Force amplification response of actin filaments under confined compression.

Actin protein is a major component of the cell cytoskeleton, and its ability to respond to external forces and generate propulsive forces through the polymerization of filaments is central to many cellular processes. The mechanisms governing actin's abilities are still not fully understood because of the difficulty in observing these processes at a molecular level. Here, we describe a technique for studying actin-surface interactions by using a surface forces apparatus that is able to directly visualize and quantify the collective forces generated when layers of noninterconnected, end-tethered actin filaments are confined between 2 (mica) surfaces. We also identify a force-response mechanism in which filaments not only stiffen under compression, which increases the bending modulus, but more importantly generates opposing forces that are larger than the compressive force. This elastic stiffening mechanism appears to require the presence of confining surfaces, enabling actin filaments to both sense and respond to compressive forces without additional mediating proteins, providing insight into the potential role compressive forces play in many actin and other motor protein-based phenomena.