Connecting the Dots: PHF13 and cohesin promote polymer-polymer phase separation of chromatin into chromosomes

How interphase chromatin compacts into mitotic chromosomes has eluded researchers for over a century. Here we show that PHF13, a tightly regulated H3K4me epigenetic reader, and cohesin, a mediator of chromatin architecture, cooperate to drive polymer-polymer-phase-separation (PPPS) and higher order compaction of chromatin into chromosomes. PHF13 interacts with cohesin, shows similar dynamics during mitosis, and their co-depletion dramatically impairs mitotic condensation. Mechanistically, we demonstrate that PHF13 stabilizes cohesin chromatin interactions and that itself oligomerizes, resulting in a polymer with increased chromatin avidity and the ability to bridge neighboring and distant nucleosomes. Consistently, molecular dynamic simulations modelling the ability of PHF13 and cohesin to drive chromatin phase separations recapitulated our in vivo observations and are in line with 2-step condensation models.