Trapping oxidative folding intermediates during translocation to the intermembrane space of mitochondria: in vivo and in vitro studies

Glutathionylated and Fe-S cluster containing hMIA40 (CHCHD4) regulates ROS and mitochondrial complex III and IV activities of the electron transport chain

Human MIA40, an intermembrane space (IMS) import receptor of mitochondria harbors twin CX9C motifs for stability while its CPC motif is known to facilitate the import of IMS bound proteins. Site-directed mutagenesis complemented by MALDI on in vivo hMIA40 protein shows that a portion of MIA40 undergoes reversible S-glutathionylation at three cysteines in the twin CX9C motifs and the lone cysteine 4 residue. We find that HEK293T cells expressing hMIA40 mutant defective for glutathionylation are compromised in the activities of complexes III and IV of the Electron Transport Chain (ETC) and enhance Reactive Oxygen Species (ROS) levels. Immunocapture studies show MIA40 interacting with complex III. Interestingly, glutathionylated MIA40 can transfer electrons to cytochrome C directly. However, Fe–S clusters associated with the CPC motif are essential to facilitate the two-electron to one-electron transfer for reducing cytochrome C. These results suggest that hMIA40 undergoes glutathionylation to maintain ROS levels and for optimum function of complexes III and IV of ETC. Our studies shed light on a novel post-translational modification of hMIA40 and its ability to act as a redox switch to regulate the ETC and cellular redox homeostasis.

Unconventional Targeting of a Thiol Peroxidase to the Mitochondrial Intermembrane Space Facilitates Oxidative Protein Folding

Thiol peroxidases are conserved hydrogen peroxide scavenging and signaling molecules that contain redox-active cysteine residues. We show here that Gpx3, the major H₂O₂ sensor in yeast, is present in the mitochondrial intermembrane space (IMS), where it serves a compartment-specific role in oxidative metabolism. The IMS-localized Gpx3 contains an 18-amino acid N-terminally extended form encoded from a non-AUG codon. This acts as a mitochondrial targeting signal in a pathway independent of the hitherto known IMS-import pathways. Mitochondrial Gpx3 interacts with the Mia40 oxidoreductase in a redox-dependent manner and promotes efficient Mia40-dependent oxidative protein folding. We show that cells lacking Gpx3 have aberrant mitochondrial morphology, defective protein import capacity, and lower inner membrane potential, all of which can be rescued by expression of a mitochondrial-only form of Gpx3. Together, our data reveal a novel role for Gpx3 in mitochondrial redox regulation and protein homeostasis.

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A pool of yeast Gpx3 localizes to mitochondria via translation from a non-AUG codon

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Loss of Gpx3 causes defects in mitochondrial architecture and membrane potential

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Gpx3 interacts with the oxidative protein folding machinery in the IMS

Oxidative folding in the mitochondrial intermembrane space: A regulated process important for cell physiology and disease

Mitochondria are fundamental organelles with a complex internal architecture that fulfill important diverse functions including iron–sulfur cluster assembly and cell respiration. Intense work for more than 30 years has identified the key protein import components and the pathways involved in protein targeting and assembly. More recently, oxidative folding has been discovered as one important mechanism for mitochondrial proteostasis whilst several human disorders have been linked to this pathway. We describe the molecular components of this pathway in view of their putative redox regulation and we summarize available evidence on the connections of these pathways to human disorders.

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Mitochondria are the cell center of iron–sulfur cluster assembly and cell respiration.

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The MIA pathway has recently been linked to Fe/S pathways, Ca^2 + uptake and apoptosis.

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Mitochondria along with the ER and peroxisomes are major sources of ROS.

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Many diseases have been linked to mitochondrial dysfunction.

Redox regulation of mitochondrial function

Redox-dependent processes influence most cellular functions, such as differentiation, proliferation, and apoptosis. Mitochondria are at the center of these processes, as mitochondria both generate reactive oxygen species (ROS) that drive redox-sensitive events and respond to ROS-mediated changes in the cellular redox state. In this review, we examine the regulation of cellular ROS, their modes of production and removal, and the redox-sensitive targets that are modified by their flux. In particular, we focus on the actions of redox-sensitive targets that alter mitochondrial function and the role of these redox modifications on metabolism, mitochondrial biogenesis, receptor-mediated signaling, and apoptotic pathways. We also consider the role of mitochondria in modulating these pathways, and discuss how redox-dependent events may contribute to pathobiology by altering mitochondrial function.