Yme2p is a mediator of nucleoid structure and number in mitochondria of the yeast Saccharomyces cerevisiae

Numtogenesis as a mechanism for development of cancer

Transfer of genetic material from cytoplasmic organelles to the nucleus, an ongoing process, has implications in evolution, aging, and human pathologies such as cancer. The transferred mitochondrial DNA (mtDNA) fragments in the nuclear genome are called nuclear mtDNA or NUMTs. We have named the process numtogenesis, defining the term as the transfer of mtDNA into the nuclear genome, or, less specifically, the transfer of mitochondria or mitochondrial components into the nucleus. There is increasing evidence of the involvement of NUMTs in human biology and pathology. Although information pertaining to NUMTs and numtogenesis is sparse, the role of this aspect of mitochondrial biology to human cancers is apparent. In this review, we present available knowledge about the origin and mechanisms of numtogenesis, with special emphasis on the role of NUMTs in human malignancies. We describe studies undertaken in our laboratory and in others and discuss the influence of NUMTs in tumor initiation and progression and in survival of cancer patients. We describe suppressors of numtogenesis and evolutionary conserved mechanisms underlying numtogenesis in cancer. An understanding the emerging field of numtogenesis should allow comprehension of this process in various malignancies and other diseases and, more generally, in human health.

ER-associated mitochondrial division links the distribution of mitochondria and mitochondrial DNA in yeast

Mitochondrial division is important for mitochondrial distribution and function. Recent data have demonstrated that ER-mitochondria contacts mark mitochondrial division sites, but the molecular basis and functions of these contacts are not understood. Here we show that in yeast, the ER-mitochondria tethering complex, ERMES, and the highly conserved Miro GTPase, Gem1, are spatially and functionally linked to ER-associated mitochondrial division. Gem1 acts as a negative regulator of ER-mitochondria contacts, an activity required for the spatial resolution and distribution of newly generated mitochondrial tips following division. Previous data have demonstrated that ERMES localizes with a subset of actively replicating mitochondrial nucleoids. We show that mitochondrial division is spatially linked to nucleoids and that a majority of these nucleoids segregate prior to division, resulting in their distribution into newly generated tips in the mitochondrial network. Thus, we postulate that ER-associated division serves to link the distribution of mitochondria and mitochondrial nucleoids in cells. DOI:http://dx.doi.org/10.7554/eLife.00422.001.

Protein quality control in organelles - AAA/FtsH story

This review focuses on organellar AAA/FtsH proteases, whose proteolytic and chaperone-like activity is a crucial component of the protein quality control systems of mitochondrial and chloroplast membranes. We compare the AAA/FtsH proteases from yeast, mammals and plants. The nature of the complexes formed by AAA/FtsH proteases and the current view on their involvement in degradation of non-native organellar proteins or assembly of membrane complexes are discussed. Additional functions of AAA proteases not directly connected with protein quality control found in yeast and mammals but not yet in plants are also described shortly. Following an overview of the molecular functions of the AAA/FtsH proteases we discuss physiological consequences of their inactivation in yeast, mammals and plants. The molecular basis of phenotypes associated with inactivation of the AAA/FtsH proteases is not fully understood yet, with the notable exception of those observed in m-AAA protease-deficient yeast cells, which are caused by impaired maturation of mitochondrial ribosomal protein. Finally, examples of cytosolic events affecting protein quality control in mitochondria and chloroplasts are given. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Protein quality control in organelles - AAA/FtsH story

This review focuses on organellar AAA/FtsH proteases, whose proteolytic and chaperone-like activity is a crucial component of the protein quality control systems of mitochondrial and chloroplast membranes. We compare the AAA/FtsH proteases from yeast, mammals and plants. The nature of the complexes formed by AAA/FtsH proteases and the current view on their involvement in degradation of non-native organellar proteins or assembly of membrane complexes are discussed. Additional functions of AAA proteases not directly connected with protein quality control found in yeast and mammals but not yet in plants are also described shortly. Following an overview of the molecular functions of the AAA/FtsH proteases we discuss physiological consequences of their inactivation in yeast, mammals and plants. The molecular basis of phenotypes associated with inactivation of the AAA/FtsH proteases is not fully understood yet, with the notable exception of those observed in m-AAA protease-deficient yeast cells, which are caused by impaired maturation of mitochondrial ribosomal protein. Finally, examples of cytosolic events affecting protein quality control in mitochondria and chloroplasts are given. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Transfection of plant mitochondria and in organello gene integration

Investigation and manipulation of mitochondrial genetics in animal and plant cells remains restricted by the lack of an efficient in vivo transformation methodology. Mitochondrial transfection in whole cells and maintenance of the transfected DNA are main issues on this track. We showed earlier that isolated mitochondria from different organisms can import DNA. Exploiting this mechanism, we assessed the possibility to maintain exogenous DNA in plant organelles. Whereas homologous recombination is scarce in the higher plant nuclear compartment, recombination between large repeats generates the multipartite structure of the plant mitochondrial genome. These processes are under strict surveillance to avoid extensive genomic rearrangements. Nevertheless, following transfection of isolated organelles with constructs composed of a partial gfp gene flanked by fragments of mitochondrial DNA, we demonstrated in organello homologous recombination of the imported DNA with the resident DNA and integration of the reporter gene. Recombination yielded insertion of a continuous exogenous DNA fragment including the gfp sequence and at least 0.5 kb of flanking sequence on each side. According to our observations, transfection constructs carrying multiple sequences homologous to the mitochondrial DNA should be suitable and targeting of most regions in the organelle genome should be feasible, making the approach of general interest.

Molecular poltergeists: mitochondrial DNA copies (numts) in sequenced nuclear genomes

The natural transfer of DNA from mitochondria to the nucleus generates nuclear copies of mitochondrial DNA (numts) and is an ongoing evolutionary process, as genome sequences attest. In humans, five different numts cause genetic disease and a dozen human loci are polymorphic for the presence of numts, underscoring the rapid rate at which mitochondrial sequences reach the nucleus over evolutionary time. In the laboratory and in nature, numts enter the nuclear DNA via non-homolgous end joining (NHEJ) at double-strand breaks (DSBs). The frequency of numt insertions among 85 sequenced eukaryotic genomes reveal that numt content is strongly correlated with genome size, suggesting that the numt insertion rate might be limited by DSB frequency. Polymorphic numts in humans link maternally inherited mitochondrial genotypes to nuclear DNA haplotypes during the past, offering new opportunities to associate nuclear markers with mitochondrial markers back in time.

Maintenance and stabilization of mtDNA can be facilitated by the DNA-binding activity of Ilv5p

Mitochondrial DNA (mtDNA) is inherited as a protein-DNA complex (the nucleoid). Proteins associated with the nucleoid are not only components directly involved in maintenance and propagation of mtDNA but can also be bi-functional enzymes whose metabolic activities are not directly related to mtDNA stability. In the yeast Saccharomyces cerevisiae, one such enzyme, Ilv5p is required for branch chain amino acid biosynthesis but also associates with the nucleoid. Deletions of ILV5 lead not only to metabolic defects but also to destabilization of mtDNA. Further, minor overproduction of Ilv5p stabilizes mtDNA in strains lacking Abf2p, a major mtDNA binding and packaging protein. Here we show that Ilv5p binds double-stranded DNA in vitro and is unaffected by the presence of saturating concentrations of Abf2p. In cells lacking Abf2p the amount of Ilv5p associated with the nucleoid increases significantly and is proportional to the mitochondrial concentration of Ilv5p. Altogether, we conclude that direct binding of Ilv5p can aid in the maintenance and stabilization of mtDNA.

Protein degradation within mitochondria: versatile activities of AAA proteases and other peptidases

Cell survival depends on essential processes in mitochondria. Various proteases within these organelles regulate mitochondrial biogenesis and ensure the complete degradation of excess or damaged proteins. Many of these proteases are highly conserved and ubiquitous in eukaryotic cells. They can be assigned to three functional classes: processing peptidases, which cleave off mitochondrial targeting sequences of nuclearly encoded proteins and process mitochondrial proteins with regulatory functions; ATP-dependent proteases, which either act as processing peptidases with regulatory functions or as quality-control enzymes degrading non-native polypeptides to peptides; and oligopeptidases, which degrade these peptides and mitochondrial targeting sequences to amino acids. Disturbances of protein degradation within mitochondria cause severe phenotypes in various organisms and can lead to the induction of apoptotic programmes and cell-specific neurodegeneration in mammals. After an overview of the proteolytic system of mitochondria, we will focus on versatile functions of ATP-dependent AAA proteases in the inner membrane. These conserved proteolytic machines conduct protein quality surveillance of mitochondrial inner membrane proteins, mediate vectorial protein dislocation from membranes, and, acting as processing enzymes, control ribosome assembly, mitochondrial protein synthesis, and mitochondrial fusion. Implications of these functions for cell-specific axonal degeneration in hereditary spastic paraplegia will be discussed.