The structure and function of glutamate receptors: Mg2+ block to X-ray diffraction

Abstract

Experiments on the action of glutamate on mammalian and amphibian nervous systems started back in the 1950s but decades passed before it became widely accepted that glutamate was the major excitatory neurotransmitter in the CNS. The pace of research greatly accelerated in the 1980s when selective ligands that identified glutamate receptor subtypes became widely available, and voltage clamp techniques, coupled with rapid perfusion, began to resolve the unique functional properties of what cloning subsequently revealed to be a large family of receptors with numerous subtypes. More recently the power of X-ray crystallography and cryo-EM has been applied to the study of glutamate receptors, revealing their atomic structures, and the conformational changes that underlie their gating. In this review I summarize the history of this field, viewed through the lens of a career in which I spent 3 decades working on the structure and function of glutamate receptor ion channels. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Keywords: Glutamate receptors; Structural biology.

Fig. 1

Mark Mayer and Gary Westbrook photographed by Phil Nelson in the fall of 1986 prior to the laboratory’s 4-year review.

Fig. 2

Voltage dependent Ca²⁺ influx through NMDA receptor channels measured under voltage clamp using arsenazo III. (A) Titration of arsenazo III with increasing concentrations of Ca²⁺ measured in a benchtop spectrophotometer. (B) Responses to NMDA with a low extracellular concentration of Mg²⁺ recorded from a spinal cord neuron under voltage clamp over the membrane potential range −100 to +40 mV. (C) The simultaneously recorded increase in [Ca²⁺]_i measured as a change in differential transmittance for the wavelength pair 570 – 660 nm.

Fig. 3

The ligand binding site of glutamate receptor ion channels. (A) The three point receptor hypothesis of Curtis and Watkins in which in 1960 they proposed that the α-carboxyl, α-amino and γ-carboxyl groups of glutamate bind to charge complimentary sites in the receptor. (B) The 1.65 Å resolution crystal structure of the GluK2 ligand site (PDB 1S50) revealed the molecular identity of these binding sites, but with one difference. There is no counter charge for the glutamate γ-carboxyl group, which instead binds via hydrogen bonds to a main chain amide, to the hydroxyl group of Thr659, and to a hydrogen bonded network of molecules which are trapped in the ligand binding cavity together with glutamate. Displacement of these water molecules allows the binding of large heterocyclic amino acids such as quisqualic acid and AMPA.