The regulated localization of ␣-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors (AMPARs) to synapses is an important component of synaptic signaling and plasticity. Regulated ubiquitination and endocytosis determine the synaptic levels of AMPARs, but it is unclear which factors conduct these processes. To identify genes that regulate AMPAR synaptic abundance, we screened for mutants that accumulate high synaptic levels of the AMPAR subunit GLR-1 in Caenorhabditis elegans. GLR-1 is localized to postsynaptic clusters, and mutants for the BTB-Kelch protein KEL-8 have increased GLR-1 levels at clusters, whereas the levels and localization of other synaptic proteins seem normal. KEL-8 is a neuronal protein and is localized to sites adjacent to GLR-1 postsynaptic clusters along the ventral cord neurites. KEL-8 is required for the ubiquitin-mediated turnover of GLR-1 subunits, and kel-8 mutants show an increased frequency of spontaneous reversals in locomotion, suggesting increased levels of GLR-1 are present at synapses. KEL-8 binds to CUL-3, a Cullin 3 ubiquitin ligase subunit that we also find mediates GLR-1 turnover. Our findings indicate that KEL-8 is a substrate receptor for Cullin 3 ubiquitin ligases that is required for the proteolysis of GLR-1 receptors and suggest a novel postmitotic role in neurons for Kelch/CUL3 ubiquitin ligases.
INTRODUCTIONGlutamate is the most abundant excitatory neurotransmitter in the brain, and glutamatergic synapses play a critical role in learning, memory, and developmental plasticity of the central nervous system (Meldrum, 2000). Ionotropic glutamate receptors (GluRs) receive and transduce glutamatergic signals on the postsynaptic face of synapses, where these multitransmembrane spanning proteins assemble into tetrameric glutamate-gated channels of differing subunit composition (Hollmann and Heinemann, 1994;Dingledine et al., 1999). The regulation of these receptors, the ␣-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) type in particular, is a critical mechanism by which neurons modulate synaptic strength (Bredt and Nicoll, 2003;Malenka, 2003;Malinow, 2003;Sheng and Hyoung Lee, 2003). In addition, GluRs play a role in several diseases of the nervous system, ranging from neurodegeneration to drug addiction and schizophrenia (Tzschentke, 2002;Mattson, 2003;Moghaddam, 2003;Aarts and Tymianski, 2004). To better understand how AMPA-type GluRs (AMPARs) function in the nervous system, it is essential to determine how the synaptic level and activity of AMPARs are regulated.AMPAR synaptic levels are controlled by proteins that interact with signaling elements on the carboxy-terminal tail sequences that are exposed to the cytosol (Dong et al., 1997;Rongo et al., 1998;Song et al., 1998;Srivastava et al., 1998;Xia et al., 1999;Sans et al., 2001). Although some of the tail sequence signaling elements function to maintain high levels of AMPARs at the synapse, other signaling elements recruit factors that result in the ubiquitination, endocytosis, ...