The UPRmt preserves mitochondrial import to extend lifespan

The mitochondrial unfolded protein response (UPRmt) is dedicated to promote mitochondrial proteostasis and is linked to extreme longevity in worms, flies, and mice. The key regulator of this process is the transcription factor, ATFS-1. In the absence of mitochondrial stress, ATFS-1 is transported to the mitochondria and degraded. During conditions of mitochondrial stress, ATFS-1 is excluded from the mitochondria and enters the nucleus to regulate the expression of UPRmt genes. However, there exists a dichotomy in regards to induction of the UPRmt and mitochondrial import. The repair proteins synthesized as a direct result of UPRmt activation must be transported into damaged mitochondria that had previously excluded ATFS-1 due to reduced import efficiency. To address this conundrum, we analyzed the role of the import machinery under conditions where the UPRmt was induced. Using in vitro biochemical assays of mitochondrial import and in vivo analysis of mitochondrial proteins, we surprisingly find that the efficiency of mitochondrial import increases when the UPRmt is activated in an ATFS-1 dependent manner, even though membrane potential is reduced. The import machinery is upregulated at the transcription and translation level, and intact import machinery is essential for UPRmt-mediated increase and lifespan extension. With age, import capacity decreases, and activation of the UPRmt delays this decline and increases longevity. Finally, we find that ATFS-1 has a significantly weaker mitochondrial targeting sequence (MTS), allowing for dynamic subcellular localization during the initial stages of UPRmt activation.