The actin network is highly dynamic and its branching is involved in controlling morphological processes. Factors such as myosin have been previously shown to regulate the density of the actin network. Following up on their previous finding that myosin 1b (Myo1b) reduces actin network density in vivo, Patricia Bassereau, Evelyne Coudrier and colleagues (Pernier et al., 2020) now use in vitro F-actin gliding assays and total internal reflection fluorescence (TIRF) microscopy to investigate actin branching by the Arp2/3 complex in the presence of Myo1b. They observe that when actin glides on Myo1b, the branch density of F-actin decreases and the branches are flattened. Actin filaments become distorted, and when branches form over time, they frequently detach and ‘run away’. This distortion and the sprouting out of the gliding mother filament is caused by the resultant force of friction that is induced by the sliding of mother and daughter filaments in different directions due to the geometry of the branching. This force can be sufficiently high to break the branch, thus reducing the overall density of the F-actin network. Additionally, and contributing to reduced F-actin density, the probability that new branches form is reduced, possibly owing to a competition between Myo1b and Arp2/3 for binding to actin. This work highlights the essential role of Myo1b in establishing the actin network architecture and in the dynamics of actin movements.
