CaM domains involved in constitutive binding to SK2 α subunits. A, Fragments of CaM tested for interaction with SK2 ABC domains in the yeast two-hybrid assay. The C-terminal 70 amino acids of CaM showed complementation, but removal of residues 78–84 or 144–148 from this fragment resulted in the loss of the interaction. B, E-F hands are not required for the constitutive interaction between CaM and SK2 ABC in the two-hybrid assay. Wild-type CaM or CaM with the D→A mutation in the indicated E-F hands were tested for interaction with the ABC domain of SK2 in the two-hybrid assay. The interaction was detected in all cases.