Mechanisms for ligand binding to GluR0 ion channels: Crystal structures ofthe glutamate and serine complexes and a closed Apo state

High-resolution structures of the ligand binding core of GluR0, a glutamate receptor ion channel from Synechocystis PCC 6803, have been solved by X-ra y diffraction. The GluR0 structures reveal homology with bacterial periplas mic binding proteins and the rat GluR2 AMPA subtype neurotransmitter recept or. The ligand binding site is formed by a cleft between two globular alpha /beta domains. L-Glutamate binds in an extended conformation, similar to th at observed for glutamine binding protein (GlnBP). However, the L-glutamate gamma -carboxyl group interacts exclusively with Asn51 in domain 1, differ ent from the interactions of ligand with domain 2 residues observed for Glu R2 and GlnBP. To address how neutral amino acids activate GluR0 gating we s olved the structure of the binding site complex with L-serine. This reveale d solvent molecules acting as surrogate ligand atoms, such that the serine OH group makes solvent-mediated hydrogen bonds with Asn51. The structure of a ligand-free, closed-cleft conformation revealed an extensive hydrogen bo nd network mediated by solvent molecules. Equilibrium centrifugation analys is revealed dimerization of the GluR0 ligand binding core with a dissociati on constant of 0.8 muM. In the crystal, a symmetrical dimer involving resid ues in domain 1 occurs along a crystallographic 2-fold axis and suggests th at tetrameric glutamate receptor ion channels are assembled from dimers of dimers. We propose that ligand-induced conformational changes cause the ion channel to open as a result of an increase in domain 2 separation relative to the dimer interface.