Roles of the C-terminal residues of calmodulin in structure and function

Electrospray ionization mass spectrometry (ESI-MS), circular dichroism (CD), nuclear magnetic resonance (NMR) spectroscopy, flow dialysis, and bioactivity measurements were employed to investigate the roles of the C-terminal residues of calmodulin (CaM). In the present study, we prepared a series of truncated mutants of chicken CaM that lack four (CCMΔ4) to eight (CCMΔ8) residues at the C-terminal end. It was found that CCMΔ4, lacking the last four residues (M145 to K148), binds four Ca²⁺ ions. Further deletion gradually decreased the ability to bind the fourth Ca²⁺ ion, and CCMΔ8 completely lost the ability. Interestingly, both lobes of Ca²⁺-sturated CCMΔ5 showed instability in the conformation, although limited part in the C-lobe of Ca²⁺-saturated CCMΔ4 was instable. Moreover, unlike CCMΔ4, structure of the C-lobe in CCMΔ5 bound to the target displayed dissimilarity to that of CaM, suggesting that deletion of M144 changes the binding manner. Deletion of the last five residues (M144 to K148) and further truncation of the C-terminal region decreased apparent capacity for target activation. Little contribution of the last four residues including M145 was observed for structural stability, Ca²⁺-binding, and target activation. Although both M144 and M145 have been recognized as key residues for the function, the present data suggest that M144 is a more important residue to attain Ca²⁺ induced conformational change and to form a proper Ca²⁺-saturated conformation.