The maintenance of a continuous mechanical network across the cytoplasm is important for cellular processes such as matrix force generation, cell shape and motility. Actin stress fibres are involved in this network, but how forces are transmitted across the cytoplasm, and whether other cellular components also contribute, has been unclear. On page , Michael Sheetz and colleagues now uncover a role for nonmuscle myosin-II (NMII) contractility in maintaining a coherent cytoplasmic actin network in spreading cells. They show that, when NMII activity is inhibited, a loss of ‘cytoplasmic coherence’ is observed during later stages of spreading and cells become fragmented; in addition, circumferential actin bundles and stress fibres are disrupted. By assessing the capacity of cells to pull on force-sensing pillars, the authors also show that NMII inhibition reduces both the level and symmetry of traction forces, which — together with the loss of cytoplasmic coherence — results in increased spreading of fragmented cells. siRNA experiments show that these activities are mediated by the A isoform of NMII (NMIIA), rather than by NMIIB or C. Finally, the authors provide evidence that microtubules and intermediate filaments are dispensable for cytoplasmic coherence and force transmission. These data expand our understanding of the cellular components that maintain a coherent cytoskeletal network in spreading cells.
