Structural modeling of the interactions between CaM mutants and SK2 CaMBD. (A) Schematic of the four α-helices within the C-lobe of CaM. Location of mutations are shown by labels and colored markers. (B–D) Comparisons of CaM_WT (green) and CaM_F90L (red; B), CaM_F93L (yellow; C), and CaM_F142L (purple; D). Side chains of key amino acid residues are shown in stick representation using the color scheme shown in A. Conformational changes due to CaM mutation are indicated by black arrows in each panel. (E) C-lobe of apo-CaM_WT (colored in green) bound to the C terminus of hSK2 channel (colored in light brown). Side chains of key amino acids are shown using space-filling representation. (F) Panel E rotated 90° to the left around the y axis. Molecular modeling was performed in Ca²⁺-free conditions.