Subtype-dependent N-Methyl-d-aspartate Receptor Amino-terminal Domain Conformations and Modulation by Spermine

Rita E Sirrieh,David M Maclean,Vasanthi Jayaraman

doi:10.1074/jbc.m115.649723

Abstract

The N-methyl-d-aspartate (NMDA) subtype of the ionotropic glutamate receptors is the primary mediator of calcium-permeable excitatory neurotransmission in the central nervous system. Subunit composition and binding of allosteric modulators to the amino-terminal domain determine the open probability of the channel. By using luminescence resonance energy transfer with functional receptors expressed in CHO cells, we show that the cleft of the amino-terminal domain of the GluN2B subunit, which has a lower channel open probability, is on average more closed than the GluN2A subunit, which has a higher open probability. Furthermore, the GluN1 amino-terminal domain adopts a more open conformation when coassembled with GluN2A than with GluN2B. Binding of spermine, an allosteric potentiator, opens the amino-terminal domain cleft of both the GluN2B subunit and the adjacent GluN1 subunit. These studies provide direct structural evidence that the inherent conformations of the amino-terminal domains vary based on the subunit and match the reported open probabilities for the receptor.

Highlights

Amino-terminal domains (ATDs) of NMDA receptors dictate open probability and bind the modulator spermine
This work provides insight into the conformational changes occurring upon spermine binding to the ATDs of the NMDA receptor
We can conclude that the upper lobes of the ATDs are stable and do not undergo any large conformational changes on average when spermine binds (Fig. 5)

Summary

Background

Amino-terminal domains (ATDs) of NMDA receptors dictate open probability and bind the modulator spermine. Subunit composition and binding of allosteric modulators to the amino-terminal domain determine the open probability of the channel. An allosteric potentiator, opens the amino-terminal domain cleft of both the GluN2B subunit and the adjacent GluN1 subunit. These studies provide direct structural evidence that the inherent conformations of the amino-terminal domains vary based on the subunit and match the reported open probabilities for the receptor. GluN1 subunits that include exon 5, a 21-residue stretch in the ATD lower lobe containing six basic residues, are no longer sensitive to spermine modulation This suggests the positive charges introduced by exon 5 function to spermine and obviate the spermine-binding site [18, 21]. It is important to note that these measurements are made in a near physiological state, i.e. with receptors that have minimal changes relative to the wild-type proteins and studied in CHO cells without being purified or isolated using lipids and/or detergents

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION