When exposed to stress (which can be environmental, such as a temperature change, or cell-intrinsic, such as the accumulation of misfolded proteins), cells activate a signalling pathway known as the integrated stress response (ISR) to minimize cellular damage. The ISR leads to global translation inhibition and induces the assembly of cytoplasmic stress granules (SGs), which form via liquid−liquid phase separation and contain mRNAs and RNA-binding proteins, among other factors. It is unclear whether mRNA recruitment to SGs promotes translation repression, although the two events coincide. The researchers now report that SG-associated mRNAs can be translated.

The authors carried out single-molecule imaging of translation in living cells using a reporter transcript that contains the 5ʹ untranslated region of ATF4 (a transcription factor whose translation is enhanced during stress) followed by a SunTag array. Individual reporter mRNAs and nascent peptides could be visualized, revealing that, in stress conditions that promote the formation of SGs, translating and non-translating mRNAs localized to the cytosol as well as to SGs. Although non-translating mRNAs were more likely to be recruited to SGs than translating mRNAs, about 30% of SG-localized ATF4-SunTag transcripts were translated.

Other reporter transcripts that are not induced by stress (as is ATF4) were also actively translated, indicating that mRNA translation in SGs is neither rare nor restricted to stress-induced transcripts. The whole translation cycle, including translation initiation, elongation and termination, occurred in SGs. Moreover, reporter transcripts that contained the regulatory region of 5ʹTOP mRNAs, which are strongly repressed during stress and accumulate in SGs, were repressed to comparable extents in the cytosol and in SGs. Lastly, translating and non-translating mRNAs were found to transit between the cytosol and SGs without associated changes in translation status.

Together, these data challenge the assumption that mRNAs found in SGs are translationally repressed and indicate that the SG environment does not induce translation repression.

https://www.cell.com/cell/fulltext/S0092-8674(20)31527-0