An Interdomain Linker Increases the Thermostability and Decreases the Calcium Affinity of the Calmodulin N-Domain^†

A hydrophobic core is a widely accepted determinant of protein stability. However, regulatory proteins undergoing ligand-induced conformational switching may expose interior residues to solvent and cannot afford to be extremely rigid. Optimizing the energetic balance between stability and binding is challenging. The addition of five interdomain residues to rat and Paramecium calmodulin N-domain fragments (residues 1−75) increased their thermostability by 9 °C and lowered their calcium affinity by a factor of 4. This demonstrates that the flexible linker regulates functional properties as well as tethering the neighboring domains and that protein stability may be increased markedly by minor modifications of the C-terminus. The sensitivity of this domain to few and conservative variations in helices A and D (D2E, S17A, T70S and M71L) is demonstrated by the rat CaM fragments having lower stability and higher calcium affinity than fragments of the same length derived from Paramecium CaM.