The Ca2+ Influence on Calmodulin Unfolding Pathway: A Steered Molecular Dynamics Simulation Study

The force-induced unfolding of calmodulin (CaM) was investigated at atomistic details with steered molecular dynamics. The two isolated CaM domains as well as the full-length CaM were simulated in N-C-terminal pulling scheme, and the isolated N-lobe of CaM was studied specially in two other pulling schemes to test the effect of pulling direction and compare with relevant experiments. Both Ca2+-loaded CaM and Ca2+-free CaM were considered in order to define the Ca2+ influence to the CaM unfolding. The results reveal that the Ca2+ significantly affects the stability and unfolding behaviors of both the isolated CaM domains and the full-length CaM. In Ca2+-loaded CaM, N-terminal domain unfolds in priori to the C-terminal domain. But in Ca2+-free CaM, the unfolding order changes, and C-terminal domain unfolds first. The force-extension curves of CaM unfolding indicate that the major unfolding barrier comes from conquering the interaction of two EF-hand motifs in both N- and C- terminal domains. Our results provide the atomistic-level insights in the force-induced CaM unfolding and explain the observation in recent AFM experiments.