Rbm24 dictates mRNA recruitment for germ plasm assembly

Ribonucleoprotein (RNP) granules are the most common membrane-less biomolecular condensates. However, the mechanisms underlying their assembly are largely unknown. The aggregation of germ plasm determines the fate of primordial germ cells (PGCs) and serves as a model for RNP granule assembly. Here, we show that maternal RNA binding protein Rbm24a is the key factor governing specific sorting of mRNAs. Mechanistically, Rbm24a complexes with Buc and interacts 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 towards the cleavage furrows where small granules fuse into large aggregates. Therefore, the loss of maternal Rbm24a causes a complete degradation of germ plasm and the disappearance of PGCs. These findings demonstrate that Rbm24a functions as a nucleating organizer of the germ plasm, highlighting an emerging mechanism for RNA-binding proteins in reading and recruiting RNA components into the phase-separated protein scaffold. To examine the global transcriptome alterations following the loss of maternal Rbm24a, we conducted bulk RNA-seq analysis on Mrbm24a and their sibling embyos at 4-cell, sphere stage and 24 hpf.