Department of Biochemistry, University at Buffalo, State University of New York, Buffalo, New York 14214, USA.
J Neurosci. 2010 Sep 15;30(37):12474-9. doi: 10.1523/JNEUROSCI.3337-10.2010.
Glutamate-gated channels mediate fundamental brain processes, yet the mechanisms by which the neurotransmitter controls channel activation are incompletely understood. Structural studies revealed that the agonist has the critical role of bridging the divide between two flexible extracellular lobes and solidified the view that agonist-induced cleft-closure drives further isomerizations, which eventually open the channel. Within the glutamate receptor family, NMDA-sensitive channels are unique in their requirement that both glycine and glutamate bind to homologous regions on GluN1 and GluN2 subunits, respectively, before the channel can open. To study the gating reaction in separation from agonist binding and dissociation, we characterized the kinetic mechanism of individual NMDA receptors whose ligand-binding clefts were locked shut by disulfide bridges engineered across lobes. We found that locking GluN1 domains had no observable consequences on receptor activity, whereas locking GluN2A domains increased channel activity without reducing the number of resolvable kinetic states. Based on these results, we suggest that glutamate but not glycine activates NMDA receptors with submaximal efficacy. Low glutamate efficacy may represent a mechanism by which the neurotransmitter maintains control over receptor kinetics despite sharing with glycine the task of activation.
谷氨酸门控通道介导基本的大脑过程，但神经递质控制通道激活的机制尚未完全理解。结构研究表明，激动剂在桥接两个柔性细胞外裂片之间的分隔中起着关键作用，并巩固了激动剂诱导的裂隙闭合驱动进一步的异构化，最终打开通道的观点。在谷氨酸受体家族中，NMDA 敏感性通道的独特之处在于，在通道能够打开之前，甘氨酸和谷氨酸分别结合到 GluN1 和 GluN2 亚基上的同源区域。为了在与激动剂结合和解离分离的情况下研究门控反应，我们对单个 NMDA 受体的动力学机制进行了表征，这些受体的配体结合裂隙通过跨裂片设计的二硫键锁定关闭。我们发现，锁定 GluN1 结构域对受体活性没有可观察到的影响，而锁定 GluN2A 结构域会增加通道活性，而不会减少可分辨动力学状态的数量。基于这些结果，我们提出谷氨酸但不是甘氨酸以亚最大功效激活 NMDA 受体。低谷氨酸功效可能代表了一种机制，即尽管与甘氨酸共享激活任务，但神经递质仍能控制受体动力学。
