The mitochondrial unfolded protein response (UPR^mt) is essential to safeguard mitochondria from proteotoxic damage by activating a dedicated transcriptional response in the nucleus to restore proteostasis^1,2. Yet, it remains unclear how the information on mitochondria misfolding stress (MMS) is signalled to the nucleus as part of the human UPR^mt (refs. ^3,4). Here, we show that UPR^mt signalling is driven by the release of two individual signals in the cytosol—mitochondrial reactive oxygen species (mtROS) and accumulation of mitochondrial protein precursors in the cytosol (c-mtProt). Combining proteomics and genetic approaches, we identified that MMS causes the release of mtROS into the cytosol. In parallel, MMS leads to mitochondrial protein import defects causing c-mtProt accumulation. Both signals integrate to activate the UPR^mt; released mtROS 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 UPR^mt genes. Together, we identify a highly controlled cytosolic surveillance mechanism that integrates independent mitochondrial stress signals to initiate the UPR^mt. These observations reveal a link between mitochondrial and cytosolic proteostasis and provide molecular insight into UPR^mt signalling in human cells.

线粒体未折叠蛋白反应 (UPR ^mt ) 对于通过激活细胞核中的专用转录反应来恢复蛋白质稳态来保护线粒体免受蛋白质毒性损伤至关重要^1,2。然而，目前尚不清楚关于线粒体错误折叠应激 (MMS) 的信息如何作为人类 UPR ^mt的一部分向细胞核发出信号（参考文献^3,4）。在这里，我们表明 UPR ^mt信号传导是由细胞质中两个单独信号的释放驱动的——线粒体活性氧 (mtROS) 和细胞质中线粒体蛋白前体的积累 (c-mtProt)。结合蛋白质组学和遗传学方法，我们发现 MMS 导致 mtROS 释放到细胞质中。与此同时，MMS 会导致线粒体蛋白输入缺陷，从而导致 c-mtProt 积累。两个信号整合以激活 UPR ^mt；释放的 mtROS 氧化细胞质 HSP40 蛋白 DNAJA1，从而导致细胞质 HSP70 向 c-mtProt 的招募增强。因此，HSP70 释放 HSF1，HSF1 易位至细胞核并激活 UPR ^mt的转录基因。我们共同确定了一种高度控制的胞质监视机制，该机制整合独立的线粒体应激信号以启动 UPR ^mt。这些观察结果揭示了线粒体和细胞质蛋白质稳态之间的联系，并为人类细胞中 UPR ^mt信号传导提供了分子洞察。