Abstract
A hallmark feature of N-methyl-d-aspartate (NMDA) receptors is their voltage-dependent block by extracellular Mg2+. The structural basis for Mg2+ block is not fully understood. Although asparagine residues in the pore-forming M2 regions of NR1 and NR2 subunits influence Mg2+ block, it has been speculated that additional residues are likely to be involved. Here, we report the unexpected finding that a tryptophan residue in the M2 region of NR2 subunits controls Mg2+ block. An NR2B(W607L) mutation abolished block and greatly increased permeation of extracellular Mg2+. A similar effect was seen with a mutation at the equivalent residue in NR2A but not with mutations at the equivalent residue or adjacent residues in NR1. In NR2B, mutations that changed NR2B(W607) to asparagine (W607N) or alanine (W607A) also greatly reduced Mg2+ block, whereas mutations that changed W607 to the aromatic residues tyrosine (W607Y) or phenylalanine (W607F) had little or no effect on Mg2+ block. Furthermore, the W607L, W607N, and W607A mutants, but not the W607Y and W607F mutants, decreased Ba2+ permeability of NMDA channels. Thus, residue NR2B(W607) may be involved in binding of divalent cations, in particular Mg2+, through a cation-π interaction with the electron-rich aromatic ring of the tryptophan. We previously suggested that NR2B(W607) may contribute to the narrow constriction of the NMDA channel. A model is now proposed in which the M2 loop of NR2B is folded in such a way that NR2B(W607) is positioned at the narrow constriction, at a level similar to NR2B(N616) and NR1(N616), with these three residues forming a binding site for Mg2+.
Footnotes
-
Send reprint requests to: Dr. Keith Williams, Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6084. E-mail:williams{at}pharma.med.upenn.edu
-
This work was supported by United States Public Health Service Grant NS35047 from the National Institute of Neurological Disorders and Stroke, a Grant-in-Aid from the American Heart Association, and grants from the Japan Health Science Foundation and the Yamanouchi Foundation for Research on Metabolic Disorders.
- Abbreviations:
- NMDA
- N-methyl-d-aspartate
- N1-DnsSpm
- N1-dansyl-spermine
- HEPES
- 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
- GluR
- glutamate receptor
- BAPTA
- 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid
- I-V
- current-voltage
- Received October 14, 1997.
- Accepted January 29, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
MolPharm articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|