When cells experience osmotic stress, they rapidly form biomolecular condensates through hyperosmotic phase separation (HOPS). However, it is not clearly understood how these condensates interact with and regulate the intracellular dynamics of RNA. Here, we utilize single-molecule fluorescence imaging to study how RNA interacts with Dcp1a-containing HOPS condensates. We found that the condensates attract RNAs that are typically recruited into and stored in P-bodies (PBs) and stress granules (SGs), suggesting a functional overlap. This selection appears to be based on general physical features, as the enrichment of RNAs within condensates correlates strongly with transcript length and less with specific RNA-protein interactions. Furthermore, HOPS condensates non-specifically enrich key RNA-processing proteins that are also core components of P-bodies and stress granules, such as effectors of translation (eIF4E and RPS20) and RNA decay (G3BP1 and CNOT1). In live cells, we observed that most RNA interactions are brief encounters at the condensate surface, with fewer, longer-lasting interactions occurring in the core. This dynamic behavior is distinct from the more stable RNA interactions with the larger PBs, highlighting the transient nature of HOPS condensates. Based on these findings, we propose a model wherein HOPS condensates function as a first-response sorting system for cellular RNAs. By briefly capturing longer granule-associated RNAs immediately after osmotic shock, they prime the cell for subsequent, slower-onset stress responses. We hypothesize that this rapid sorting represents a buffering stage that facilitates the later assembly of more stable stress granules, while also enabling rapid reversal if the hyperosmotic stress subsides, providing an immediate strategy for managing RNA during fluctuating environmental stresses.
Gao et al. (Sun,) studied this question.