BPS2026 – Coarse-grained simulations of cytokinetic ring anchoring protein Cdc15 phase separation

The assembly of Cdc15 into cytokinetic nodes plays a significant role in the organization and anchoring of the cytokinetic actomyosin ring to the plasma membrane. The intrinsically disordered region (IDR) of Cdc15 is regulated by multiple cell polarity kinases and dephosphorylation has been shown to induce liquid-liquid phase separation [1]. A computational model of single Cdc15 dimer suggested how IDR phosphorylation may inactivate Cdc15 by blocking its membrane binding and oligomerization via its F-BAR domain [1] but did not explore the phase separation of many proteins. Here, we investigate the phase separation of multiple IDRs with coarse-grained molecular dynamics simulations. We use the hydrophobicity scale (HPS) model, in which aminoacids are represented by single beads and their interactions quantified with a hydrophobicity factor. To confirm the different phase separation properties of phosphorylated and dephosphorylated Cdc15, we simulated condensation of the IDRs in a slab geometry (elongated rectangular box with periodic boundary conditions) and quantified the separation of proteins into condensed and dilute phases. The results show that phosphorylation decreases the phase separation critical temperature T_c value, indicating that it inhibits phase separation. The HPS coarse-grained model is thus in agreement with the experimentally observed difference between the phase separation of dephosphorylated Cdc15 and its highly phosphorylated state. [1] Bhattacharjee, R., Hall, A.R., Mangione, M.C., Igarashi, M.G., Roberts-Galbraith, R.H., Chen, J.S., Vavylonis, D., and Gould, K.L., (2023). “Multiple polarity kinases inhibit phase separation of F-BAR protein Cdc15 and antagonize cytokinetic ring assembly in fission yeast”. Elife , 12, e83062.