Myosin II–independent contraction of actin filaments in membrane nanotubes

The ability to generate active stresses within filamentous actin matrices is a fundamental and evolutionarily conserved process driving locomotion and morphogenetic changes in cells. The generation of pushing forces by actin polymerization is reasonably well understood, and is known to drive lamellipodia based motility and filopodial extension. Actin filaments decorated with myosin motors can also generate contractile stresses as in the cell cortex or in cytokinetic rings. In this article we use membrane nanotubes pulled out of axonal shaft to investigate actin dynamics and force generation. We report cyclic growth and retraction dynamics of actin within the tube and correlated contraction events giving rise to sustained load and fail cycles. The contraction mechanism operates independent of myosin II motor proteins. Furthermore, we analyzed the dynamics of actin within the tube, including under various biochemical or genetic perturbations. By combining these results with physical modeling, we argue that stresses generated in the actin filaments by the binding of actin depolymerizing factor (ADF/cofilin) proteins can explain the cyclic load-fail behavior.