Motor-free contractility in light-controlled in-vitro protein networks

Contractile structures in the cytoskeleton are essential for various functions of living cells such as cell division and motility. Most of these structures utilize motor proteins for generating active contractility. However, recent experimental and theoretical studies have shown evidence and plausibility of motor-free contractile networks in the cytoskeleton. In this work, we are presenting a simple in-vitro motor-free model system capable of producing contractile structures.

Our system is composed of cortical proteins from ciliates of Tetrahymena which generate contractile visco-elastic networks when triggered by Ca²⁺ ions. The simple 2 species system can exhibit phase transitions from local individual protein clusters to contractile visco-elastic networks as a function of Ca²⁺ concentration. To study the network dynamics systematically, we also performed controlled experiments by using caged Ca²⁺ which enabled us to control Ca²⁺ concentration with an incident light source. Interestingly, by triggering the system with light pulses, we can generate an out-of-equilibrium viscoelastic network that breaks spatial symmetry by periodically contracting and relaxing. With the help of a physical model and numerical simulations, we provide an explanation for the observed behavior of the system.