Fig. 6. GluN2A and GluN2B receptors exhibit different conformational landscape.

Structural analyses were performed using 29 available full-length NMDAR structures. a Lower panel: Localization of the various regions of interest. Letters (c, d and e) indicate the receptor’ regions where structural parameters were measured, and refer to plots in corresponding panels (c, d and e). Upper panel: Zoom of the (d) region encompassing the lower lobes of the GluN1-GluN2 NTD heterodimer. The estimated region for GluN1-K178AzF photocrosslinking is highlighted as a reaction sphere of 9 Å radius from GluN1-K178AzF Cβ^57,70. N: reactive nitrene radical, NTD N-terminal domain, LBD ligand-binding domain, TMD transmembrane domain, UL upper lobe, LL lower lobe. b GluN2A and GluN2B receptors differ by their accessibility for inter-subunit cross-linking by the photocrosslinker GluN1-K718AzF. The more atoms lie within the 9 Å of AzF Cβ, the closer are the lower lobes within the NTD heterodimer. Horizontal bars represent mean values. Values of n (number of structures) are (from left to right): 12, 8, 16, 6, 16 and 6. c The NTD intra-dimer upper lobe-upper lobe (UL-UL) interface adopts various compactions in GluN2B, but not GluN2A, receptors. The smaller the value of compaction, the lower the distance between the two UL protomers. Boxes display interquartile range, median is shown as center line, whiskers extend from the hinge to the largest and smallest value no further than plus (upper whisker) or minus (lower whisker) 1.5 x IQR (interquartile range). Values of n (number of structures) are (from left to right): 12, 8, 16, 6, 16 and 6. d The NTD intra-dimer lower lobe-lower lobe (LL) interface displays distinct range of LL separation between GluN2A and GluN2B receptors. Boxes display interquartile range, median is shown as center line, whiskers extend from the hinge to the largest and smallest value no further than plus (upper whisker) or minus (lower whisker) 1.5 x IQR (interquartile range). Values of n (number of structures) are (from left to right): 12, 8, 16, 6, 16 and 6. e The range of LBD inter-dimer rolling motion⁴⁷ is greater in GluN2B than in GluN2A receptors. Boxes display interquartile range, median is shown as center line, whiskers extend from the hinge to the largest and smallest value no further than plus (upper whisker) or minus (lower whisker) 1.5 x IQR (interquartile range). Values of n (number of structures) are (from left to right): 12, 8, 16, 6, 16 and 6. f NTD-LBD conformational coupling is sampled differently in GluN2A and GluN2B receptors. The graph plots the NTD intra-dimer lower lobe-lower lobe (LL) distance as a function the LBD inter-dimer rolling (parameters described in panels d and e, respectively). The dotted lines correspond to linear regressions for all states of GluN2A (blue, R = 0.725) and GluN2B (red, R = 0.729) receptors. Black arrows indicate the change in level of receptor activity (channel Po), increasing (+) or decreasing (−), according to the receptor conformational state.