Identification by mutagenesis of a Mg²⁺ -block site of the NMDA receptor channel

THE N-methyl-D-aspartate (NMDA) receptor channel is highly permeable to Ca²⁺ but is blocked by Mg²⁺ in a voltage-dependent manner^1-4. These characteristics are essential for the NMDA receptor channel to mediate the induction of long-term potentiation of synaptic efficacy, a form of activity-dependent synaptic plasticity thought to underlie memory, learning and development^5-8. Recent studies have revealed the molecular and functional diversity of the NMDA receptor channel subunits, which are classified into the ε and ζ families according to the amino-acid sequence homology^9-12. Here we report that replacement by glutamine of asparagine 598 in putative transmembrane segment M2 of the ζ1 subunit, strongly reduces the sensitivity of the heteromeric ε 2/ζ1 NMDA receptor channel to Mg²⁺ block. The corresponding mutation of the ε2 subunit has a similar effect. Furthermore, the heteromeric ε 2/ζl NMDA receptor channel with the mutation on both subunits shows greatly reduced sensitivity to MK-801, a channel blocker of the NMDA receptor channel^13,14, but is still susceptible to inhibition by Zn ^2+15,16. These findings suggest that the conserved asparagine residue in segment M2 constitutes a Mg²⁺-block site of the NMDA receptor channel, and that the MK-801 site overlaps the Mg²⁺ site.