bims-cytox1 2020-05-17 papers

Issue of 2020–05–17
five papers selected by
Gavin McStay, Staffordshire University

Mitochondrion. 2020 May 05. pii: S1567-7249(20)30012-X. [Epub ahead of print]53 57-65

Protein turnover rates in plant mitochondria.

Shaobai Huang, Lei Li, Jakob Petereit, A Harvey Millar.

Plant mitochondria operate as the powerhouses for cellular energy production by using the combined functions of both imported and organelle-synthesised proteins. Homeostasis of mitochondrial proteins is controlled by both synthesis and degradation processes which together define protein turnover in this organelle. Better understanding of plant mitochondrial protein turnover will provide information on protein quality control inside these organelles and its importance for proper function and regulation of mitochondrial machinery. This review discusses methods used for measurement of turnover rates of plant mitochondrial proteins and presents our current understanding of these rates for key mitochondrial proteins and protein complexes. The assembly and maintenance of mitochondrial OXPHOS complexes, in particular Complexes I and V, will be discussed in detail based on the evidence for differential protein turnover rates of the same protein subunits in different mitochondrial fractions. The impact of the loss of specific plant mitochondrial proteases on proteolysis events and rates of mitochondrial protein turnover will be highlighted. The challenges and future directions for investigation of plant mitochondrial protein turnover are also discussed.

Keywords: Plant mitochondria; Proteases; Protein complex assembly; Protein turnover; Proteostasis

DOI: https://doi.org/10.1016/j.mito.2020.04.011

Trends Biochem Sci. 2020 May 11. pii: S0968-0004(20)30091-8. [Epub ahead of print]

Cytosolic Events in the Biogenesis of Mitochondrial Proteins.

Yury S Bykov, Doron Rapaport, Johannes M Herrmann, Maya Schuldiner.

While targeting of proteins synthesized in the cytosol to any organelle is complex, mitochondria present the most challenging of destinations. First, import of nuclear-encoded proteins needs to be balanced with production of mitochondrial-encoded ones. Moreover, as mitochondria are divided into distinct subdomains, their proteins harbor a number of different targeting signals and biophysical properties. While translocation into the mitochondrial membranes has been well studied, the cytosolic steps of protein import remain poorly understood. Here, we review current knowledge on mRNA and protein targeting to mitochondria, as well as recent advances in our understanding of the cellular programs that respond to accumulation of mitochondrial precursor proteins in the cytosol, thus linking defects in targeting-capacity to signaling.

Keywords: RNA-binding proteins; chaperones; mitochondrial precursor; mitochondrial protein import; mitochondrial targeting sequence; nascent-chain associated complex

DOI: https://doi.org/10.1016/j.tibs.2020.04.001

J Exp Biol. 2020 May 11. pii: jeb.223776. [Epub ahead of print]

Stable mitochondrial CICIII2 supercomplex interactions in reptiles compared to homeothermic vertebrates.

Amanda Bundgaard, Andrew M James, Michael E Harbour, Michael P Murphy, Angela Fago.

The association of complex I (CI), complex III (CIII) and complex IV (CIV) of the mitochondrial electron transport chain into stable high-molecular weight supercomplexes (SCs) has been observed in several prokaryotes and eukaryotes, but among vertebrates it has only been examined in mammals. The biological role of these SCs is unclear but suggestions so far include enhanced electron transfer between complexes, decreased production of the reactive oxygen species (ROS) O2 ·- and H2O2, or enhanced structural stability. Here, we provide the first overview on the stability, composition and activity of mitochondrial SCs in representative species of several vertebrate classes to determine patterns of SC variation across endotherms and ectotherms. We found that the stability of the CICIII2 SC and the inclusion of CIV within SC varied considerably. Specifically, when solubilized by the detergent DDM, mitochondrial CICIII2 SCs were unstable in endotherms (birds and mammals) and highly stable in reptiles. Using mass-spectrometric complexomics, we confirmed that the CICIII2 is the major SC in the turtle, and that 90% of CI is found in this highly stable SC. Interestingly, the presence of stable SCs did not prevent mitochondrial H2O2 production and was not associated with elevated respiration rates of mitochondria isolated from the examined species. Together, these data show that SC stability varies among vertebrates and is greatest in poikilothermic reptiles and weakest in endotherms. This pattern suggests an adaptive role of SCs to varying body temperature, but not necessarily a direct effect on electron transfer or in the prevention of ROS production.

Keywords: Bioenergetics; Complexomics; Mass spectrometry; Mitochondria; Oxidative phosphorylation; Reactive oxygen species

DOI: https://doi.org/10.1242/jeb.223776

PLoS One. 2020 ;15(5): e0233177

Atco, a yeast mitochondrial complex of Atp9 and Cox6, is an assembly intermediate of the ATP synthase.

Leticia Veloso Ribeiro Franco, Chen-Hsien Su, Julia Burnett, Lorisa Simas Teixeira, Alexander Tzagoloff.

Mitochondrial oxidative phosphorylation (oxphos) is the process by which the ATP synthase conserves the energy released during the oxidation of different nutrients as ATP. The yeast ATP synthase consists of three assembly modules, one of which is a ring consisting of 10 copies of the Atp9 subunit. We previously reported the existence in yeast mitochondria of high molecular weight complexes composed of mitochondrially encoded Atp9 and of Cox6, an imported structural subunit of cytochrome oxidase (COX). Pulse-chase experiments indicated a correlation between the loss of newly translated Atp9 complexed to Cox6 and an increase of newly formed Atp9 ring, but did not exclude the possibility of an alternate source of Atp9 for ring formation. Here we have extended studies on the functions and structure of this complex, referred to as Atco. We show that Atco is the exclusive source of Atp9 for the ATP synthase assembly. Pulse-chase experiments show that newly translated Atp9, present in Atco, is converted to a ring, which is incorporated into the ATP synthase with kinetics characteristic of a precursor-product relationship. Even though Atco does not contain the ring form of Atp9, cross-linking experiments indicate that it is oligomeric and that the inter-subunit interactions are similar to those of the bona fide ring. We propose that, by providing Atp9 for biogenesis of ATP synthase, Atco complexes free Cox6 for assembly of COX. This suggests that Atco complexes may play a role in coordinating assembly and maintaining proper stoichiometry of the two oxphos enzymes.

DOI: https://doi.org/10.1371/journal.pone.0233177

Int J Mol Sci. 2020 May 12. pii: E3414. [Epub ahead of print]21(10):

Yeast Mitochondrial Translation Initiation Factor 3 Interacts with Pet111p to Promote COX2 mRNA Translation.

Ivan Chicherin, Sergey Levitskii, Maria V Baleva, Igor A Krasheninnikov, Maxim V Patrushev, Piotr Kamenski.

Mitochondrial genomes code for several core components of respiratory chain complexes. Thus, mitochondrial translation is of great importance for the organelle as well as for the whole cell. In yeast, mitochondrial translation initiation factor 3, Aim23p, is not essential for the organellar protein synthesis; however, its absence leads to a significant quantitative imbalance of the mitochondrial translation products. This fact points to a possible specific action of Aim23p on the biosynthesis of some mitochondrial protein species. In this work, we examined such peculiar effects of Aim23p in relation to yeast mitochondrial COX2 mRNA translation. We show that Aim23p is indispensable to this process. According to our data, this is mediated by Aimp23p interaction with the known specific factor of the COX2 mRNA translation, Pet111p. If there is no Aim23p in the yeast cells, an increased amount of Pet111p ensures proper COX2 mRNA translation. Our results demonstrate the additional non-canonical function of initiation factor 3 in yeast mitochondrial translation.

Keywords: initiation factor; mitochondria; translation; translational activator

DOI: https://doi.org/10.3390/ijms21103414