Phase separation drives cortical enrichment of the F-BAR proteins Rga7 and Rga8 to maintain cell integrity

Polarized cell growth necessitates the dynamic remodeling of the plasma membrane, a process requiring BAR domain-containing proteins. While classical BAR proteins, with their crescent-shaped structure, are well characterized, the mechanisms underlying the localization and function of elongated F-BAR proteins remain unclear. Here, we demonstrate that the F-BAR domains of the fission yeast proteins Rga7 and Rga8 undergo liquid-liquid phase separation (LLPS). These domains form oligomers via hydrophobic interactions and assemble into condensates through electrostatic interactions mediated by the charged residues at their tips. Mutants deficient in phase separation fail to localize properly at cell poles, leading to defective polar distribution of key regulators, including the Rho GTPases, the exocyst complex, and glucan synthases, all crucial for maintaining cell integrity. We further show that Rga8 requires the actin transport system for tip localization, whereas Rga7 accumulates at cell tips via diffusion. The absence of both Rga7 and Rga8 causes cell lysis. Hence, our findings establish LLPS as a fundamental mechanism for the cortical accumulation and function of F-BAR proteins, providing new insights into their role in membrane dynamics.