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What we learn about the dynamics of actin by growing it from
cylinders, stars and diamonds
Actin is a major component of biological cells and forms filaments
that constantly polymerize and depolymerize. This activity is used by
the cell to generate forces in order to crawl on a surface or to pinch
part of its membrane during endocytosis. The cortex, a dense actin
network, endows its viscoelastic properties to the cell and protects
it from unwanted deformations. The in vitro reconstitution of dense
actin networks on micron-size spheres was a seminal step to understand
these processes. Here I will describe two experiments where we
reconstituted actin networks around non-spherical micro-objects. In
the first one, actin grows from the faces of magnetic cylinders
arranged in a string. A load is applied by the attraction of
neighboring cylinders to monitor the actin network’s growth under
stress and measure its elasticity. We evidenced a peculiar non-linear
elastic behavior of the network and a direct interaction between
growth velocity and viscoelastic behavior. In the second experiment,
we revisit the problem of the symmetry breaking occurring in an actin
gel growing from a spherical object by growing the actin from star and
diamond-shaped prisms. I will show unexpected results on where the
fracture arises that downplay the role of mechanical stress in this
phenomenon.