Solution Structure of the Gloeobacter Violaceus Ligand-Gated Ion Channel Probed by Small-Angle Neutron Scattering

Abstract

Pentameric ligand-gated ion channels transform chemical signals into electrical ones, a process during which they undergo subtle conformational cycling resulting in rapid, reversible gating of an intrinsic transmembrane pore. The pH-gated bacterial channel GLIC has proved a valuable model system for this receptor family, in part due to its accessibility to X-ray crystallography - which has provided structures of closed, open, and intermediate conformations. However, such models raise critical questions as to whether the X-ray structures represent actual functional states, and if the cycle includes additional conformations such as highly contracted or expanded states. Small-angle neutron scattering (SANS) offers an approach to characterizing solution-phase structure, increasingly applicable to biomolecular problems on this scale. Here we characterized GLIC by SANS, in combination with inline size-exclusion chromatography (SEC), match-out deuterated detergents, and computational modeling of multiple putative structural models. SEC-SANS under resting conditions (pH 7) enabled the characterization of non-aggegated species which fit predictions from resting-state models, particularly following unrestrained molecular dynamics simulation of X-ray structures. Curves collected under activating conditions (pH 3) best fit a mixture of resting- and open-state models, whereas they diverged from expanded-pore models. in addition to supporting less-expanded models of the open state, this work demonstrates that SEC-SANS, in combination with molecular modeling and simulations, can distinguish solution-phase functional states of an ion channel. As neutron source brilliance increases and inline SEC-SANS set-ups become increasingly accessible, such methods will offer valuable tools for the elucidation of receptor conformational cycling and pharmaceutical development.