Glutamate gates NMDA, AMPA and kainate ionotropic receptors. For kainate receptors, comparatively little is known about how their function is regulated by subunit composition or anchoring molecules such as postsynaptic density proteins (PSD/SAP). Our recent work demonstrates that PSD-95/SAP9O colocalizes with kainate receptors at synapses, binds the kainate receptor KA2 and GIuR6 subunits, and significantly modifies GluR6 and KA2/G1uR6 physiology (Garcia et al., 1998). We find that tyrosine phoshorylation correlates with the modification of physiological response and that potential regulatory molecules, src and mss4 (a guanine nucleotide exchange factor) preferentially associate with, respectively, KA2 and PSD-95/SAP9O. Furthermore, we also find that both associations are regulated by the KA2-PSD-95/SAP9O interaction, and additionally that SAP97 is unable to bind KA2-containing receptors. Together, the results imply that PSD proteins may function not only in anchoring receptors at synaptic Sites, but also in regulating receptor activity and which receptor subtypes are present in a given site. This proposal will establish mechanisms by which PSD-95/SAP9O regulates kainate receptor function and targeting. We will assess how it regulates receptor activity and if regulation depends on receptor subunit composition, determining agonist affinities, desensitization properties and sensitivity to inhibitors. For potential mechanisms, we will examine how binding of ligands, such as GKAP, to the GK domain may promote the association of the SH3 domain of PSD-95 with KA2, the molecular sites on KA2 and PSD-95 that determine tyrosine phosphorylation, as well as the sites on mss4 and PSD-95/SAP9O that regulate mss4-PSD-95/SAP9O-KA2 interactions will be identified. The physiological effects of these interactions and those of src on KA2 and PSD-95, will be established by expressing active and inactive enzymes with receptor subunits (wild-type vs. mutated). To determine potential mss4 regulation of receptor targeting, the surface expression of receptors and receptor distribution (clustered vs. unclustered) will be evaluated. These studies are expected to provide novel information on how functional properties can be determined through mechanisms controlling the clustering/targeting of receptors of specific subunit composition.
