Single-molecule force spectroscopy reveals force-enhanced binding of calcium ions by gelsolin

Force is increasingly recognized as an important element in controlling biological processes.
Forces can deform native protein conformations leading to protein-specific effects.
Protein–protein binding affinities may be decreased, or novel protein–protein interaction sites
may be revealed, on mechanically stressing one or more components. Here we demonstrate
that the calcium-binding affinity of the sixth domain of the actin-binding protein gelsolin (G6)
can be enhanced by mechanical force. Our kinetic model suggests that the calcium-binding
affinity of G6 increases exponentially with force, up to the point of G6 unfolding. This implies
that gelsolin may be activated at lower calcium ion levels when subjected to tensile forces.
The demonstration that cation–protein binding affinities can be force-dependent provides
a new understanding of the complex behaviour of cation-regulated proteins in stressful
cellular environments, such as those found in the cytoskeleton-rich leading edge and at
cell adhesions.