Actin network architecture can ensure robust centering or sensitive decentering of the centrosome

Abstract

The orientation of cell polarity depends on the position of the centrosome, the main microtubule‐organizing center (MTOC). Microtubules (MTs) transmit pushing forces to the MTOC as they grow against the cell periphery. How the actin network regulates these forces remains unclear. Here, in a cell‐free assay, we used purified proteins to reconstitute the interaction of a microtubule aster with actin networks of various architectures in cell‐sized microwells. In the absence of actin filaments, MTOC positioning was highly sensitive to variations in microtubule length. The presence of a bulk actin network limited microtubule displacement, and MTOCs were held in place. In contrast, the assembly of a branched actin network along the well edges centered the MTOCs by maintaining an isotropic balance of pushing forces. An anisotropic peripheral actin network caused the MTOC to decenter by focusing the pushing forces. Overall, our results show that actin networks can limit the sensitivity of MTOC positioning to microtubule length and enforce robust MTOC centering or decentering depending on the isotropy of its architecture. image The structural and biochemical complexity of the cytoplasm has hampered analyses of the mechanical role of the actin network in positioning of the microtubule‐organizing center (MTOC). Here, a reconstitution assay reveals that variations of actin network density in bulk or along the periphery can focus or balance pushing forces within the microtubules network. Lipid‐coated microwells, artificial MTOCs, actin filaments and microtubules allow in vitro reconstitution of centrosome positioning. Bulk actin filaments can immobilize the MTOC by limiting lateral translocation of microtubules. The reconstituted cortical actin network can ensure MTOC centering by preventing microtubule slippage along the cell boundary. The cortical actin network limits the sensitivity of MTOC positioning to variations of microtubule length. Asymmetry of the cortical actin network directs the decentering of the MTOC positioning by focusing pushing forces.