Stress granules (SGs) are formed in the cytoplasm by liquid-liquid phase separation (LLPS) of translationally-stalled mRNA and RNA-binding proteins during stress response. Understanding the mechanisms governing SG assembly requires imaging SG formation in real time. Here we used live cell imaging and super-resolution imaging to visualize SG assembly in human cells. We found that IGF2BP proteins formed microscopically visible clusters almost instantaneously upon osmotic stress, prior to the recruitment of G3BP1 and TIA1. The rapid clustering of IGF2BP1 was ATP-independent and was mediated by its KH3/4 di-domains and an intrinsically disordered region (IDR), whereas ATP depletion inhibited the recruitment of G3BP1 and TIA1. Moreover, we detected cytoplasmic clusters of IGF2BP1 below the optical resolution in normal cells and found IGF2BP1 forming a dense granule associated with multiple clusters of poly(A) mRNA in mature SGs. Thus, ATP-independent, rapid clustering of IGF2BP nucleates SG assembly during osmotic stress.

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