An In Vitro Assembly System Identifies Roles for RNA Nucleation and ATP in Yeast Stress Granule Formation

Stress granules (SGs) are condensates of mRNPs that form in response to stress. SGs arise by multivalent protein-protein, protein-RNA, and RNA-RNA interactions. However, the role of RNA-RNA interactions in SG assembly remains understudied. Here, we describe a yeast SG reconstitution system that faithfully recapitulates SG assembly in response to trigger RNAs. SGs assembled by stem-loop RNA triggers are ATP-sensitive, regulated by helicase/chaperone activity, and exhibit the hallmarks of maturation observed for SG proteins that phase-separate in vitro. Additionally, the fraction of total RNA that phase-separates in vitro is sufficient to trigger SG formation. However, condensation of NFT1 mRNA, an enriched transcript in this population, can only assemble an incomplete SG. These results suggest that networks of distinct transcripts are required to form a canonical SG and provide a platform for dissecting the interplay between the transcriptome and ATP-dependent remodeling in SG formation.