Liquid-to-solid phase transition ofoskarRNP granules is essential for their function in theDrosophilagermline

Abstract

SummaryAsymmetric localization ofoskarRNP granules to the oocyte posterior is crucial for abdominal patterning and germline formation of theDrosophilaembryo. We show thatoskarRNP granules in the oocyte are condensates with solid-like physical properties. Using purifiedoskarRNA and scaffold proteins Bruno and Hrp48, we confirmin vitrothatoskargranules undergo a liquid-to-solid phase transition. Whereas the liquid phase allows RNA incorporation, the solid phase precludes incorporation of additional RNA while allowing RNA-dependent partitioning of specific proteins. Genetic modification of scaffold granule proteins, or tethering the intrinsically disordered region of human Fused in Sarcoma tooskarmRNA, allowed modulation of granule material propertiesin vivo. The resulting liquid-like properties impairedoskarlocalization and translation with severe consequences on embryonic development. Our study reflects how physiological phase transitions shape RNA-protein condensates to regulate localization and expression of a maternal RNA that instructs germline formation.