BLM helicase protein negatively regulates stress granule formation through unwinding RNA G-quadruplex structures

Stress granules (SGs), are cytoplasmic biomolecular condensates that regulate the cellular RNA metabolism in health and disease. These membrane-less organelles are formed under stress by complex interactions of RNA-binding proteins (RBPs) and mRNAs that are mostly halted from translation. The SG-transcriptome may organized in different structures, such as RNA G-Quadruplexes (rG4s). rG4s are guanine-rich RNA non-canonical secondary structures that have regulatory roles in central cellular processes. Although formation of rG4s within 5’UTR of mRNAs is suggested to block ribosome scanning, which lead these G4-enriched transcripts to SGs, little is known about direct roles of rG4s in SG biology. Here, we hypothesize that rG4s may play a direct role in SG dynamics. Using small-molecules that specifically bind to rG4s, and imaging of fixed and live G3BP1-GFP-expressing U2OS cells under different stresses and conditions, we characterize a direct function of rG4s in the assembly of SGs.

Our results show in fixed cells an enrichment of rG4s in G3BP1-GFP-enriched SGs by fluorescently-labeled preferential rG4 binders, bio-TASQ and QUMA-1. The colocalization between rG4s and SGs is found under different stresses. Moreover, we demonstrate that QUMA-1 and the rG4 stabilizer, cPDS, reduce the assembly dynamics of SGs in live cells.

Thus, these results suggest that RBPs-rG4 interactions contribute to the SG formation and rG4s regulate SG biology through this aspect in addition to the already known translation-associated aspects. This study sheds a new light on the regulatory roles of rG4-RBP interactions in general. Particularly, this work contributes to a better understanding of the molecular mechanisms underlying SG biology.