Oxidative protein biogenesis and redox regulation in the mitochondrial intermembrane space

Mitochondria are organelles that play a central role in cellular metabolism, as they are responsible for processes such as iron/sulfur cluster biogenesis, respiration and apoptosis. Here, we describe briefly the various protein import pathways for sorting of mitochondrial proteins into the different subcompartments, with an emphasis on the targeting to the intermembrane space. The discovery of a dedicated redox-controlled pathway in the intermembrane space that links protein import to oxidative protein folding raises important questions on the redox regulation of this process. We discuss the salient features of redox regulation in the intermembrane space and how such mechanisms may be linked to the more general redox homeostasis balance that is crucial not only for normal cell physiology but also for cellular dysfunction

The yeast voltage-dependent anion channel Porin: more IMPORTant than just metabolite transport

Porin is crucial for metabolite flux in mitochondria. In this issue of Molecular Cell, Sakaue et al. (2019) and Ellenrieder et al. (2019) describe an unexpected role for Porin in mitochondrial protein import by regulating the oligomeric state of the major protein import gate, the TOM complex, and the inner membrane insertion of metabolite carriers

MIMAS is a new giant multifunctional player in the mitochondrial megacomplex playground

Mitochondria are rich in multi-protein assemblies that are usually dedicated to one function. In this issue of Cell Reports, Horten et al.1 describe a 3-nanometer megacomplex in the mitochondrial inner membrane, which serves multiple functions integrating mitochondria biogenesis and metabolism

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

AbstractMitochondria 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 30years 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

Oxidative protein folding in the mitochondrial intermembrane space

Disulfide bond formation is a crucial step for oxidative folding and necessary for the acquisition of a protein's native conformation. Introduction of disulfide bonds is catalyzed in specialized subcellular compartments and requires the coordinated action of specific enzymes. The intermembrane space of mitochondria has recently been found to harbor a dedicated machinery that promotes the oxidative folding of substrate proteins by shuttling disulfide bonds. The newly identified oxidative pathway consists of the redox-regulated receptor Mia40 and the sulfhydryl oxidase Erv1. Proteins destined to the intermembrane space are trapped by a disulfide relay mechanism that involves an electron cascade from the incoming substrate to Mia40, then on to Erv1, and finally to molecular oxygen via cytochrome c. This thiol–disulfide exchange mechanism is essential for the import and for maintaining the structural stability of the incoming precursors. In this review we describe the mechanistic parameters that define the interaction and oxidation of the substrate proteins in light of the recent publications in the mitochondrial oxidative folding field

Targeting and insertion of membrane proteins in mitochondria

Mitochondrial membrane proteins play an essential role in all major mitochondrial functions. The respiratory complexes of the inner membrane are key for the generation of energy. The carrier proteins for the influx/efflux of essential metabolites to/from the matrix. Many other inner membrane proteins play critical roles in the import and processing of nuclear encoded proteins (∼99% of all mitochondrial proteins). The outer membrane provides another lipidic barrier to nuclear-encoded protein translocation and is home to many proteins involved in the import process, maintenance of ionic balance, as well as the assembly of outer membrane components. While many aspects of the import and assembly pathways of mitochondrial membrane proteins have been elucidated, many open questions remain, especially surrounding the assembly of the respiratory complexes where certain highly hydrophobic subunits are encoded by the mitochondrial DNA and synthesised and inserted into the membrane from the matrix side. This review will examine the various assembly pathways for inner and outer mitochondrial membrane proteins while discussing the most recent structural and biochemical data examining the biogenesis process

MiR-195 regulates mitochondrial function by targeting mitofusin-2 in breast cancer cells

Mitochondrial dynamics is a highly dysregulated process in cancer. Apoptosis and mitochondrial fission are two concurrent events wherein increased mitochondrial fragmentation serves as a hallmark of apoptosis. We have shown earlier that miR-195 exerts pro-apoptotic effects in breast cancer cells. Herein, we have demonstrated miR-195 as a modulator of mitochondrial dynamics and function. Imaging experiments upon miR-195 treatment have shown that mitochondria undergo extensive fission. We validated mitofusin2 as a potential target of miR-195. Which may provide a molecular explanation for the respiratory defects induced by miR-195 over-expression in breast cancer cells? Active, but not total, mitochondrial mass, was reduced with increasing levels of miR-195. We have further shown that miR-195 enhances mitochondrial SOD-2 expression but does not affect PINK1 levels in breast cancer cells. Collectively, we have revealed that miR-195 is a modulator of mitochondrial dynamics by targeting MFN2 thereby impairing mitochondrial function. Concomitantly, it enhances the scavenger of reactive oxygen species (SOD-2) to maintain moderate levels of oxidative stress. Our findings suggest a therapeutic potential of miR-195 in both ER-positive as well as ER-negative breast cancer cells

Involvement of separate domains of the cellulosomal protein S1 of Clostridium thermocellum in binding to cellulose and in anchoring of catalytic subunits to the cellulosome

AbstractFragments of the 25OkDa SI subunit of the Clostridium thermocellum cellulosome were obtained by protease-induced or spontaneous degradation. All detectable fragments, down to a mass of about 30 kDa, retained the ability to bind to 125I-labelled endoglucanase CelD, one of the catalytic subunits of the cellulosome. Several fragments were able to bind both to cellulose and to CElD. However, some fragments that could still bind to CelD did not have the ability to bind to cellulose. Therefore, S1, a putative scaffolding protein of the cellulosome, is likely to carry two separate types of domains, one of which binds to cellulose, while the other type binds to the various catalytic subunits of the complex

The mitochondrial intermembrane space: the most constricted mitochondrial sub-compartment with the largest variety of protein import pathways

The mitochondrial intermembrane space (IMS) is the most constricted sub-mitochondrial compartment, housing only about 5% of the mitochondrial proteome, and yet is endowed with the largest variability of protein import mechanisms. In this review, we summarize our current knowledge of the major IMS import pathway based on the oxidative protein folding pathway and discuss the stunning variability of other IMS protein import pathways. As IMS-localized proteins only have to cross the outer mitochondrial membrane, they do not require energy sources like ATP hydrolysis in the mitochondrial matrix or the inner membrane electrochemical potential which are critical for import into the matrix or insertion into the inner membrane. We also explore several atypical IMS import pathways that are still not very well understood and are guided by poorly defined or completely unknown targeting peptides. Importantly, many of the IMS proteins are linked to several human diseases, and it is therefore crucial to understand how they reach their normal site of function in the IMS. In the final part of this review, we discuss current understanding of how such IMS protein underpin a large spectrum of human disorders

Interaction of the duplicated segment carried by Clostridium thermocellum cellulases with cellulosome components

AbstractThe function of the non-catalytic, duplicated segment found in C. thermocellum cellulases was investigated. Rabbit antibodies reacting with the duplicated segment of endoglucanase CelD cross-reacted with a variety of cellulosome components ranging between 50 and 100 kDa. 125I-labeled forms of CelD and of xylanase XynZ carrying the duplicated segment bound to a set of cellulosome proteins ranging between 66 and 250 kDa, particularly to the 250 kDa SL (or S1) subunit. 125I-labeled forms of CelD and XynZ devoid of the duplicated segment failed to bind to any cellulosome protein. The duplicated segment appears thus to serve to anchor the various cellulosome subunits to the complex by binding to SL, which may be a scaffolding element of the cellulosome