Recent studies have argued that the m6A modification of mRNAs promotes mRNA recruitment to stress granules through the interaction with YTHDF proteins (Anders et al., 2018; Ries et al., 2019). However, mRNAs that contain multiple m6A modified sites partition similarly into stress granules in both wild-type and m6A-deficient cells by single-molecule FISH suggesting m6A modifications play a minor role in mRNA partitioning into stress granules. Moreover, multiple linear regression analysis suggests m6A modification plays a minimal role in stress granule recruitment. Finally, the artificial tethering of 25 YTHDF proteins on reporter mRNAs leads to only a modest increase in mRNA partitioning to stress granules. These results indicate m6A modification makes a small, but measurable, contribution to recruiting specific mRNAs to stress granules.
17Polycomb group (PcG) proteins are master regulators of development and differentiation. 18 Mutation and dysregulation of PcG genes cause developmental defects and cancer. PcG 19 proteins form condensates in the nucleus of cells and these condensates are the physical sites 20 of PcG-targeted gene silencing. However, the physiochemical principles underlying the PcG 21 condensate formation remain unknown. Here we show that Polycomb repressive complex 1 22 (PRC1) protein Cbx2, one member of the Cbx family proteins, contains a long stretch of 23 intrinsically disordered region (IDR). Cbx2 undergoes phase separation to form condensates. 24Cbx2 condensates exhibit liquid-like properties and can concentrate DNA and nucleosomes. We 25 demonstrate that the conserved residues within the IDR promote the condensate formation in 26 vitro and in vivo. We further indicate that H3K27me3 has minimal effects on the Cbx2 27 condensate formation while depletion of core PRC1 subunits facilitates the condensate 28 formation. Thus, our results reveal that PcG condensates assemble through liquid-liquid phase 29 separation (LLPS) and suggest that PcG-bound chromatin is in part organized through phase-30 separated condensates. 31 decrease of YFP-Cbx2 through diffusion. We did not detect YFP-Cbx2 condensates in the 131 lysate. We expected that formaldehyde crosslinking would preserve Cbx2 condensates. After 132 sonication and centrifugation, YFP-Cbx2 condensates would be within the pellets. To test this 133 speculation, prior to lysis, we cross-linked cells with formaldehyde. Lysates were prepared from 134 the cross-linked cells and subjected to sonication. Using fluorescence microscopy, we observed 135
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