MOLECULAR DISSECTION OF THE AGONIST BINDING-SITE OF AN AMPA RECEPTOR

Two discontinuous segments (S1 and S2), separated by membrane-associat ed domains, in ionotropic glutamate receptor (GluR) subunits show sequ ence similarity to bacterial periplasmic amino acid-binding proteins, suggesting an evolutionary and structural relationship, Experimental e vidence arguing for and against the inferred extracellular location of the S1 and S2 domains in GluRs has been presented. Here, we report th at an extracellularly expressed fusion protein consisting of the S1 an d S2 domains of a-amino-5-methyl-3-hydroxyisoxazolone-4-propionate (AM PA)-selective glutamate receptor GluR-D joined together via a hydrophi lic linker peptide specifically reproduces the AMPA-binding properties of GLuR-D, whereas the separately expressed segments do not bind liga nd. This provides direct evidence that the S1 and S2 segments of GluR- D contain the structural determinants necessary and sufficient for sel ective agonist binding, Dissection of a functional neurotransmitter bi nding site as a soluble protein separate from the integral membrane ch annel will facilitate new approaches to analyse the structure of GluRs .