G3BP1 promotes intermolecular RNA-RNA interactions during RNA condensation.

Ribonucleoprotein (RNP) granules are biomolecular condensates requiring RNA and proteins to assemble. Stress granules are RNP granules formed upon increases in non-translating messenger ribonucleoprotein particles (mRNPs) during stress. G3BP1 and G3BP2 proteins are proposed to assemble stress granules through multivalent crosslinking of RNPs. We demonstrate that G3BP1 also has "condensate chaperone" functions, which promote the assembly of stress granules but are dispensable following initial condensation. Following granule formation, G3BP1 is dispensable for the RNA component of granules to persist in vitro and in cells when RNA decondensers are inactivated. These results demonstrate that G3BP1 functions as an "RNA condenser," a protein that promotes intermolecular RNA-RNA interactions stabilizing RNA condensates, leading to RNP granule persistence. Moreover, the stability of RNA-only granules highlights the need for active mechanisms limiting RNP condensate stability and lifetime.