Abstract

Regulation of microtubule dynamics is essential for diverse cellular functions, and proteins that bind to dynamic microtubule ends can regulate network dynamics. Here we show that two conserved microtubule end-binding proteins, CLIP -170 and EB3 , undergo phase separation and form dense liquid-networks. When CLIP -170 and EB3 act together the multivalency of the network increases, which synergistically increases the amount of protein in the dense phase. In vitro and in cells, these liquid networks can concentrate tubulin. In vitro, in the presence of microtubules, phase separation of EB3 /CLIP-170 can enrich tubulin all along the microtubule. In this condition, microtubule growth speed increases up to two-fold and the frequency of depolymerization events are strongly reduced compared to conditions in which there is no phase separation. Our data show that phase separation of EB3 /CLIP-170 adds a novel layer of regulation to the control of microtubule growth dynamics.