A Multiprotein Trafficking Complex Composed of SAP97, CASK, Veli, and Mint1 Is Associated with Inward Rectifier Kir2 Potassium Channels

Abstract

Strong inward rectifier potassium (Kir2) channels are important in the control of cell excitability, and their functions are modulated by interactions with intracellular proteins. Here we identified a complex of scaffolding/trafficking proteins in brain that associate with Kir2.1, Kir2.2, and Kir2.3 channels. By using a combination of affinity interaction pulldown assays and co-immunoprecipitations from brain and transfected cells, we demonstrated that a complex composed of SAP97, CASK, Veli, and Mint1 associates with Kir2 channels via the C-terminal PDZ-binding motif. We further demonstrated by using in vitro protein interaction assays that SAP97, Veli-1, or Veli-3 binds directly to the Kir2.2 C terminus and recruits CASK. Co-immunoprecipitations indicated that specific Veli isoforms participate in forming distinct protein complexes in brain, where Veli-1 stably associates with CASK and SAP97, Veli-2 associates with CASK and Mint1, and Veli-3 associates with CASK, SAP97, and Mint1. Additionally, immunocytochemistry of rat cerebellum revealed overlapping expression of Kir2.2, SAP97, CASK, Mint1, with Veli-1 in the granule cell layer and Veli-3 in the molecular layer. We propose a model whereby Kir2.2 associates with distinct SAP97-CASK-Veli-Mint1 complexes. In one complex, SAP97 interacts directly with the Kir2 channels and recruits CASK, Veli, and Mint1. Alternatively, Veli-1 or Veli-3 interacts directly with the Kir2 channels and recruits CASK and SAP97; association of Mint1 with the complex requires Veli-3. Expression of Kir2.2 in polarized epithelial cells resulted in targeting of the channels to the basolateral membrane and co-localization with SAP97 and CASK, whereas a dominant interfering form of CASK caused the channels to mislocalize. Therefore, CASK appears to be a central protein of a macromolecular complex that participates in trafficking and plasma membrane localization of Kir2 channels.