orbital degrees of freedom of magnetic atoms
on surfaces, going beyond the spin Hamiltonian description successfully used in previous
STM studies of nanosized magnetic structures
(19, 30, 32, 33). Aside from a consistent description of the electronic and magnetic ground
state, the role of nonthermal spin relaxation
mechanisms can be determined based on independent input obtained through the multiplet analysis of the x-ray spectra and pump-probe measurements.
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S.S. and P.G. acknowledge support from the Swiss Competence
Centre for Materials Science and Technology (CCMX). J.D.
acknowledges funding by an Ambizione grant of the Swiss
National Science Foundation. I.G.R., S.B., C.P.L., and A.J.H.
thank B. Melior for expert technical assistance. C.P.L and A.J.H.
thank the Office of Naval Research for financial support. S.G.,
O.R.A., and B.A.J. thank the National Energy Research Scientific
Computing Center (NERSC) for computational resources. O.R.A.
was supported by the National Science Foundation under
grant DMR-1006605. B.A.J. thanks the Aspen Center for Physics
and National Science Foundation grant 1066293 for hospitality
while doing the calculations which appear in this paper. We
thank A. Cavallin for helping in developing the Igor code used
to analyze the XAS spectra. I.G.R., S.B., R.M.M., C.P.L., and
A.J.H. performed the STM experiments and data analysis. S.G.,
O.R.A., and B.A.J. carried out the DFT calculations. S.R., F.D.,
L. G., S.B., J. D., C. P., and F.N. carried out the XMCD experiments.
F.D., S.R., S.S., and P.G. analyzed the XMCD data. S.S. wrote
the multiplet calculation code and performed the simulations.
All authors discussed the results and participated in writing the
manuscript. A.J.H., P.G., and H.B. initiated and directed this
research. The authors declare that they have no competing
Materials and Methods
Figs. S1 to S7
3 March 2014; accepted 21 April 2014
Published online 8 May 2014;
ION CHANNEL STRUCTURE
Crystal structure of a
heterotetrameric NMDA receptor
Erkan Karakas and Hiro Furukawa*
N-Methyl-D-aspartate (NMDA) receptors belong to the family of ionotropic glutamate
receptors, which mediate most excitatory synaptic transmission in mammalian brains.
Calcium permeation triggered by activation of NMDA receptors is the pivotal event for
initiation of neuronal plasticity. Here, we show the crystal structure of the intact
heterotetrameric GluN1-GluN2B NMDA receptor ion channel at 4 angstroms. The NMDA
receptors are arranged as a dimer of GluN1-GluN2B heterodimers with the twofold
symmetry axis running through the entire molecule composed of an amino terminal
domain (ATD), a ligand-binding domain (LBD), and a transmembrane domain (TMD). The
ATD and LBD are much more highly packed in the NMDA receptors than non-NMDA
receptors, which may explain why ATD regulates ion channel activity in NMDA receptors
but not in non-NMDA receptors.
Brain development and function rely on euronal communication at a specialized junction called the synapse. In response to an action potential, neurotransmitters are released from the presynapse and activate
ionotropic and metabotropic receptors at the post-
synapse to generate a postsynaptic potential. Such
synaptic transmission is a basis for experience-
dependent changes in neuronal circuits. The
majority of excitatory neurotransmission in the
human brain is mediated by transmission of a
simple amino acid, L-glutamate (1), which activ-
ates metabotropic and ionotropic glutamate re-
ceptors (mGluRs and iGluRs, respectively). iGluRs
are ligand-gated ion channels that comprise
three major families, a-amino-3-hydroxy-5-methyl-4-
isoxazole propionic acid (AMPA) (GluA1-4), kai-
nate (GluK1-5), and N-Methyl-D-aspartate (NMDA)
receptors (GluN1, GluN2A-D, and GluN3A-B). Non-
NMDA receptors can form functional homo-
tetramers that respond only to L-glutamate. In
contrast, NMDA receptors are obligatory het-
erotetramers mainly composed of two copies each
of GluN1 and GluN2, which activate upon con-
current binding of glycine or D-serine to GluN1
and L-glutamate to GluN2 and relief of a mag-
nesium block of the ion channel pore by mem-
brane depolarization (2). Opening of NMDA
receptor channels results in an influx of calcium
ions that triggers signal transduction cascades
that control the strength of neural connectivity
or neuroplasticity. Hyper- or hypo-activation of
NMDA receptors is implicated in neurological
disorders and diseases including Alzheimer’s
Cold Spring Harbor Laboratory, W. M. Keck Structural Biology
Laboratory, One Bungtown Road, Cold Spring Harbor,
NY 11724, USA.
*Corresponding author. E-mail: email@example.com
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