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Abstract

Crosslinked and bundled actin filaments form networks that are essential
for the mechanical properties of living cells. Reconstituted actin
networks have been extensively studied not only as a model system for
the cytoskeleton, but also to understand the interplay between
microscopic structure and macroscopic viscoelastic properties of
network-forming soft materials. These constitute a broad class of
materials with countless applications in science and industry. So far,
it has been widely assumed that reconstituted actin networks represent
equilibrium structures. Here, we show that fully polymerized
actin/fascin bundle networks exhibit surprising age-dependent changes in
their viscoelastic properties and spontaneous dynamics, a feature
strongly reminiscent of out-of-equilibrium, or glassy, soft materials.
Using a combination of rheology, confocal microscopy and space-resolved
dynamic light scattering, we demonstrate that actin networks build up
stress during their formation and then slowly relax towards equilibrium
owing to the unbinding dynamics of the crosslinking molecules.