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The N-Terminal Domains of both NR1 and NR2 Subunits Determine Allosteric Zn²⁺ Inhibition and Glycine Affinity of N-Methyl-D-aspartate Receptors

Abteilung Neurochemie, Max-Planck-Institut für Hirnforschung, Frankfurt am Main, Germany (C.M., I.M., H.B., B.L.); and AG Molekulare und Zelluläre Neurophysiologie, Technische Universität Darmstadt, Darmstadt, Germany (I.M., B.L.)

The N-methyl-D-aspartate (NMDA) subtype of ionotropic glutamate receptors (iGluRs) is a tetrameric protein composed of homologous NR1 and NR2 subunits, which require the binding of glycine and glutamate, respectively, for efficient channel gating. The extracellular N-terminal domains (NTDs) of iGluR subunits show sequence homology to the bacterial periplasmic leucine/isoleucine/valine binding protein (LIVBP) and have been implicated in iGluR assembly, trafficking, and function. Here, we investigated how deletion of the NR1- and NR2-NTDs affects the expression and function of NMDA receptors. Both proteolytic cleavage of the NR1-NTD from assembled NR1/NR2 receptors and coexpression of the NTD-deleted NR1 subunit with wild-type or NTD-deleted NR2 subunits resulted in agonist-gated channels that closely resembled wild-type receptors. This indicates that the NTDs of both NMDA receptor subunits are not essential for receptor assembly and function. However, deletion of either the NR1 or the NR2 NTD eliminated high-affinity, allosteric inhibition of agonist-induced currents by Zn²⁺ and ifenprodil, consistent with the idea that interdomain interactions between these domains are important for allosteric receptor modulation. Furthermore, by replacing the NR2A-NTD with the NR2B NTD, and vice versa, the different glycine affinities of NR1/NR2A and NR1/NR2B receptors were found to be determined by their respective NR2-NTDs. Together, these data show that the NTDs of both the NR1 and NR2 subunits determine allosteric inhibition and glycine potency but are not required for NMDA receptor assembly.

Address correspondence to: Bodo Laube, Technische Universität Darmstadt, AG Molekulare und zelluläre Neurophysiologie, Schnittspahnstr. 3, 64287 Darmstadt, Germany. E-mail: laube{at}mpih-frankfurt.mpg.de

This article has been cited by other articles:

				C. Madry, H. Betz, J. R. P. Geiger, and B. Laube Supralinear potentiation of NR1/NR3A excitatory glycine receptors by Zn2+ and NR1 antagonist PNAS, August 26, 2008; 105(34): 12563 - 12568. [Abstract] [Full Text] [PDF]

				T. Schuler, I. Mesic, C. Madry, I. Bartholomaus, and B. Laube Formation of NR1/NR2 and NR1/NR3 Heterodimers Constitutes the Initial Step in N-Methyl-D-aspartate Receptor Assembly J. Biol. Chem., January 4, 2008; 283(1): 37 - 46. [Abstract] [Full Text] [PDF]