Structural basis for the Ca²⁺/CaM-mediated regulation of CASK-CaMK

As a member of the Ca²⁺/CaM-dependent protein kinase family, CASK contains an N-terminal CaMK domain that is regulated by Ca²⁺-bound CaM, while the underlying mechanism remains to be elucidated. Here, we determine the crystal structures of CASK-CaMK in different states: apo CASK-CaMK, CASK-CaMK in complex with CaM (the CaM-CASK complex) and CASK-CaMK in complex with CaM and Mint1 (the CaM-CASK-Mint1 complex). CASK-CaMK exhibits an inhibitory conformation with the αR2 helix of the autoregulatory domain (ARD) inserting into its nucleotide-binding pocket. Contrary to conventional CaM-mediated binding paradigms, in the CaM-CASK complex, only the C-terminal lobe of CaM (C-CaM) engages with the ARD of CASK-CaMK. This C-CaM binding induces the formation of an extended ARD α-helix that reshapes the nucleotide-binding pocket of CASK-CaMK to enhance its nucleotide-binding capacity. Correspondingly, in the CaM-CASK-Mint1 complex, similar C-CaM binding to CASK-CaMK leads to a slight opening of the CASK-Mint1 complex, and only the Mint1-CID (CASK-interaction domain) core is resolved association with CASK-CaMK. Taken together, CaM likely regulates CASK-CaMK through a C-CaM-dependent mechanism to tune its nucleotide-binding and target-recognition capacities.