The actin and microtubule (MT) cytoskeletal networks are integral to the structure and organisation of the interior of a cell, and although often studied independently, are highly intertwined. Although the growth and dynamics of MTs are known to be affected by actin, the mechanical crosstalk between MTs and different types of actin structures is not yet fully understood. In this study (Gélin et al., 2023), Manuel Théry and colleagues develop an in vitro reconstitution assay that allows them to spatially separate MT growth from actin networks to study the physical interactions between growing MTs and pre-existing actin structures. Here, the authors find that MTs align with and grow along linear bundles of actin filaments, despite a lack of MT–actin crosslinking proteins. When MTs encounter denser branched actin networks, they cannot penetrate them and instead the force produced by polymerisation ‘pushes’ the untethered MTs backwards. Interestingly, when MTs are tethered to an adjacent actin network or cross-linked to actin with the protein Tau, the force of polymerisation builds up pressure and the growing end of the MT is capable of penetrating the dense actin meshwork. These findings suggest that the ability of MTs to penetrate dense actin networks requires tethering to prevent backward translocation and allow a build-up of mechanical pressure, providing insights into how MT–actin interactions are mechanically regulated.
