Nury H, Bocquet N, Poupon CL, Raynal B, Haouz A, Corringer PJ, Delarue M
Crystal Structure of the Extracellular Domain of a Bacterial Ligand-Gated Ion Channel. [JOURNAL ARTICLE]
J Mol Biol 2009 Nov 13.
The crystal structure of the extracellular domain of the pentameric ligand-gated ion-channel from Gloeobacter violaceus (GLIC) was solved at neutral pH at 2.3 A resolution in two crystal forms, showing a surprising hexameric quaternary structure with a six-fold axis replacing the expected five-fold axis. While each subunit retain the usual beta-sandwich immunoglobulin-like fold, small deviations from the whole GLIC structure indicate zones of differential flexibility. The changes in interface between two adjacent subunits in the pentamer and the hexamer can be described with in a downward translation by one inter-strand distance and a global rotation of the second subunit, using the first one for superposition. While global characteristics of the interface, such as the buried accessible surface area, do not change very much, most of the atom-atom interactions are rearranged. It thus appears that the transmembrane domain is necessary for the proper oligomeric assembly of GLIC and that there is an intrinsic plasticity or polymorphism in possible subunit-subunit interfaces at the extracellular domain level, the latter behaving as a monomer in solution. Possible functional implications of this novel structural data are discussed in the context of the allosteric transition of this family of proteins. In addition we propose a novel way to quantify elastic energy stored in the interface between subunits, which indicates a tenser interface for the open form than for the closed form (rest state). The hexameric or pentameric forms of the extra-cellular domain have a similar negative curvature in their subunit-subunit interface, while AchBPs have a smaller and positive curvature that increases from the apo to the holo form.
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