# Structural basis and functional analysis of NMDA receptor regulation by calmodulin

## TL;DR

This paper explores how calmodulin interacts with NMDA receptors to regulate calcium-dependent channel desensitization, using structural and functional experiments.

## Contribution

The study reveals novel structural interactions between calmodulin and specific NMDA receptor subunit regions that are crucial for channel desensitization.

## Key findings

- GluN1-C0 binds to both N-lobe and C-lobe of Ca2+-CaM, while GluN2A-C0 binds only to the C-lobe.
- Mutations in GluN1 and GluN2A residues disrupt calmodulin binding and reduce calcium-dependent desensitization.
- A structural model suggests four calmodulin molecules bind per NMDA receptor tetramer to induce desensitization.

## Abstract

The synaptic plasticity mechanisms that are thought to underlie learning and memory require Ca2+ influx mediated by N-methyl-D-aspartate receptors composed of glycine-binding GluN1 and glutamate-binding GluN2 subunits. Calmodulin (CaM) binding to the cytosolic regions in both GluN1 (residues 841–865, called GluN1-C0) and GluN2A (residues 1004–1023, called GluN2A-C0) may be important for Ca2+-dependent channel desensitization (CDD). Here, we report NMR, ITC and electrophysiological experiments to probe the structure and functional role of Ca2+-bound CaM (Ca2+-CaM) binding to both GluN1 and GluN2A subunits. Our ITC studies show that the GluN1-C0 peptide binds to both the N-lobe and C-lobe of Ca2+-CaM, whereas the GluN2A-C0 peptide binds to only the Ca2+-CaM C-lobe. Our NMR analysis reveals GluN2A residues (W1014 and V1018) interact with exposed hydrophobic residues in the Ca2+-CaM C-lobe. The NMR structure of Ca2+-CaM bound to the GluN1-C0 peptide indicates the two CaM lobes bind to opposite sides of the GluN1-C0 helix (C-lobe contacts M848, F852, A853 and N-lobe contacts A854, V855, W858). The GluN1 mutant F852E and the GluN2A mutant W1014E both perturbed CaM binding in ITC studies, and also diminished electrophysiologically-measured CDD, suggesting CaM interaction with these residues contributes to CDD. We propose a structural mechanism of CDD wherein channel desensitization is caused by the binding of four CaM per N-methyl-D-aspartate receptor subunit tetramer.