m6A-binding YTHDF proteins promote stress granule formation.

Diverse RNAs and RNA-binding proteins form phase-separated, membraneless granules in cells under stress conditions. However, the role of the prevalent mRNA methylation, m6A, and its binding proteins in stress granule (SG) assembly remain unclear. Here, we show that m6A-modified mRNAs are enriched in SGs, and that m6A-binding YTHDF proteins are critical for SG formation. Depletion of YTHDF1/3 inhibits SG formation and recruitment of mRNAs to SGs. Both the N-terminal intrinsically disordered region and the C-terminal m6A-binding YTH domain of YTHDF proteins are important for SG formation. Super-resolution imaging further reveals that YTHDF proteins appear to be in a super-saturated state, forming clusters that often reside in the periphery of or at the junctions between SG core clusters, and potentially promote SG formation by reducing the activation energy barrier and critical size for SG condensate formation. Our results suggest a new function of the m6A-binding YTHDF proteins in regulating SG formation.