Rbm24a dictates mRNA recruitment for germ granule assembly

Ribonucleoprotein (RNP) granules are the most common membrane-less biomolecular condensates. However, mechanisms underlying their assembly are largely unknown. The aggregation of germ granules determines the fate of primordial germ cells (PGCs) and serves as a model for studying RNP granule assembly. Here, we show that maternal RNA binding protein Rbm24a is the key factor governing specific sorting of mRNAs. Mechanistically, Rbm24a complexes and interacts with Buc to dictate the specific grasp of germ plasm mRNAs into phase-separated condensates. Germ plasm particles lacking Rbm24a and mRNAs fail to undergo kinesin-dependent transport toward cleavage furrows where small granules fuse into large aggregates. Therefore, the loss of maternal Rbm24a causes a complete degradation of the germ plasm and the disappearance of PGCs. These findings demonstrate that Rbm24a/Buc complex functions as a nucleating organizer of germ granules, highlighting an emerging mechanism for RNA-binding proteins in reading and recruiting RNA components into the phase-separated protein scaffold.