Guillén-Boixet, J; Kopach, A; Holehouse, AS; Wittmann, S; Jahnel, M; Schlüßler, R; Kim, K; Trussina, I; Wang, J; Mateju, D; Poser, I; Maharana, S; Ruer-Gruß, M; Richter, D; Zhang, X; Chang, YT; Guck, J; Honigmann, A; Mahamid, J; Hyman, AA; Pappu, RV; Alberti, S; Franzmann, TM; ,
Stressed cells shut down translation, release mRNA molecules from polysomes, and form stress granules (SGs) via a network of interactions that involve G3BP. Here we focus on the mechanistic underpinnings of SG assembly. We show that, under non-stress conditions, G3BP adopts a compact auto-inhibited state stabilized by electrostatic intramolecular interactions between the intrinsically disordered acidic tracts and the positively charged arginine-rich region. Upon release from polysomes, unfolded mRNAs outcompete G3BP auto-inhibitory interactions, engendering a conformational transition that facilitates clustering of G3BP through protein-RNA interactions. Subsequent physical crosslinking of G3BP clusters drives RNA molecules into networked RNA/protein condensates. We show that G3BP condensates impede RNA entanglement and recruit additional client proteins that promote SG maturation or induce a liquid-to-solid transition that may underlie disease. We propose that condensation coupled to conformational rearrangements and heterotypic multivalent interactions may be a general principle underlying RNP granule assembly.