The architecture of the actin network can balance the pushing forces produced by growing microtubules

The position of centrosome, the main microtubule-organizing center (MTOC), is instrumental in the definition of cell polarity. It is defined by the balance of tension and pressure forces in the network of microtubules (MTs). As MTs polymerize against the cell periphery, pressure increases and produces pushing forces on the MTOC. How the mechanical interplay between MTs and the actin network is involved in the regulation of these forces remains poorly understood, in particular because its investigation is technically limited by the structural and biochemical complexity of the cell cytoplasm. Here, in a cell-free assay, we used purified proteins to reconstitute the interaction of an aster of dynamic MTs with actin networks of various compositions and architectures in cell-sized microwells. In the absence of actin filaments, the positioning of the MTOC was highly sensitive to variations in MT length. The presence of a bulk actin filament network limited MTs deformation and displacement, and MTOCs were hold in place. In contrast, the assembly of a dense and branched actin network along the edges of the wells centered the MTOCs by preventing MT slippage and thus maintaining an isotropic balance of pushing forces. In agreement with this, an asymmetric peripheral actin network caused the MTOC to decenter by creating an asymmetry in the pushing forces. Numerical simulations demonstrated that steric hindrance by actin networks, at the tip or along the entire length of MTs, can modulate MTOC positioning, as observed in the experiments. Overall, our results show that actin networks can limit the sensitivity of MTOC positioning to MT length and enforce robust MTOC centering or decentering depending on its architecture.