The scaffolding protein CASK mediates the interaction between rabphilin3a and β-neurexins

CASK, a member of the membrane-associated guanylate kinase (MAGUK) superfamily, binds to the carboxyl-terminus of β-neurexins on the intracellular side of the presynaptic membrane. The guanylate kinase-like (GUK) domains of MAGUKs lack kinase activities, but might be important for mediating specific protein–protein interaction. By a yeast two-hybrid approach, we identified an interaction between the GUK domain of CASK and the C2B domain of rabphilin3a, a presynaptic protein involved in synaptic vesicle exocytosis. The interaction was confirmed by in vitro GST pull-down and co-immunoprecipitation assays. It was proposed that presynaptic vesicles might be guided to the vicinity of points of exocytosis defined by β-neurexins via the interaction between rabphilin3a–CASK–β-neurexins.