Assembly and ligand binding properties of the water-soluble extracellular domains of the glutamate receptor 1 subunit

High resolution structural studies of models of glutamate receptors (GluRs) have been limited to monomeric models of the ligand-binding site, To obtai n oligomeric models of glutamate receptors that can reveal more complete st ructural information, we examined the assembly and ligand binding propertie s of two truncated versions of the GluR1 subunit, The first version, GluR1- WS, consisted of only the N-terminal extracellullar segment (Ala(1)-Glu(520 )) bridged by a synthetic linker to the second extracellular domain (Asn(61 5)-Gly(790)). The second version, GluR1-M1, consisted of the first N-termin al extracellular domain (Ala(1)-Glu(520)) bridged by a synthetic linker to a second segment containing the second extracellular domain, the third tran smembrane domain, and the intracellular C-terminal domain (Asn(615)-Leu(889 )). When expressed in Xenopus oocytes, GluR-WS was secreted and water-solub le; GluR1-M1 was displayed on the surface of oocytes, GluR1-WS exhibited a velocity sedimentation profile that was consistent with assembly of homooli gomers and bound the glutamate receptor agonist alpha -amino-3-hydroxy-5-me thyl-4-isoxazole propionic acid with high affinity, These findings show tha t the extracellular domains of GluR1 that are sufficient for ligand binding apparently are sufficient for subunit assembly and might be a suitable tar get for structural studies of a water-soluble GluR1 oligomer.