This study constitutes the first description of an unconventional MAGUK protein, CASK, which directly interacts with NaV1.5 channel and controls its surface expression at the lateral membrane by regulating ion channel trafficking.

Background: Membrane-associated guanylate kinase (MAGUK) proteins function as adaptor proteins to mediate the recruitment and scaffolding of ion channels in the plasma membrane in various cell types. In the heart, the protein CASK (Calcium/CAlmodulin-dependent Serine protein Kinase) negatively regulates the main cardiac sodium channel, NaV1.5, which carries the sodium current (INa) by preventing its anterograde trafficking. CASK is also a new member of the dystrophin-glycoprotein complex, and like syntrophin, binds to the C-terminal domain of the channel. Objective: To unravel the mechanisms of CASK-mediated negative INa regulation, and interaction with the dystrophin-glycoprotein complex in cardiac myocytes. Methods: CASK adenoviral truncated constructs with sequential single functional domain deletions were designed for overexpression in cardiac myocytes: CASK CAMKII , CASK L27A , CASK L27B , CASK PDZ , CASK SH3 , CASK HOOK , and CASK GUK. Combination of whole-cell patch-clamp recording, total internal reflection fluorescence microscopy (TIRFm) and biochemistry experiments were conducted in cardiac myocytes to study the functional consequences of domain deletions. Results: We show that both L27B and GUK domains are required for the negative regulatory effect of CASK on INa and NaV1.5 surface expression and that the HOOK domain is essential for interaction with the cell adhesion dystrophin-glycoprotein complex. Conclusions: This study demonstrates that the multi-modular structure of CASK confers an ability to simultaneously interact with several targets within cardiomyocytes. Through its L27B, GUK, and HOOK domains, CASK potentially provides the ability to control channel delivery at adhesion points in cardiomyocyte.