ISRIB Promotes Aggregation of TIA-1 by Modulating Its Phase Separation <i>in Vitro</i>

Stress granules (SGs) are non-membranous biomolecular condensates formed through liquid-liquid phase separation (LLPS) of RNA-binding proteins (RBPs) and RNA under stress conditions. T-cell intracellular antigen-1 (TIA-1), a major RBP of SGs, can undergo LLPS via its low-complexity domain, contributing to SG nucleation. Integrated stress response inhibitor (ISRIB), a small molecule that enhances eIF2B activity and inhibits the integrated stress response, has been widely studied for its therapeutic potential in neurodegenerative diseases. However, little is known about how ISRIB directly affects the behavior of SG proteins. Here, we show that ISRIB enhances TIA-1 phase separation and promotes its aggregation in vitro. Interestingly, this effect was mitigated in the presence of RNA or cell lysates, suggesting that RNA-binding plays a protective role against ISRIB-induced aggregation. These findings imply that ISRIB alters the physical properties of SGs in an RNA-dependent manner, raising important considerations for their therapeutic application.