Fig. 1. HSF1 is the key determinant in recruiting hSF2/ASF and hSRp30c to nuclear stress granules. (A) hSF2/ASF-GFP or hSRp30c-GFP proteins (green) were transiently expressed in HeLa cells and HSF1 (red) was detected by immunofluorescence. Both proteins display a typical speckled pattern at 37°C, and are targeted to HSF1 granules upon stress. (B) hSF2/ASF-GFP or hSRp30c-GFP (green) were co-expressed in HeLa cells with either the HSP70 protein or with a dominant negative mutant of HSF1 (DBD+TRIM-myc) (red). HSP70 and the myc-tagged HSF1 mutant were subsequently detected by immunofluorescence. Overexpression of HSP70 does not alter SR protein distribution at 37°C (left panels). By contrast, at 42°C, HSP70 overexpression prevents the targeting of both hSF2/ASF and hSRp30c to the granules (right panel). Likewise, hSF2/ASF and hSRp30c are not recruited to the granules formed by the DBD+TRIM mutant at 37°C or at 42°C. Bars, 5 µm.