CASK has been implicated in synaptic protein targeting, synaptic organization, and transcriptional regulation. Here, three more CASK associated proteins, GRIP1, PKCepsilon, and RGS4, were initially identified by immunoprecipitation and mass analysis, and confirmed by immunoprecipitation-immunoblotting assay using rat brain extract. Via the interaction with GRIP1, GluR2/3 was also co-immunoprecipitated by CASK antibody from rat brain. The PDZ and SH3-GK domains of CASK were demonstrated as the associated domains for GRIP1 and PKCepsilon, respectively. The associations between CASK, PKCepsilon, and RGS4 were up-regulated in the adult brain compared with postnatal day 11 rat brain. In contrast, the associations of CASK with Mint1, GRIP1, and GluR2/3 were down-regulated in the adult brain. These results suggest that CASK protein complex is developmentally regulated by unknown signals. In conclusion, our study suggests that the CASK protein complex not only functions as a scaffold but also recruits signaling molecules and may contribute to signal transduction.