bims-cytox1 2023-06-11 papers

Issue of 2023‒06‒11
three papers selected by
Gavin McStay
Liverpool John Moores University

EMBO Rep. 2023 Jun 05. e56430

Human Tim8a, Tim8b and Tim13 are auxiliary assembly factors of mature Complex IV.

Alexander J Anderson, Jordan J Crameri, Ching-Seng Ang, Tess R Malcolm, Yilin Kang, Megan J Baker, Catherine S Palmer, Alice J Sharpe, Luke E Formosa, Katherine Ganio, Michael J Baker, Christopher A McDevitt, Michael T Ryan, Megan J Maher, Diana Stojanovski.

Human Tim8a and Tim8b are paralogous intermembrane space proteins of the small TIM chaperone family. Yeast small TIMs function in the trafficking of proteins to the outer and inner mitochondrial membranes. This putative import function for hTim8a and hTim8b has been challenged in human models, but their precise molecular function(s) remains undefined. Likewise, the necessity for human cells to encode two Tim8 proteins and whether any potential redundancy exists is unclear. We demonstrate that hTim8a and hTim8b function in the assembly of cytochrome c oxidase (Complex IV). Using affinity enrichment mass spectrometry, we define the interaction network of hTim8a, hTim8b and hTim13, identifying subunits and assembly factors of the Complex IV COX2 module. hTim8-deficient cells have a COX2 and COX3 module defect and exhibit an accumulation of the Complex IV S2 subcomplex. These data suggest that hTim8a and hTim8b function in assembly of Complex IV via interactions with intermediate-assembly subcomplexes. We propose that hTim8-hTim13 complexes are auxiliary assembly factors involved in the formation of the Complex IV S3 subcomplex during assembly of mature Complex IV.

Keywords: Complex IV; mitochondria; protein assembly; protein trafficking; small TIMs

DOI: https://doi.org/10.15252/embr.202256430

FEBS Lett. 2023 Jun 05.

Defective mitochondrial import as a challenge for cellular protein homeostasis.

Klaudia K Maruszczak, Selvaraj Ayyamperumal, Agnieszka Chacinska.

Mitochondria are organelles indispensable for the correct functioning of eukaryotic cells. Their significance for cellular homeostasis is manifested by the existence of complex quality control pathways that monitor organellar fitness. Mitochondrial biogenesis relies on the efficient import of mitochondrial precursor proteins, a large majority of which are encoded by nuclear DNA and synthesized in the cytosol. This creates a demand for highly specialized import routes that comprise cytosolic factors and organellar translocases. The passage of newly encoded mitochondrial precursor proteins through the cytosol to the translocase of the outer mitochondrial membrane (TOM) is under tight surveillance. As a result of mitochondrial import defects, mitochondrial precursor proteins accumulate in the cytosol or clog the TOM complex, which in turn stimulates cellular stress responses to minimize the consequences of these challenges. These responses are critical for maintaining protein homeostasis under conditions of mitochondrial stress. The present review summarizes recent advances in the field of mitochondrial protein import quality control and discusses the role of this quality control within the network of cellular mechanisms that maintain the cellular homeostasis of proteins.

Keywords: cellular stress responses; mitochondria; mitochondrial dysfunction; mitochondrial quality control; protein aggregates; protein homeostasis

DOI: https://doi.org/10.1002/1873-3468.14677

Nature. 2023 Jun 07.

A cytosolic surveillance mechanism activates the mitochondrial UPR.

F X Reymond Sutandy, Ines Gößner, Georg Tascher, Christian Münch.

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 proteostasis1,2. Yet, it remains unclear how the information on mitochondria misfolding stress (MMS) is signalled to the nucleus as part of the human UPRmt (refs. 3,4). Here, we show that UPRmt 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 UPRmt; 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 UPRmt genes. Together, we identify a highly controlled cytosolic surveillance mechanism that integrates independent mitochondrial stress signals to initiate the UPRmt. These observations reveal a link between mitochondrial and cytosolic proteostasis and provide molecular insight into UPRmt signalling in human cells.

DOI: https://doi.org/10.1038/s41586-023-06142-0