The fusion of actin bundles driven by interacting motor proteins.

The cooperative action of many molecular motors is essential for dynamic processes such as cell motility and mitosis. This action can be studied by using motility assays which track the motion of cytoskeletal filaments over a surface coated with motor proteins. Here, we propose to use a motility assay consisting of a-polar actin bundles subjected to the action of myosin II motors where no external loading is applied. In this work we focus on those bundles undergoing fusion with other nearby bundles. Specifically, we investigate the role of the bundles' dimension on the transition from bidirectional to directional motion and on the properties of their motion during fusion. Our experimental data reveal that only small bundles exhibit dynamic transition to directional motion, implying that the forces acting on them exceed the threshold value necessary to induce the transition. Moreover, these bundles accelerate along their trajectory, suggesting that the forces acting on them increase while approaching each other. We show that these forces do not originate from external loading but rather arise from the action of the motors on the bundles. These forces are transmitted through the medium over micron-scale distances without being cut off. Moreover, we show that the forces propagate to distances that are proportional to the size of the bundles, or equivalently, to the number of motors, which they interact with.