Regulation of zebrafish dorsoventral patterning by phase separation of RNA-binding protein Rbm14

RNA-binding proteins with intrinsically disordered regions (IDRs) such as Rbm14 can phase separate in vitro. To what extent the phase separation contributes to their physiological functions is however unclear. Here we show that zebrafish Rbm14 regulates embryonic dorsoventral patterning through phase separation. Zebrafish rbm14 morphants displayed dorsalized phenotypes associated with attenuated BMP signaling. Consistently, depletion of mammalian Rbm14 downregulated BMP regulators and effectors Nanog, Smad4/5, and Id1/2, whereas overexpression of the BMP-related proteins in the morphants significantly restored the developmental defects. Importantly, the IDR of zebrafish Rbm14 demixed into liquid droplets in vitro despite poor sequence conservation with its mammalian counterpart. While its phase separation mutants or IDR failed to rescue the morphants, its chimeric proteins containing an IDR from divergent phase separation proteins were effective. Rbm14 complexed with proteins involved in RNA metabolism and phase separated into cellular ribonucleoprotein compartments. Consistently, RNA deep sequencing analysis on the morphant embryos revealed increased alternative splicing events as well as large-scale transcriptomic downregulations. Our results suggest that Rbm14 functions in ribonucleoprotein compartments through phase separation to modulate multiple aspects of RNA metabolism. Furthermore, IDRs conserve in phase separation ability but not primary sequence and can be functionally interchangeable.