Atomic Force Microscopy Study of the Conformational Change in Immobilized Calmodulin

Maintaining the biological functionality of immobilized proteins is the key to the success of numerous protein-based biomedical devices. To that end, we studied the conformational change in calmodulin (CaM) immobilized on chemical patterns. 1-Cysteine-mutated calmodulin was immobilized on a mercapto-terminated surface through cysteine-Hg-mercapto coupling. Utilizing atomic force microscopy (AFM), the average height of immobilized calmodulin was determined to be 1.87 ± 0.19 nm. After incubation in EGTA solution, the average height of the protein changed to 2.26 ± 0.21 nm, indicating the conformational change of CaM to Apo-CaM. Immobilized CaM also demonstrated a conformational change upon the reaction with known calmodulin antagonist chlorpromazine (CPZ). After incubation in CPZ solution, the average height of CPZ-bound CaM increased to 2.32 ± 0.20 nm, demonstrating that immobilized CaM has a similar response to that in bulk solution. These results show that the immobilization of calmodulin on a solid support does not interfere with the ability of the protein to bind calcium and calmodulin antagonists. Our results demonstrate the feasibility of employing AFM to probe and understand protein conformational changes.