Mechanism of conductance control and neurosteroid binding in NMDA receptors.

Ion-channel activity reflects a combination of open probability and unitary conductance¹. Many channels display subconductance states that modulate signalling strength^2,3, yet the structural mechanisms governing conductance levels remain incompletely understood. Here we report that conductance levels are controlled by the bending patterns of pore-forming transmembrane helices in the heterotetrameric neuronal channel GluN1a-2B N-methyl-D-aspartate receptor (NMDAR). Our single-particle electron cryomicroscopy (cryo-EM) analyses demonstrate that an endogenous neurosteroid and synthetic positive allosteric modulator (PAM), 24S-hydroxycholesterol (24S-HC), binds to a juxtamembrane pocket in the GluN2B subunit and stabilizes the fully open-gate conformation, where GluN1a M3 and GluN2B M3' pore-forming helices are bent to dilate the channel pore. By contrast, EU1622-240 binds to the same GluN2B juxtamembrane pocket and a distinct juxtamembrane pocket in GluN1a to stabilize a sub-open state whereby only the GluN2B M3' helix is bent. Consistent with the varying extents of gate opening, the single-channel recordings predominantly show full-conductance and subconductance states in the presence of 24S-HC and EU1622-240, respectively. Another class of neurosteroid, pregnenolone sulfate, engages a similar GluN2B pocket, but two molecules bind simultaneously, revealing a diverse neurosteroid recognition pattern. Our study identifies that the juxtamembrane pockets are critical structural hubs for modulating conductance levels in NMDAR.