Probing Activated and Non-Activated Single Calmodulin Molecules under a Piconewton Compressive Force

Interrogating the protein structure–function inter-relationship under a piconewton force manipulation has been highly promising and informative. Although protein conformational changes under pulling force manipulations have been extensively studied, protein conformational changes under a compressive force have not been explored in detail. Using our home-modified sensitive and high signal-to-noise atomic force microscopy (AFM) approach, we have applied a piconewton compressive force, manipulating a Calmodulin (CaM) molecule to characterize two different forms of CaM, the Ca2+-ligated activated form and the Ca2+ free non-activated form (apo-CaM). We observed sudden and spontaneous structural rupture of apo-CaM under compressive force applied by an AFM tip, though no such events were recorded in the case of Ca2+-ligated activated CaM form. The sudden spontaneous structural rupture under a piconewton force compression has never been reported before, which presents an unexplored function that is likely important for protein–protein interactions and cell signaling functions.