Molecular rearrangements of the extracellular vestibule in NMDAR channels during gating.

Many N-methyl-D-aspartate receptor (NMDAR) channel blockers that have therapeutic potential can be trapped in the closed state. Using a combination of the substituted cysteine accessibility method and open channel blockers, we found that the M3 segment forms the core of the extracellular vestibule, including a deep site for trapping blockers. The M3 segment, as well as more superficial parts of the extracellular vestibule, undergo extensive remodeling during channel closure, but do not define the activation gate, which is located deeper in the pore. Rather, the pore walls lining the extracellular vestibule constrict during channel closure. This movement is essential for coupling ligand binding to activation gate opening and accounts for the different mechanisms of open channel block, including trapping.