Structure and function of AMPA receptors

Abstract

The application of X-ray diffraction has allowed the structure of the ligand-binding core of AMPA receptors to be determined. Here I review the insights that this has given into the molecular mechanisms of activation and desensitization of these receptors.

Figure 1. Schematic diagram of the domain architecture of single GluR2 receptor polypeptide

The amino terminus, which begins the amino terminal domain (ATD), is located extracellularly. The ligand-binding core, also located in the extracellular space is composed of discontinuous polypeptide segments S1 and S2. The ion channel is formed by the membrane-embedded domains 1, P, 2 and 3 while the carboxy terminal domain (CTD) is located on the inside of the cell. The GluR2 S1S2 constructs are generated by deleting the ATD, coupling the end of S1 to the beginning of S2 via a Gly-Thr linker and deleting the final transmembrane segment by ending the polypeptide near the end of S2.

Figure 2. The GluR2 S1S2 ligand-binding core dimer

A, view of the dimer perpendicular to the molecular two-fold axis with protomer A on the left and protomer B on the right. The two juxtamembrane linkers are on the ‘bottom’ of the dimer and the amino terminus of S1 is on the ‘top’. Residue 483, which when changed from a leucine to a tyrosine gives rise to a non-desensitizing phenotype, is located in the dimer interface and is sandwiched between Leu748 and Lys 752 on helix J of the two-fold-related subunit. B, view of the dimer from the ‘top’, parallel to the molecular two-fold axis, showing the two symmetry-related interaction sites between the protomers (Sun et al. 2002).

Figure 3. Liganded states of the GluR2 S1S2 L483Y dimer

During the rearrangement between the antagonist-bound state and the agonist-bound state, the ‘linker’ regions that are proximal to the ion channel gate undergo an increase in their separation of ca 8 Å. Shown here are two different liganded states of the GluR2 S1S2 L483Y dimer. In red is the structure of the complex formed with the antagonist DNQX and in green is the structure of the complex formed with AMPA. Here, we suggest that the domain closure of each subunit that occurs upon agonist binding is coupled to a separation of the linker regions and that it is this conformational change that opens the ion channel.

Figure 4. Strength of the dimer is inversely correlated with the free energy of receptor desensitization

A graph showing that the strength of the dimer interface, as measured by the free energy of dimer dissociation, is inversely correlated to the free energy of receptor desensitization. In other words, the more tightly the dimer is held together, the less favourable is receptor desensitization. Mutations like L483Y and the small molecule cyclothiazide block desensitization by stabilizing the dimer interface, preventing it from rearranging, and thus ‘forcing’ the receptor to exclusively couple the conformational change of domain closure to the gating or opening of the ion channel.

Figure 5. Schematic diagram showing the relationships between the conformational changes at the ligand-binding core, the dimer interface and the ion channel gate

Here, only two subunits of the tetrameric receptor are shown. One subunit is in front, with the ligand-binding domains in grey and blue, and the second, two-fold-related subunit is at the back, with the ligand-binding domains in pink and purple. Glutamate is schematized as a red sphere, binding in the cleft between domain 1 (D1) and domain 2 (D2) (Sun et al. 2002). In this figure, the mechanism shows the receptor entering a desensitized state from either a closed state or from an open state, although other mechanisms have been suggested.