X-Ray Structures of the Open and Resting Forms of the Same Bacterial Pentameric Ligand-Gated Ion Channel

Abstract

Pentameric ligand-gated ion channels (pLGIC) mediate fast chemical transmission of nerve signals. The structure of GLIC, a bacterial proton-gated homolog, has been established in its open and Locally Closed (LC) conformations at acidic pH. Here we report its crystal structure at neutral pH, revealing for the first time the two end-points of the gating mechanism in the same pLGIC using X-ray structures. The structural variability in the neutral pH structure observed in the crystal due to the presence of four copies in the asymmetric unit can be used to analyse the intrinsic fluctuations in this state. It is found that they have a marked tendency to occur in the direction of the closed-to-open transition. In the extra-cellular domain (ECD), an important quaternary change is observed, involving both a twist and a blooming motion, while the transmembrane channel (TMD) is closed in an LC-manner. On the tertiary level, detachment of inner and outer beta-sheets in the ECD reshapes two essential cavities at the ECD-ECD and ECD-TMD interfaces. The first one is the ligand-binding cavity, the other is a new one that matches a known Ca++ or Zn++ binding site in some pLGICs. These results shed new light on the allosteric transitions of pLGIC and their pharmacology. In addition, a plausible pathway between the two forms is presented and discussed.