, 2002). Thus, the role of GABARAP and associated proteins in GABAAR targeting to the synapse is likely to be indirect, possibly through stabilizing the γ-subunit-containing intracellular pools of these receptors. Another GABAAR binding protein
that specifically associates with γ-subunits is GODZ (Golgi-specific DHHC zinc finger domain protein; Keller et al., 2004). This protein regulates palmitoylation of γ-subunits, and is required for the assembly of GABAARs and their transport to the cell see more surface (Fang et al., 2006). This protein is, however, also located away from the postsynaptic membrane, within the Golgi apparatus, and unlikely therefore to play a direct role in GABAAR-targeting to the synapse. Paradoxically, direct association between GABAARs and proteins such as gephyrin that clearly co-localize with them at synapses has traditionally been difficult to demonstrate using biochemical approaches. Gephryn is highly enriched at GABAergic synapses, forming submembraneous aggregates due to its self-association into trimers and dimers mediated by its N-terminal G-domains and C-terminal E-domains (Sola et al., 2001; Schwarz et al., 2001). It is unclear whether gephyrin interacts with GABAARs directly, via low-affinity binding, such as its binding to the α2-subunit (Tretter et al., 2008), or indirectly, via as yet unidentified GABAAR-associated proteins, or both. While direct interaction
with GABAARs remains to be confirmed in vivo, the role of gephryn in synaptic localization of GABAARs is strongly supported ABT199 by prominent loss of α2- and γ-subunit-containing synaptic pools in gephyrin-knockout
mice (Kneussel et al., 1999). Gephyrin interacts with a number of other proteins including collybistin, a guanylate exchange factor (GEF) for Cdc42 (Kins et al., 2000), cytoskeletal protein tubulin (Prior et al., GABA Receptor 1992), tubulin-associated protein dynein light chain (DLC; Fuhrmann et al., 2002), the actin-binding proteins profilin I and II (Mammoto et al., 1998), actin-associated proteins Mena and VASP (Giesemann et al., 2003) and a glutamate receptor-associated protein GRIP-1 (Yu et al., 2008). Of these, the gephyrin–collybistin interaction is the best characterized (Harvey et al., 2004; and see above). This correlates well with the phenotype of collybistin-knockout mice. These mice have increased levels of anxiety and impaired spatial learning associated with a selective loss of GABAARs in the hippocampus and basolateral amygdala (Papadopoulos et al., 2007). Reversible low-affinity interactions between GABAARs and gephyrin at the synapse may be necessary for the observed high mobility of GABAARs within the plane of the plasma membrane (Jacob et al., 2005; Lévi et al., 2008). Using a variety of imaging techniques, GABAARs have been shown to diffuse rapidly, in and out of synaptic contact regions (Jacob et al., 2005; Thomas et al., 2005; Bogdanov et al., 2006).