Synaptic targeting and localization of Discs-large is a stepwise process controlled by different domains of the protein

Background: Membrane-associated guanylate kinases (MAGUKs) assemble ion cha nnels, cell-adhesion molecules and components of second messenger cascades into synapses, and are therefore potentially important for co-ordinating sy naptic strength and structure. Here, we have examined the targeting of the Drosophila MAGUK Discs-large (DLG) to larval neuromuscular junctions. Results: During development, DLG was first found associated with the muscle subcortical compartment and plasma membrane, and later was recruited to th e postsynaptic membrane. Using a transgenic approach, we studied how mutati ons in various domains of the DLG protein affect DLG targeting. Deletion of the HOOK region - the region between the Src homology 3 (SH3) domain and t he guanylate-kinase-like (GUK) domain - prevented association of DLG with t he subcortical network and rendered the protein largely diffuse. Loss of th e first two PDZ domains led to the formation of large clusters throughout t he plasma membrane, with scant targeting to the neuromuscular junction. Pro per trafficking of DLG missing the GUK domain depended on the presence of e ndogenous DLG. Conclusions: Postsynaptic targeting of DLG requires a HOOK-dependent associ ation with extrasynaptic compartments, and interactions mediated by the fir st two PDZ domains. The GUK domain routes DLG between compartments, possibl y by interacting with recently identified cytoskeletal-binding partners.