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Biogenesis of the bc1 complex in mitochondria
Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mitochondria perform a variety of tasks, but the function they are most prominent for is the energy conversion to form ATP, the universal energy equivalent of the cell. The majority of this ATP is created by the oxidative phosphorylation system, consisting of the respiratory chain and the ATP synthase. These elaborate machineries channel electrons through the respiratory complexes and thereby generate an electrochemical gradient across the inner mitochondrial membrane. This, so called proton motive force, is in turn utilized by the ATP Synthase to produce ATP.

A particularity of the oxidative phosphorylation complexes is that their subunits are derived from two genetic sources. As a result, and the fact that the respiratory chain complexes contain redox cofactors, the biogenesis of these enzymes is challenging and involves multiple, highly coordinated and regulated assembly steps. For the obligate homodimeric bc1 complex, a handful of assembly factors are known and its assembly can be divided into distinct assembly intermediates. In this work we provided insights into the maturation of the catalytic subunit cytochrome b. We revealed that the insertion of the redox active heme b groups is sequential and that it depends on the interaction with the early assembly factor Cbp4. With successful insertion of both heme bs, the binding of the structural subunit Qcr7 is necessary for stabilization and further assembly.

Furthermore, we were able to delineate the dimerization event in detail. We could establish that the interaction of the two matrix subunits, Cor1 and Cor2, with the bc1 complex assembly intermediate II, as well as the dissociation of Cbp4, are the triggering point for dimerization.

In our subsequent work we investigated the roles of the fairly uncharacterized assembly factor Bca1 and its interplay with the structural subunit Qcr7. We could demonstrate that Bca1 interacts early and transiently during assembly and is an important factor for efficient assembly. Additionally, we could show that Qcr7 is not only a structural subunit but also serves as an assembly checkpoint for the maturation of the bc1 complex.

With our work we could illustrate the necessity for basic biochemical research within the model organism yeast, as the fundamental molecular mechanisms are well conserved. This is exemplified by our work on UQCC3, the human orthologue of the bc1 complex assembly factor Cbp4.

Place, publisher, year, edition, pages
Stockholm: Department of Biochemistry anb Biophysics, Stockholm University , 2020. , p. 68
Keywords [en]
Respiratory chain, bc1 complex assembly, mitochondrial protein biogenesis, molecular biology, yeast
National Category
Natural Sciences Biochemistry and Molecular Biology Chemical Sciences
Research subject
Biochemistry
Identifiers
URN: urn:nbn:se:su:diva-180531ISBN: 978-91-7911-116-8 (print)ISBN: 978-91-7911-117-5 (electronic)OAI: oai:DiVA.org:su-180531DiVA, id: diva2:1423536
Public defence
2020-06-11, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.

Available from: 2020-05-18 Created: 2020-04-15 Last updated: 2020-05-26Bibliographically approved
List of papers
1. Assembly factors monitor sequential hemylation of cytochrome b to regulate mitochondria! translation
Open this publication in new window or tab >>Assembly factors monitor sequential hemylation of cytochrome b to regulate mitochondria! translation
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2014 (English)In: Journal of Cell Biology, ISSN 0021-9525, E-ISSN 1540-8140, Vol. 205, no 4, p. 511-524Article in journal (Refereed) Published
Abstract [en]

Mitochondrial respiratory chain complexes convert chemical energy into a membrane potential by connecting electron transport with charge separation. Electron transport relies on redox cofactors that occupy strategic positions in the complexes. How these redox cofactors are assembled into the complexes is not known. Cytochrome b, a central catalytic subunit of complex III, contains two henne bs. Here, we unravel the sequence of events in the mitochondrial inner membrane by which cytochrome b is hemylated. Heme incorporation occurs in a strict sequential process that involves interactions of the newly synthesized cytochrome b with assembly factors and structural complex III subunits. These interactions are functionally connected to cofactor acquisition that triggers the progression of cytochrome b through successive assembly intermediates. Failure to hemylate cytochrome b sequesters the Cbp3-Cbp6 complex in early assembly intermediates, thereby causing a reduction in cytochrome b synthesis via a feedback loop that senses hemylation of cytochrome b.

National Category
Cell Biology
Research subject
Biochemistry
Identifiers
urn:nbn:se:su:diva-105918 (URN)10.1083/jcb.201401009 (DOI)000336639000008 ()
Funder
Swedish Research CouncilCarl Tryggers foundation Knut and Alice Wallenberg FoundationNIH (National Institute of Health), GM101386
Note

AuthorCount:6;

Available from: 2014-07-08 Created: 2014-07-08 Last updated: 2020-04-27Bibliographically approved
2. Timing of dimerization of the bc1 complex during mitochondrial respiratory chain assembly
Open this publication in new window or tab >>Timing of dimerization of the bc1 complex during mitochondrial respiratory chain assembly
2020 (English)In: Biochimica et Biophysica Acta - Bioenergetics, ISSN 0005-2728, E-ISSN 1879-2650, Vol. 1861, no 5-6, article id 148177Article in journal (Refereed) Published
Abstract [en]

The mitochondrial bc1 complex plays an important role in mitochondrial respiration. It transfers electrons from ubiquinol to the soluble electron shuttle cytochrome c and thereby contributes to the proton motive force across the inner mitochondrial membrane. In the yeast Saccharomyces cerevisiae, each monomer consists of three catalytic and seven accessory subunits. The bc1 complex is an obligate homo-dimer in all systems. It is currently not known when exactly during the assembly dimerization occurs. In this study, we determined that the dimer formation is an early event. Specifically, dimerization is mediated by the interaction of a stable tetramer formed by the two Cor subunits, Cor1 and Cor2, that joins assembly intermediate II, containing the fully hemylated cytochrome b and the two small accessory proteins, Qcr7 and Qcr8. Addition of cytochrome c1 and Qcr6 can either occur concomitantly or independently of dimerization. These results reveal a strict order in assembly, where dimerization occurs after stabilization of co-factor acquisition by cytochrome b. Finally, assembly is completed by addition of the remaining subunits.

Keywords
Complex III assembly, Cytochrome b, Dimerization, Mitochondrial respiration, Oxidative phosphorylation, bc(1) complex
National Category
Other Chemistry Topics Biochemistry and Molecular Biology
Research subject
Biochemistry
Identifiers
urn:nbn:se:su:diva-179941 (URN)10.1016/j.bbabio.2020.148177 (DOI)000523619700008 ()
Available from: 2020-03-24 Created: 2020-03-24 Last updated: 2020-04-29Bibliographically approved
3. A mutation in the human CBP4 ortholog UQCC3 impairs complex III assembly, activity and cytochrome b stability
Open this publication in new window or tab >>A mutation in the human CBP4 ortholog UQCC3 impairs complex III assembly, activity and cytochrome b stability
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2014 (English)In: Human Molecular Genetics, ISSN 0964-6906, E-ISSN 1460-2083, Vol. 23, no 23, p. 6356-6365Article in journal (Refereed) Published
Abstract [en]

Complex III (cytochrome bc(1)) is a protein complex of the mitochondrial inner membrane that transfers electrons from ubiquinol to cytochrome c. Its assembly requires the coordinated expression of mitochondrial-encoded cytochrome b and nuclear-encoded subunits and assembly factors. Complex III deficiency is a severe multisystem disorder caused by mutations in subunit genes or assembly factors. Sequence-profile-based orthology predicts C11orf83, hereafter named UQCC3, to be the ortholog of the fungal complex III assembly factor CBP4. We describe a homozygous c.59T > A missense mutation in UQCC3 from a consanguineous patient diagnosed with isolated complex III deficiency, displaying lactic acidosis, hypoglycemia, hypotonia and delayed development without dysmorphic features. Patient fibroblasts have reduced complex III activity and lower levels of the holocomplex and its subunits than controls. They have no detectable UQCC3 protein and have lower levels of cytochrome b protein. Furthermore, in patient cells, cytochrome b is absent from a high-molecular-weight complex III. UQCC3 is reduced in cells depleted for the complex III assembly factors UQCC1 and UQCC2. Conversely, absence of UQCC3 in patient cells does not affect UQCC1 and UQCC2. This suggests that UQCC3 functions in the complex III assembly pathway downstream of UQCC1 and UQCC2 and is consistent with what is known about the function of Cbp4 and of the fungal orthologs of UQCC1 and UQCC2, Cbp3 and Cbp6. We conclude that UQCC3 functions in complex III assembly and that the c.59T > A mutation has a causal role in complex III deficiency.

National Category
Biochemistry and Molecular Biology Genetics Medical Genetics
Research subject
Biochemistry
Identifiers
urn:nbn:se:su:diva-113233 (URN)10.1093/hmg/ddu357 (DOI)000346777400018 ()
Note

AuthorCount:15;

Available from: 2015-02-27 Created: 2015-01-26 Last updated: 2020-04-27Bibliographically approved
4. The role of the assembly factor Bca1 and the structural subunit Qcr7 during bc1 complex biogenesis
Open this publication in new window or tab >>The role of the assembly factor Bca1 and the structural subunit Qcr7 during bc1 complex biogenesis
(English)Manuscript (preprint) (Other academic)
Keywords
bc1 complex assembly, cytochrome b, complex III, oxidative phosphorylation, assembly factors
National Category
Biochemistry and Molecular Biology
Research subject
Biochemistry
Identifiers
urn:nbn:se:su:diva-180248 (URN)
Available from: 2020-03-30 Created: 2020-03-30 Last updated: 2020-04-27Bibliographically approved

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