  The cytoskeleton of eukaryotic cells provides mechanical support and governs
intracellular transport. These functions rely on the complex mechanical
properties of networks of semiflexible protein filaments. Recent theoretical
interest has focused on mesoscopic properties of such networks and especially
on the effect of local, non-affine bending deformations on mechanics. Here, we
study the impact of local network deformations on the scale-dependent mobility
of probe particles in entangled networks of semiflexible actin filaments by
high-bandwidth microrheology. We find that micron-sized particles in these
networks experience two opposing non-continuum elastic effects: entropic
depletion reduces the effective network rigidity, while local non-affine
deformations of the network substantially enhance the rigidity at low
frequencies. We show that a simple model of lateral bending of filaments
embedded in a viscoelastic background leads to a scaling regime for the
apparent elastic modulus G'(\omega) \sim \omega^{9/16}, closely matching the
experiments. These results provide quantitative evidence for how different a
semiflexible polymer network can feel for small objects, and they demonstrate
how non-affine bending deformations can be dominant for the mobility of
vesicles and organelles in the cell.
