  The actin cortex is an active biopolymer network underneath the plasma
membrane at the periphery of mammalian cells. It is a major regulator of cell
shape through the generation of active cortical tension. In addition, the
cortex constitutes a mechanical shield that protects the cell during mechanical
agitation. Cortical mechanics is tightly controlled by the presence of actin
cross-linking proteins, that dynamically bind and unbind actin filaments.
Cross-linker actin bonds are weak non-covalent bonds whose bond lifetime is
likely affected by mechanical tension in the actin cortex making cortical
composition inherently mechanosensitive. Here, we present a quantitative study
of changes in cortex composition and turnover dynamics upon short-lived peaks
in active and passive mechanical tension in mitotic HeLa cells. Our findings
disclose a twofold mechanical reinforcement strategy of the cortex upon tension
peaks entailing i) a direct catch-bond mechanosensitivity of cross-linkers
filamin and $\alpha$-actinin and ii) an indirect cortical mechanosensitivity
that triggers actin cortex reinforcement via enhanced polymerization of actin.
We thereby disclose a `molecular safety belt' mechanism that protects the
cortex from injury upon mechanical challenges.
