Phase separated liquid vimentin droplets stabilize actin fibers through wetting

The cytoskeleton is composed of F-actin, microtubules, and intermediate filaments (IFs). Vimentin is the most ubiquitous IF. It is involved in wound healing, tissue fibrosis and cancer metastasis, all of which require rapid vimentin filaments assembly. In this paper, we report that un-polymerized vimentin forms liquid condensates that appear to enable rapid filament growth. Given the transient nature of these droplets, we focus on properties of vimentin-Y117L, a mutant which does not form filaments, enabling us to study these droplets in detail. They dissolve under 1,6-Hexanediol treatment and under decreasing concentration, confirming that they are liquid, and phase separated. These condensates extensively wet actin fibers, rendering them resistant to actin-depolymerizing drugs. We show similar behavior occurs in wild type vimentin during its assembly into filaments.