Reentrant Phase Transition Drives Dynamic Substructure Formation in Ribonucleoprotein Droplets.

Intracellular ribonucleoprotein (RNP) granules are membrane-less droplet organelles that are thought to regulate posttranscriptional gene expression. While liquid-liquid phase separation may drive RNP granule assembly, the mechanisms underlying their supramolecular dynamics and internal organization remain poorly understood. Herein, we demonstrate that RNA, a primary component of RNP granules, can modulate the phase behavior of RNPs by controlling both droplet assembly and dissolution in vitro. Monotonically increasing the RNA concentration initially leads to droplet assembly by complex coacervation and subsequently triggers an RNP charge inversion, which promotes disassembly. This RNA-mediated reentrant phase transition can drive the formation of dynamic droplet substructures (vacuoles) with tunable lifetimes. We propose that active cellular processes that can create an influx of RNA into RNP granules, such as transcription, can spatiotemporally control the organization and dynamics of such liquid-like organelles.