The functional and molecular determinants of stress-induced mRNA condensation

Stress-induced condensation of mRNA and protein into massive, heterogeneous clumps is widely conserved across eukarya, but the physiological mechanism of mRNA condensation and recruitment to the stress granules they mature into is unclear. The release of ribosome-free mRNA following stress-induced polysome collapse is thought to be the trigger of stress granule formation. Yet, coalescence of proteins into stress granules can be blocked without affecting protein condensation, raising the question: Is the same true of mRNAs? Can we separate mRNA condensation from stress granule formation? Here, we show blocking translation initiation triggers specific mRNA condensation. Global translation initiation inhibition, such as following temperature stress or pharmacological depletion of an essential translation factor, causes most mRNA to condense. In contrast, we demonstrate reporter transcripts engineered with blocked translation initiation are specifically condensed, even under unstressed conditions. The well-translated stress response messages relatively escape condensation during stress. Interestingly, blocking translation initiation via depletion of different factors can either trigger condensation or prevent it. Therefore, increased concentration of ribosome-free mRNA is insufficient to trigger condensation. In sum, blocked translation initiation causes mRNA condensation even in the absence of stress, which reframes stress granule formation as an accentuation of normal mRNA physiology and suggests a functional role for mRNA condensation in accelerating translation of stress-induced mRNA.