An Early mtUPR: Redistribution of the Nuclear Transcription Factor Rox1 to Mitochondria Protects against Intramitochondrial Proteotoxic Aggregates

Abstract

SummaryThe mitochondrial proteome is built mainly by import of nuclear-encoded precursors, which are targeted mostly by cleavable presequences. Presequence processing upon import is essential for proteostasis and survival, but the consequences of dysfunctional protein maturation are unknown. We find that impaired presequence processing causes accumulation of precursors inside mitochondria that form aggregates, which escape degradation and unexpectedly do not cause cell death. Instead, cells survive via activation of a mitochondrial unfolded protein response (mtUPR)-like pathway that is triggered very early after precursor accumulation. In contrast to classical stress pathways, this immediate response maintains mitochondrial protein import, membrane potential, and translation through translocation of the nuclear HMG-box transcription factor Rox1 to mitochondria. Rox1 binds mtDNA and performs a TFAM-like function pivotal for transcription and translation. Induction of early mtUPR provides a reversible stress model to mechanistically dissect the initial steps in mtUPR pathways with the stressTFAM Rox1 as the first line of defense.

Open questions on the mitochondrial unfolded protein response

“…Unfortunately, these mechanistic analyses are still strongly underrepresented in the field of mitochondrial stress research. In a recent study employing the model system yeast accumulation of unfolded proteins in the mitochondrial matrix induced a strong transcriptional response [20] (Fig. 3).…”

“…arsenite [38], paraquat [32,39,40], oligomycin [35][36][37]41], antimycin A [35,37,41], piericidin [35], rotenone [40], actinonin [31], FCCP/CCCP [23,31,37], dinitrophenol [42], chloramphenicol [43], doxycycline [31,43], MitoBloCK-6 [31]), depletion of mtDNA/ethidium bromide treatment [39,41,42,44], mutations in the mitochondrial ribosome [24], knockdown or knockout of mitochondrial proteins by RNAi or CRISPR/Cas9 [TIM23 [32,39], SPG-7 [32,39,42,45], LON [34], presequence protease (PreP) [46], HSP70 [25,42,45], HSP60 [42], mitochondrial aspartyl-tRNA synthetase [47]], heat shock [42,45] or the use of temperature-sensitive alleles [e.g. mia40-4int, mas1, zc-32 (an unknown gene) [20,21,48]; Fig. 2].…”

“…In contrast to the proposed model in C. elegans, Rox1 localizes to the nucleus under nonstress conditions, but is imported into mitochondria upon mitochondrial perturbations. Mechanistic analysis revealed that Rox1 binds mitochondrial DNA and upholds organellar transcription and translation upon mtUPR [20]. Also for the function of Rox1, several details need further investigation, for example the exact mechanism how Rox1 binds and protects mtDNA, but it already challenges the existing models and invites to revisit mtUPR paradigms.…”

More than just a ticket canceller: The mitochondrial processing peptidase matures complex precursor proteins at internal cleavage sites

“…Dysfunctional preprotein maturation in mitochondria was observed in models of Alzheimer's disease (Mossmann et al, 2014). It results in proteome instability, aggregation of incorrectly processed precursors and proteotoxic stress (Poveda-Huertes et al, 2020).…”

Author's View: a nuclear transcription factor relocalizing to mitochondria rescues cells from proteotoxic aggregates

“…7 Introduction of PMPCB patient mutations into the homologous Mas1 protein in Saccharomyces cerevisiae resulted in a temperature sensitive phenotype, in which MPP activity was impaired upon growth at elevated temperature. 6,8 These cells enabled not only investigation of the cellular consequences of defective MPP processing but also analysis of the fate of unprocessed precursors. Upon MPP inactivation unprocessed precursors accumulated in the mitochondrial matrix.…”