natl.Acad.Sci.USA 110: 18716-18721
- PubMed: 24167270
- DOI: 10.1073/pnas.1313156110
- Primary Citation of Related Structures:
- PubMed Abstract:
Cryoelectron microscopy and X-ray crystallography have recently been used to generate structural models that likely represent the unliganded closed-channel conformation and the fully liganded open-channel conformation of different members of the nico ...
Cryoelectron microscopy and X-ray crystallography have recently been used to generate structural models that likely represent the unliganded closed-channel conformation and the fully liganded open-channel conformation of different members of the nicotinic-receptor superfamily. To characterize the structure of the closed-channel conformation in its liganded state, we identified a number of positions in the loop between transmembrane segments 2 (M2) and 3 (M3) of a proton-gated ortholog from the bacterium Gloeobacter violaceus (GLIC) where mutations to alanine reduce the liganded-gating equilibrium constant, and solved the crystal structures of two such mutants (T25'A and Y27'A) at pH ~4.0. At the level of backbone atoms, the liganded closed-channel model presented here differs from the liganded open-channel structure of GLIC in the pre-M1 linker, the M3-M4 loop, and much more prominently, in the extracellular half of the pore lining, where the more pronounced tilt of the closed-channel M2 α-helices toward the pore's long axis narrows the permeation pathway. On the other hand, no differences between the liganded closed-channel and open-channel models could be detected at the level of the extracellular domain, where conformational changes are expected to underlie the low-to-high proton-affinity switch that drives gating of proton-bound channels. Thus, the liganded closed-channel model is nearly indistinguishable from the recently described "locally closed" structure. However, because cross-linking strategies (which could have stabilized unstable conformations) and mutations involving ionizable side chains (which could have affected proton-gated channel activation) were purposely avoided, we favor the notion that this structure represents one of the end states of liganded gating rather than an unstable intermediate.
Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, Department of Biochemistry, Institute for Genomic Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, IL 61801.