If you were twirling double-dutch ropes and someone shook the ropes back and forth from the middle, you and your partner would be pulled together. These are the kinds of large-scale behaviors of cellular polymers that David Weitz studies at Harvard University.
To measure the flexibility of actin, Weitz and his colleagues exploited the stiffness of scruin, a glue-like protein that holds the actin fibers together.1 In this scenario, "the joints are stiffer than the fibers, so the properties of the network are determined by the properties of the fibers," says theorist and coauthor L. Mahadevan. They sandwiched actin-scruin gels between two plates and measured the movement of one plate after applying a force to it.
The researchers found that the elasticity could be easily tuned. Above a certain threshold, small changes in the number of crosslinks caused a large change in the stiffness. "It becomes a completely...
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