Force assay for cell motility

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The functional and dynamic activity of the plasma membrane is revealed by temporal variation of experimentally measured parameters. For example, changes in flux through ion-channels is observed by measuring ionic currents; displacement currents reveal movement of voltage sensors; the diffusivity of membrane components is measured upon tracking them with targeted fluorescent probes. Here we measure the change in membrane force arising from work done by chemical motors in the cytoskeleton at the plasma membrane, and show that this force reveals functional information on the remodeling networks within the cytoplasm at the membrane. In our assay we use optical tweezers to monitor the magnitude and time course of the membrane force at the tip of an F-actin filled membrane tube where the geometry (radius: 100-200 nm, axial length: 10 to 20 µm) limits the area of the probed network and isolates it spatially from the soma. We describe one transient observed within the force signal –a sawtooth- and reasoned that it arose from de-polymerization and polymerization of F-actin that occurred at the tip of this protrusion. We estimate the on- and off-rates of monomeric actin, the number of working filaments and the force per filament from the magnitude and time course of the membrane force. If our reasoning is correct then this was the first time these parameters were determined in a living cell. This signature transient has since been reported by others upon monitoring the membrane force at tips of filopodia.

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