mRNA caps accumulate in stress granules and are essential for their formation

Stress granules (SGs) are essential cytoplasmic, membraneless organelles that form in response to cellular stress, functioning to prevent mRNA translation and protect mRNA from damage. However, the mechanism of SG formation remains largely unknown. Here, utilizing a systems-level technique for quantification of RNA cap epitranscriptome, we found that mRNA Cap1 and non-canonical caps are predominantly enriched in SGs, with the composition of mRNA caps in SGs of mammalian cells differing between different stress conditions. Knockdown of RNA guanine-7-methyltransferase (RNMT) and phosphorylated CTD interacting factor 1(PCIF1) both resulted in substantial changes in the content and composition of mRNA caps and RNMT knockdown caused failure of SG formation under different stress conditions. Furthermore, proteomic, Co-IP and confocal immunofluorescence analysis of these knockdown cells and SGs revealed that mRNAs partition into SGs through cap-protein interactions. These findings collectively revealed the significant role of mRNA cap in SG formation and stress response in mammalian cells.