RNAseq UPRmt
The mitochondrial unfolded protein response (UPRmt) is essential to safeguard mitochondria from proteotoxic damage by activating a dedicated transcriptional response in the nucleus to restore proteostasis. Yet it remains unclear how the information on mitochondria misfolding stress (MMS) is signalled to the nucleus as part of the human UPRmt. Here, we show that UPRmt signalling is driven by the release of two individual signals in the cytosol – mitochondrially-produced reactive oxygen species (ROS) and accumulation of mitochondrial preproteins in the cytosol (c-mtProt). Combining proteomics and genetic approaches, we identified that MMS causes the release of ROS from mitochondria into the cytosol, which is essential for UPRmt signalling. In parallel, MMS leads to mitochondrial protein import defects causing c-mtProt accumulation. Both signals integrate to activate the UPRmt: released ROS oxidize the cytosolic HSP40 protein DNAJA1, which leads to enhanced recruitment of cytosolic HSP70 to c-mtProt. Consequently, HSP70 releases HSF1, which translocates to the nucleus and activates transcription of UPRmt genes. Strikingly, artificial ROS production and c-mtProt accumulation was sufficient to activate the UPRmt, revealing a highly-controlled cytosolic surveillance mechanism that integrates independent mitochondrial stress signals to initiate the UPRmt via HSF1 activation in human cells. These observations reveal a novel link between mitochondrial and cytosolic proteostasis and provide molecular insight into UPRmt signalling in humans.