Phosphoregulation provides functional specificity to biomolecular condensates in the cell cycle and cell polarity

Cytoplasmic patterning is a feature of many cells from embryos to neurons and fungi. Biomolecular condensation is a way of organizing cytosol in which proteins and nucleic acids coassemble into compartments. How molecular identity of condensates is achieved is not well understood. In the multinucleate, filamentous fungus Ashbya gossypii , the RNA-binding protein Whi3 regulates the cell cycle and cell polarity through forming macromolecular structures that behave like condensates. Whi3 has distinct spatial localizations and mRNA targets making it a powerful model for how, when and where specific identities are established for condensates. Using mass-spectrometry, we identified residues on Whi3 that are differentially phosphorylated under specific conditions and generated mutants which ablate this regulation. This yielded separation of function alleles that were functional for either cell polarity or nuclear cycling but not both. This study shows that phosphorylation of individual residues on molecules in biomolecular condensates can provide specificity that give rise to distinct functional identities in the same cell. Summary Residue specific phosphorylation of the RNA-binding protein Whi3 is used to specifically regulate subsets of functionally distinct condensates in the multinucleate fungus Ashbya gossypii .