Structural insights into the diverse actions of magnesium on NMDA receptors.

Magnesium (Mg²⁺) is a key regulatory ion of N-methyl-ᴅ-aspartate (NMDA) receptors, including conferring them to function as coincidence detectors for excitatory synaptic transmission. However, the structural basis underlying the Mg²⁺ action on NMDA receptors remains unclear. Here, we report the cryo-EM structures of GluN1-N2B receptors and identify three distinct Mg²⁺-binding pockets. Specifically, site Ⅰ is located at the selectivity filter where an asparagine ring forms coordination bonds with Mg²⁺ and is responsible for the voltage-dependent block. Sites Ⅱ and Ⅲ are located at the N-terminal domain (NTD) of the GluN2B subunit and involved in the allosteric potentiation and inhibition, respectively. Site Ⅱ consists of three acidic residues, and the combination of three mutations abolishes the GluN2B-specific Mg²⁺ potentiation, while site Ⅲ overlaps with the Zn²⁺ pocket, and mutations here significantly reduce the inhibition. Our study enhances the understanding of multifaceted roles of Mg²⁺ in NMDA receptors and synaptic plasticity.