Rbm24a dictates mRNA recruitment for germ granule assembly in zebrafish

The germ granules are ribonucleoprotein (RNP) biomolecular condensates that determine the fate of primordial germ cells (PGCs) and serve as a model for studying RNP granule assembly. Here, we show that the maternal RNA-binding protein Rbm24a is a key factor governing the specific sorting of mRNAs into germ granules. Mechanistically, Rbm24a interacts with the germ plasm component Buc to dictate the specific recruitment 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 the 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 a phase-separated protein scaffold.