Contributions by N-terminal Domains to NMDA Receptor Currents
To investigate the role of the N-terminal domains (NTDs) in NMDA receptor signaling we used kinetic analyses of one-channel currents and compared the reaction mechanism of recombinant wild-type GluN1/GluN2A and GluN1/GluN2B receptors with those observed for NDT-lacking receptors. We found that truncated receptors maintained the fundamental gating mechanism characteristic of NMDA receptors, which includes a multi-state activation sequence, desensitization steps, and mode transitions. This result establishes that none of the functionally-defined NMDA receptor activation events require the NTD. Notably, receptors that lacked the entire NTD layer retained isoform-specific kinetics. Together with previous reports, these results demonstrate that the entire gating machinery of NMDA receptors resides within a core domain that contains the ligand-binding and the channel-forming transmembrane domains, whereas the NTD and C-terminal layers serve modulatory functions, exclusively.