In vertebrates the vast majority of the excitatory neurotransmission is mediated by glutamate receptors. Are they stand-alone molecules? How is their function regulated to ensure proper signaling for normal physiology and behavior? To address these questions I have undertaken combined molecular, genetic, electrophysiological and behavioral analyses centered on GLR-1 AMPA-class ionotropic glutamate receptor (iGluR), one of the major components of glutamate receptor signaling in simple nematode Caenorhabditis elegans. This dissertation describes my identification and characterization of two families of GLR-1 auxiliary subunits.

By taking both reverse and forward genetic approaches I isolated STG-1 and STG-2, two TARPs (Transmembrane AMPA receptor Regulatory Proteins), as the essential set of obligate accessory proteins for worm GLR-1 AMPA receptors. I showed that the elimination of both STG-1 and STG-2 causes disruption of AMPAR-mediated synaptic currents and worm behavior despite normal surface expression and clustering of the receptors. By reconstituting AMPAR function in heterologous systems, I demonstrated that both STG-1 and STG-2 can functionally substitute for vertebrate TARPs to modify receptor function, indicative of their evolutionarily conserved role. In addition, I also showed that STG-1 and STG-2 differentially alter the kinetics of C. elegans GLR-1 and vertebrate GluR1 (GluA1) receptors.

By taking a forward genetic approach in a sensitized genetic background, I identified a CUB-domain protein - SOL-2 that contributes to the function of GLR-1 AMPARs. I showed that the deletion of SOL-2 significantly reduces the AMPAR-mediated currents and elicits behavioral changes. I also demonstrated that SOL-2 is required to bind to and stabilize the extracellular part of SOL-1, a previously identified obligate auxiliary subunit of GLR-1, and this binding complex co-localizes with GLR-1. These data suggest that a protein complex containing CUB-domains has a scaffolding role and contributes to the function of AMPARs.

Together, I identified two families of accessory proteins - TARPs and CUB-domain proteins that are required for the function of AMPARs, supporting the hypothesis that glutamate receptors exert their function by forming signaling complexes with auxiliary subunits instead of working alone.