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Fuste J.M.,Karolinska Institutet | Wanrooij S.,Gothenburg University | Jemt E.,Gothenburg University | Granycome C.E.,MRC Mitochondrial Biology Unit | And 7 more authors.
Molecular Cell | Year: 2010

Mitochondrial DNA is replicated by a unique enzymatic machinery, which is distinct from the replication apparatus used for copying the nuclear genome. We examine here the mechanisms of origin-specific initiation of lagging-strand DNA synthesis in human mitochondria. We demonstrate that the mitochondrial RNA polymerase (POLRMT) is the primase required for initiation of DNA synthesis from the light-strand origin of DNA replication (OriL). Using only purified POLRMT and DNA replication factors, we can faithfully reconstitute OriL-dependent initiation in vitro. Leading-strand DNA synthesis is initiated from the heavy-strand origin of DNA replication and passes OriL. The single-stranded OriL is exposed and adopts a stem-loop structure. At this stage, POLRMT initiates primer synthesis from a poly-dT stretch in the single-stranded loop region. After about 25 nt, POLRMT is replaced by DNA polymerase γ, and DNA synthesis commences. Our findings demonstrate that POLRMT can function as an origin-specific primase in mammalian mitochondria. © 2010 Elsevier Inc. All rights reserved. Source

Szilagyi Z.,Gothenburg University | Banyai G.,Gothenburg University | Lopez M.D.,Gothenburg University | McInerny C.J.,University of Glasgow | And 2 more authors.
Molecular and Cellular Biology | Year: 2012

Temporal changes in transcription programs are coupled to control of cell growth and division. We here report that Mediator, a conserved coregulator of eukaryotic transcription, is part of a regulatory pathway that controls mitotic entry in fission yeast. The Mediator subunit cyclin-dependent kinase 8 (Cdk8) phosphorylates the forkhead 2 (Fkh2) protein in a periodic manner that coincides with gene activation during mitosis. Phosphorylation prevents degradation of the Fkh2 transcription factor by the proteasome, thus ensuring cell cycle-dependent variations in Fkh2 levels. Interestingly, Cdk8-dependent phosphorylation of Fkh2 controls mitotic entry, and mitotic entry is delayed by inactivation of the Cdk8 kinase activity or mutations replacing the phosphorylated serine residues of Fkh2. In addition, mutations in Fkh2, which mimic protein phosphorylation, lead to premature mitotic entry. Therefore, Fkh2 regulates not only the onset of mitotic transcription but also the correct timing of mitotic entry via effects on the Wee1 kinase. Our findings thus establish a new pathway linking the Mediator complex to control of mitotic transcription and regulation of mitotic entry in fission yeast. © 2012, American Society for Microbiology. Source

Sterky F.H.,Karolinska Institutet | Ruzzenente B.,Max Planck Institute For Biologie Des Alterns | Ruzzenente B.,University of Padua | Gustafsson C.M.,Max Planck Institute For Biologie Des Alterns | And 3 more authors.
Biochemical and Biophysical Research Communications | Year: 2010

LRPPRC (also called LRP130) is an RNA-binding pentatricopeptide repeat protein. LRPPRC has been recognized as a mitochondrial protein, but has also been shown to regulate nuclear gene transcription and to bind specific RNA molecules in both the nucleus and the cytoplasm. We here present a bioinformatic analysis of the LRPPRC primary sequence, which reveals that orthologs to the LRPPRC gene are restricted to metazoan cells and that all of the corresponding proteins contain mitochondrial targeting signals. To address the subcellular localization further, we have carefully analyzed LRPPRC in mammalian cells and identified a single isoform that is exclusively localized to mitochondria. The LRPPRC protein is imported to the mitochondrial matrix and its mitochondrial targeting sequence is cleaved upon entry. © 2010 Elsevier Inc. Source

Uhler J.P.,Gothenburg University | Spahr H.,Max Planck Institute For Biologie Des Alterns | Farge G.,University Blaise Pascal | Clavel S.,University of Nice Sophia Antipolis | And 6 more authors.
Biochemical and Biophysical Research Communications | Year: 2014

UBTD1 is a previously uncharacterized ubiquitin-like (UbL) domain containing protein with high homology to the mitochondrial Dc-UbP/UBTD2 protein. Here we show that UBTD1 and UBTD2 belong to a family of proteins that is conserved through evolution and found in metazoa, funghi, and plants. To gain further insight into the function of UBTD1, we screened for interacting proteins. In a yeast-2-hybrid (Y2H) screen, we identified several proteins involved in the ubiquitylation pathway, including the UBE2D family of E2 ubiquitin conjugating enzymes. An affinity capture screen for UBTD1 interacting proteins in whole cell extracts also identified members of the UBE2D family. Biochemical characterization of recombinant UBTD1 and UBE2D demonstrated that the two proteins form a stable, stoichiometric complex that can be purified to near homogeneity. We discuss the implications of these findings in light of the ubiquitin proteasome system (UPS). © 2013. Source

Atanassova N.,Karolinska Institutet | Atanassova N.,Scripps Research Institute | Fuste J.M.,Karolinska Institutet | Fuste J.M.,Gothenburg University | And 13 more authors.
Human Molecular Genetics | Year: 2011

A large number of mutations in the gene encoding the catalytic subunit of mitochondrial DNA polymerase γ (POLγA) cause human disease. The Y955C mutation is common and leads to a dominant disease with progressive external ophthalmoplegia and other symptoms. The biochemical effect of the Y955C mutation has been extensively studied and it has been reported to lower enzyme processivity due to decreased capacity to utilize dNTPs. However, it is unclear why this biochemical defect leads to a dominant disease. Consistent with previous reports, we show here that the POLγA:Y955C enzyme only synthesizes short DNA products at dNTP concentrations that are sufficient for proper function of wild-type POLγA. In addition, we find that this phenotype is overcome by increasing the dNTP concentration, e.g. dATP. At low dATP concentrations, the POLγA:Y955C enzyme stalls at dATP insertion sites and instead enters a polymerase/exonuclease idling mode. The POLγA:Y955C enzyme will compete with wild-type POLγA for primer utilization, and this will result in a heterogeneous population of short and long DNA replication products. In addition, there is a possibility that POLγA:Y955C is recruited to nicks of mtDNA and there enters an idling mode preventing ligation. Our results provide a novel explanation for the dominant mtDNA replication phenotypes seen in patients harboring the Y955C mutation, including the existence of site-specific stalling. Our data may also explain why mutations that disturb dATP pools can be especially deleterious for mtDNA synthesis. © The Author 2011. Published by Oxford University Press. All rights reserved. Source

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