labelling-visualization-mitochondrial-genome-expression-products

Labelling and Visualization of Mitochondrial Genome Expression Products in Baker's Yeast <em>Saccharomyces cerevisiae</em>

Mitochondria are essential organelles of eukaryotic cells capable of aerobic respiration. They contain circular genome and gene expression apparatus. A ...

M.V. Lomonosov Moscow State University

Protein biosynthesis in mitochondria is organized in a bacterial manner. However, during evolution, mitochondrial translation mechanisms underwent many organelle-specific changes. In particular, almost all mitochondrial translation factors, being orthologous to bacterial proteins, are characterized by some unique elements of primary or secondary structure. In the case of the organellar initiation factor 3 (IF3), these elements are several dozen amino acids long N- and C-terminal extensions. This study focused on the terminal extensions of baker's yeast mitochondrial IF3, Aim23p. By in vivo deletion and complementation analysis, we show that at least one extension is necessary for Aim23p function. At the same time, human mitochondrial IF3 is fully functional in yeast mitochondria even without both terminal extensions. While  IF3 itself is poorly active in yeast mitochondria, adding Aim23p terminal extensions makes the resulting chimeric protein as functional as the cognate factor. Our results show that the terminal extensions of IF3 have evolved as the "adaptors" that accommodate the translation factor of bacterial origin to the evolutionary changed protein biosynthesis system in mitochondria.

Biological and Evolutionary Significance of Terminal Extensions of Mitochondrial Translation Initiation Factor 3.

After billions of years of evolution, mitochondrion retains its own genome, which gets expressed in mitochondrial matrix. Mitochondrial translation machinery rather differs from modern bacterial and eukaryotic cytosolic systems. Any disturbance in mitochondrial translation drastically impairs mitochondrial function. In budding yeast , deletion of the gene coding for mitochondrial translation initiation factor 3 - , leads to an imbalance in mitochondrial protein synthesis and significantly delays growth after shifting from fermentable to non-fermentable carbon sources. Molecular mechanism underlying this adaptation to respiratory growth was unknown. Here, we demonstrate that slow adaptation from glycolysis to respiration in the absence of Aim23p is accompanied by a gradual increase of cytochrome c oxidase activity and by increased levels of Tma19p protein, which protects mitochondria from oxidative stress.

 Strain Lacking Mitochondrial IF3 Shows Increased Levels of Tma19p during Adaptation to Respiratory Growth.

The initiation of protein synthesis in bacteria is ruled by three canonical factors: IF1, IF2, and IF3. This system persists in human mitochondria; however, it functions in a rather different way due to specialization and adaptation to the organellar micro-environment. We focused on human mitochondrial IF3, which was earlier studied in vitro, but no knock-out cellular models have been published up to date. In this work, we generated human HeLa cell lines deficient in the MTIF3 gene and analyzed their mitochondrial function. Despite the lack of IF3mt in these cells, they preserved functional mitochondria capable of oxygen consumption and protein synthesis; however, the translation of ATP6 mRNA was selectively decreased which compromised the assembly of ATP synthase. Together with the analogous results obtained earlier for baker's yeast mitochondrial IF3, our findings point to a functional divergence of mitochondrial initiation factors from their bacterial ancestors.

Initiation Factor 3 is Dispensable For Mitochondrial Translation in Cultured Human Cells.

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  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  mRNA translation, Pet111p. If there is no Aim23p in the yeast cells, an increased amount of Pet111p ensures proper  mRNA translation. Our results demonstrate the additional non-canonical function of initiation factor 3 in yeast mitochondrial translation.

Article	Year
Biological and Evolutionary Significance of Terminal Extensions of Mitochondrial Translation Initiation Factor 3. International journal of molecular sciences\| PubMed ID: 30518034	2018
Strain Lacking Mitochondrial IF3 Shows Increased Levels of Tma19p during Adaptation to Respiratory Growth. Cells\| PubMed ID: 31248014	2019
Initiation Factor 3 is Dispensable For Mitochondrial Translation in Cultured Human Cells. Scientific reports\| PubMed ID: 32346061	2020
Yeast Mitochondrial Translation Initiation Factor 3 Interacts with Pet111p to Promote mRNA Translation. International journal of molecular sciences\| PubMed ID: 32408541	2020

Igor A. Krasheninnikov

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