Mitochondrial SSBP1 protects cells from proteotoxic stresses by potentiating stress-induced HSF1 transcriptional activity.

Heat-shock response is an adaptive response to proteotoxic stresses including heat shock, and is regulated by heat-shock factor 1 (HSF1) in mammals. Proteotoxic stresses challenge all subcellular compartments including the mitochondria. Therefore, there must be close connections between mitochondrial signals and the activity of HSF1. Here, we show that heat shock triggers nuclear translocation of mitochondrial SSBP1, which is involved in replication of mitochondrial DNA, in a manner dependent on the mitochondrial permeability transition pore ANT-VDAC1 complex and direct interaction with HSF1. HSF1 recruits SSBP1 to the promoters of genes encoding cytoplasmic/nuclear and mitochondrial chaperones. HSF1-SSBP1 complex then enhances their induction by facilitating the recruitment of a chromatin-remodelling factor BRG1, and supports cell survival and the maintenance of mitochondrial membrane potential against proteotoxic stresses. These results suggest that the nuclear translocation of mitochondrial SSBP1 is required for the regulation of cytoplasmic/nuclear and mitochondrial proteostasis against proteotoxic stresses.