RASSF9: A modulator of PAR3 condensates in polarity signaling

The establishment of apical-basal polarity is fundamental for epithelial tissue integrity and function. The core polarity protein PAR3 (Partitioning defective 3) plays a pivotal role in assembling the polarity complex for polarity establishment and maintenance. While liquid-liquid phase separation (LLPS) has recently emerged as a key mechanism governing the spatial organization of PAR3, how this process is modulated remains largely unknown. Here, we identify RASSF9, a member of the Ras-association domain family (RASSF) often implicated in tumor progression, as a novel interacting partner of PAR3 through biochemical assays. Pathological mutations RASSF9 E58K and H60R in Lung Squamous Cell Carcinoma significantly disrupt the interaction between RASSF9 and PAR3. Furthermore, we show that purified RASSF9 protein undergoes LLPS in vitro. Strikingly, RASSF9 acts as a potent promoter of PAR3 phase separation, significantly enhancing the formation and size of PAR3 condensates. Mutations that impair coacervation of RASSF9 or its interaction with PAR3, diminish its promotion of PAR3 phase separation. Our findings identify RASSF9 as a modulator of PAR3 condensates and suggest a phase separation-mediated framework that may contribute to maintaining cell polarity and restraining oncogenic signaling.