Because the GluR6 and KA2 ATD heterodimer is formed with very high affinity, it is unlikely that this assembly undergoes large conformational changes in an intact receptor. Because the dimer is stabilized by contacts find more mediated by both the R1 and R2 domains, it is also unlikely that the individual subunits in a dimer could undergo substantial

changes in domain closure in response to small ligands. By contrast, although we can set only a lower limit of 3–5 μM on the Kd for formation of the tetrameric ATD dimer of dimers assembly, it is very likely that the tetramer is a dynamic assembly in which the two arms formed by ATD dimers can move relative to each other. It is thus tempting to speculate that if the ATD interacts with other proteins in the synaptic cleft, this could affect receptor clustering and mobility and in addition regulate ion channel activity via conformational changes propagated to the ligand binding domain. The AMPA receptor ATD has been reported to bind to N-cadehrins (Saglietti et al., 2007) and neuronal pentraxins thereby contributing to excitatory synapotogenesis (O’Brien et al., 1999, PARP inhibitor Ripley et al., 2011, Sia et al., 2007 and Xu et al., 2003). Likewise, the GluN1 ATD and the extracellular domain of EphB mediate

EphrinB and NMDA receptor interaction (Dalva et al., 2000 and Takasu et al., 2002). More recently, the Delta2 receptor ATD has been shown to form trans-synaptic interactions via cerebelin-1 Florfenicol precursor

protein and neurexin ( Matsuda et al., 2010 and Uemura et al., 2010). The exact nature and stoichiometry of these interactions is not known but will be influenced by the different stabilities of the high-affinity dimer and low-affinity dimer of dimers interfaces in individual iGluR subtypes. The GluR6 and KA2 ATDs were expressed in adherent and suspension cultures of wild-type HEK293T cells for SEC-UV/RI/MALS and AUC studies and purified as described previously (Kumar and Mayer, 2010 and Kumar et al., 2009). For crystallization the proteins were expressed in N-acetyl glucosaminyltransferase I-deficient GnTI− HEK293 cells and digested with Endo H (Reeves et al., 2002). Complete descriptions are given in Supplemental Experimental Procedures. X-ray diffraction data sets were collected using synchrotron radiation at the Advanced Photon Source (GM/CA CAT; beamline 23-ID-B) and were indexed, integrated, and scaled using HKL2000 (Otwinowski and Minor, 1997). The GluR6Δ1 homodimer and GluR6Δ1/KA2 heterodimer structures were solved by molecular replacement using the program PHASER (McCoy et al., 2007) and search probes composed of monomers of rat GluR6 (PDB ID: 3H6H) and KA2 (PDB ID: 3OM0) ATDs. The structures were iteratively built and refined with riding hydrogens using Coot (Emsley and Cowtan, 2004) and Phenix (Adams et al.