When I read papers like the one that gave us today’s image, I think that one day my wish of jumping into a cell to float in the cytoplasm wearing goggles and swimmies may actually come true. Physical manipulation of proteins gets me so excited about how far our tools and technology have come. In this paper, biologists physically yanked on actin filaments to show how tension affects the presence and function of an actin-modulating protein.
Actin has many regulatory proteins that do a variety of things, such as promoting filament nucleation, branching, and severing. Cofilin is a ubiquitous protein that functions in actin filament severing and reorganization. Cofilin binds to the actin filament itself and induces a slight twist in the actin, which makes it easily severed. A recent paper describes the use of optical tweezers and manipulations to show that the binding of cofilin to actin, and in turn its severing of actin, is regulated by tension in the filaments. Hayakawa and colleagues bound one end of an actin filament to a glass coverslip and manipulated the other end using optical tweezers. When the filament was put under tension, the actin filament was not severed (or it took longer, in some cases). In another manipulation, a fine glass pipette was used to pull bundles of actin. Tension applied to the actin filaments caused a drop in the binding rate of cofilin to actin filaments, as seen in the images above. Top row shows actin (left) and cofilin (right, fat arrows) in a tension-relieved actin bundle, while bottom row shows actin and reduced cofilin binding in an actin bundle that was stretched by 20%.
Hayakawa, K., Tatsumi, H., & Sokabe, M. (2011). Actin filaments function as a tension sensor by tension-dependent binding of cofilin to the filament originally published in The Journal of Cell Biology, 195 (5), 721-727 DOI: 10.1083/jcb.201102039