Cells are often exposed to stress conditions such as temperature changes, oxidative stress, viral infection, or presence of toxins. A sudden drop in oxygen, overheating or toxin will trigger a series of molecular changes, leading to cell growth arrest, the production of stress-protective factors, and the formation of stress granule—proteins and RNA molecules squeezed together to form membrane-less organelles. Although the function of stress granules is still largely unknown, it is believed that they only contain RNA that is not translated into proteins.
Now, a new study overturns the long-standing view that mRNA in stress particles can indeed make proteins. The related research results are published in the journal Cell with the title of "Single-Molecule Imaging Reveals Translation of mRNAs Localized to Stress Granules".
Figure 1. Single-molecule imaging reveals translation of mRNAs localized to stress granules. (Mateju D, et al. 2020)
In order to protect against unfavorable conditions and minimize the damage during stress, cells activate an evolutionarily conserved pathway, called integrated stress response. This pathway triggers the phosphorylation of the translation initiation factor eIF2a, which results in the inhibition of translation of most mRNAs. With the reprogramming of translation, the integrated stress response also results in the assembly of stress granules, phase-separated membrane-less organelles, in the cytoplasm. A fraction of cytoplasmic mRNAs are located in stress granules upon their assembly, and translation inhibition and RNA length are related to stress granules. In the process of cellular stress response, many mRNAs cluster inside stress granules—an observation that led scientists to believe that these mRNAs stopped getting translated when the cell is threatened.
Although most transcripts are inhibited during stress, there are transcripts such as activating transcription factor 4 (ATF4) that are necessary for mounting the stress response and are preferentially translated when eIF2ɑ is phosphorylated. In order to describe the relationship between translational status and stress granule recruitment of mRNAs during stress, researchers generated ATF4-SunTag reporter transcripts for single-molecule imaging of translation in living cells. Using this method, they find direct evidence that mRNAs localized to stress granules can undergo translation. ATF4-SunTag transcripts located in stress granules can initiate translation, elongate, and terminate, suggesting that the stress granule environment does not inhibit any step of the translation cycle.
Although there is a slight over-representation of non-translating mRNAs in stress granules, this can be explained by the fact that non-translating mRNAs are preferentially recruited into stress granules, rather than silencing of translation in stress granules. For instance, mRNA accumulation in stress granules has been shown to correlate with the expected time of ribosome runoff from different lengths of mRNA. Importantly, the analysis of single mRNA translation dynamics shows that localization to stress granules does not prevent translation initiation or efficient elongation. In conclusion, the data suggest that stress granules have a preference for recruitment of non-translating mRNAs, but stress granules do not directly inhibit any aspect of mRNA translation.
These findings shed light on unprecedented details of cellular stress responses. With the development of single molecule imaging techniques and fluorescent biosensors, as well as the identification of separation-of-function mutants or chemical biology tools, it will play a key role toward understanding the function of biomolecular condensates.