NMDA receptors (NMDARs) are glutamate-gated ion channels that mediate fast excitatory synaptic transmission in the nervous system. Applying glutamate to outside-out patches containing a single NMDAR, we find that agonist-bound receptors transition to the open state via two conformations, an “unconstrained pre-active” state that contributes to fast synaptic events and a “constrained pre-active” state that does not. To define how glutamate drives these conformations, we decoupled the ligand-binding domains from specific transmembrane segments for GluN1 and GluN2A. Displacements of the pore-forming M3 segments define the energy of fast opening. However, to enter the unconstrained conformation and contribute to fast signaling, the GluN2 pre-M1 helix must be displaced before the M3 segments move. This pre-M1 displacement is facilitated by the flexibility of the S2-M4 of GluN1 and GluN2A. Thus, outer structures—pre-M1 and S2-M4—work in concert to remove constraints and prime the channel for rapid opening, facilitating fast synaptic transmission.

NMDA 受体 (NMDAR) 是谷氨酸门控离子通道，可介导神经系统中的快速兴奋性突触传递。将谷氨酸应用于包含单个 NMDAR 的外部贴片，我们发现激动剂结合受体通过两种构象转变为开放状态，一种有助于快速突触事件的“不受约束的前激活”状态和“受约束的前激活”状态声明没有。为了定义谷氨酸如何驱动这些构象，我们将配体结合域与 GluN1 和 GluN2A 的特定跨膜片段分离。成孔 M3 段的位移定义了快速打开的能量。然而，为了进入不受约束的构象并有助于快速信号传导，GluN2 pre-M1 螺旋必须在 M3 片段移动之前被置换。GluN1 和 GluN2A 的 S2-M4 的灵活性促进了这种前 M1 置换。因此，外部结构——pre-M1 和 S2-M4——协同工作以消除约束并为快速打开的通道做好准备，促进快速突触传递。