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  • 1.
    Heublein, Manfred
    et al.
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Ndi, Mama
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Vazquez-Calvo, Carmela
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Vögtle, F.-Nora
    Ott, Martin
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Alternative Translation Initiation at a UUG Codon Gives Rise to Two Functional Variants of the Mitochondria! Protein Kgd42019In: Journal of Molecular Biology, ISSN 0022-2836, E-ISSN 1089-8638, Vol. 431, no 7, p. 1460-1467Article in journal (Refereed)
    Abstract [en]

    Kgd4 is a novel subunit of the mitochondria! a-ketoglutarate dehydrogenase complex (KGDH). In yeast, the protein is present in two forms of unknown origin, as there is only one open reading frame and no alternative splicing. Here, we show that the two forms of Kgd4 derive from one mRNA that is translated by employing two alternative start sites. The standard, annotated AUG codon gives rise to the short form of the protein, while an upstream UUG codon is utilized to generate the larger form. However, both forms can be efficiently imported into mitochondria and stably incorporate into KGDH to support its activity. Translation of the long variant depends on sequences directly upstream of the alternative initiation site, demonstrating that translation initiation and its efficiency are dictated by the sequence context surrounding a specific codon. In summary, the two forms of Kgd4 follow a very unusual biogenesis pathway, supporting the notion that translation initiation in yeast is more flexible than it is widely recognized.

  • 2.
    Marin-Buera, Lorena
    et al.
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Ndi, Mama
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Meunier, Brigitte
    Ott, Martin
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Functional characterization of disease-causing cytochrome b mutationsManuscript (preprint) (Other academic)
  • 3.
    Mata Forsberg, Manuel
    et al.
    Stockholm University, Faculty of Science, Department of Molecular Biosciences, The Wenner-Gren Institute.
    Björkander, Sophia
    Stockholm University, Faculty of Science, Department of Molecular Biosciences, The Wenner-Gren Institute.
    Pang, Yanhong
    Lundqvist, Ludwig
    Ndi, Mama
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Ott, Martin
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Escribá, Irene Buesa
    Jaeger, Marie-Charlotte
    Roos, Stefan
    Sverremark-Ekström, Eva
    Stockholm University, Faculty of Science, Department of Molecular Biosciences, The Wenner-Gren Institute.
    Extracellular membrane vesicles from lactobacilli dampen IFN-γ responses in a monocyte-dependent mannerIn: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322Article in journal (Refereed)
  • 4.
    Ndi, Mama
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Structure and Biogenesis of Membrane Proteins2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Membrane proteins make up about one-third of the cellular proteome. The diverse roles that membrane proteins have in cells include major life-sustaining processes, making them major drug targets. The respiratory chain comprises a series of complexes of membrane proteins residing in the inner mitochondrial membrane, which serve as major drivers of ATP synthesis. Assembly of the respiratory chain complexes (RCC) requires coordinated synthesis of nuclear and mitochondrial subunits. Cbp3-Cbp6 complex binds to the mitoribosome as translational activator for cytochrome b synthesis and binds the nascent polypeptide to facilitate its hemylation. Cbp3 consists of an N-terminal domain specific to mitochondrial homologues and a conserved C-terminal ubiquinol-cytochrome c chaperone domain. In this thesis I present the first crystal structure of the C-terminal domain from a bacterial homologue that has enabled us to identify the interaction sites of yeast Cbp3 with Cbp6 and cytochrome b using site-specific photo-crosslinking. Our finding suggests that Cbp3 contacts the mitoribosome via the N-terminal domain in a manner that positions the substrate binding site close to the tunnel exit. In the second project, we have analyzed the effects of disease causing cytochrome b mutations, on bc1 complex assembly. We found that complex III assembly is blocked at either intermediate 0 or I due to impaired insertion of bL or bH heme respectively, which indicates that assembly processes are involved in disease development. We then focused on NADH; a product of alpha-ketoglutarate dehydrogenase complex (KGDH) catalyzed citric acid cycle reaction and one of the substrates that supply electron to the respiratory chain. Kgd4 is a novel subunit of this enzyme complex and two functional variants (Kgd4S and Kgd4L) of unknown origins exist in yeast. We report in our work that Kgd4L originates from a UUG alternative start site, 90 nucleotides upstream and in frame of the annotated start codon. The sequence context upstream of UUG determines the efficiency of recognition of this alternative start codon. Finally, Na+/H+ antiporters are present in all species and are involved in regulation of intracellular pH, cell volume and sodium concentration. ATP formed during oxidative phosphorylation serves as energy source for Na+/K+ ATPase to generate Na+ gradient across the inner mitochondrial membrane, which drives local Na+/H+ antiporters. We show that K305 is involved in proton transport and responsible for the electrogenicity of NapA, while human NHA2 shows electroneutral antiporter activity.

  • 5.
    Ndi, Mama
    et al.
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Marin-Buera, Lorena
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Salvatori, Roger
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Singh, Abeer Prakash
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Ott, Martin
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Biogenesis of the bc(1) Complex of the Mitochondria! Respiratory Chain2018In: Journal of Molecular Biology, ISSN 0022-2836, E-ISSN 1089-8638, Vol. 430, no 21, p. 3892-3905Article, review/survey (Refereed)
    Abstract [en]

    The oxidative phosphorylation system contains four respiratory chain complexes that connect the transport of electrons to oxygen with the establishment of an electrochemical gradient over the inner membrane for ATP synthesis. Due to the dual genetic source of the respiratory chain subunits, its assembly requires a tight coordination between nuclear and mitochondrial gene expression machineries. In addition, dedicated assembly factors support the step-by-step addition of catalytic and accessory subunits as well as the acquisition of redox cofactors. Studies in yeast have revealed the basic principles underlying the assembly pathways. In this review, we summarize work on the biogenesis of the bc(1) complex or complex III, a central component of the mitochondrial energy conversion system.

  • 6.
    Ndi, Mama
    et al.
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Masuyer, Geoffrey
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics. University of Bath, UK.
    Dawitz, Hannah
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Carlström, Andreas
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Michel, Mirco
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics. Stockholm University, Science for Life Laboratory (SciLifeLab).
    Elofsson, Arne
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics. Stockholm University, Science for Life Laboratory (SciLifeLab).
    Rapp, Mikaela
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Stenmark, Pål
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics. Lund University, Sweden.
    Ott, Martin
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Structural basis for Cbp3 interaction with newly synthesized cytochrome b during mitochondrial respiratory chain assembly2019In: Journal of Biological Chemistry, ISSN 0021-9258, E-ISSN 1083-351XArticle in journal (Refereed)
    Abstract [en]

    Assembly of the mitochondrial respiratory chain requires the coordinated synthesis of mitochondrial and nuclear encoded subunits, redox co-factor acquisition, and correct joining of the subunits to form functional complexes. The conserved Cbp3–Cbp6 chaperone complex binds newly synthesized cytochrome b and supports the ordered acquisition of the heme co-factors. Moreover, it functions as a translational activator by interacting with the mitoribosome. Cbp3 consists of two distinct domains, an N-terminal domain present in mitochondrial Cbp3 homologs, and a highly conserved C-terminal domain comprising a ubiquinol–cytochrome c chaperone region. Here, we solved the crystal structure of this C-terminal domain from a bacterial homolog at 1.4 Å resolution, revealing a unique all-helical fold. This structure allowed mapping of the interaction sites of yeast Cbp3 with Cbp6 and cytochrome b via site-specific photo-crosslinking. We propose that mitochondrial Cbp3 homologs carry an N-terminal extension that positions the conserved C-terminal domain at the ribosomal tunnel exit for an efficient interaction with its substrate, the newly synthesized cytochrome b protein.

  • 7.
    Uzdavinys, Povilas
    et al.
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Coincon, Mathieu
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Nji, Emmanuel
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Ndi, Mama
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Winkelmann, Iven
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    von Ballmoos, Christoph
    Drew, David
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Dissecting the proton transport pathway in electrogenic Na+/H+ antiporters2017In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 114, no 7, p. E1101-E1110Article in journal (Refereed)
    Abstract [en]

    Sodium/proton exchangers of the SLC9 family mediate the transport of protons in exchange for sodium to help regulate intracellular pH, sodium levels, and cell volume. In electrogenic Na+/H+ antiporters, it has been assumed that two ion-binding aspartate residues transport the two protons that are later exchanged for one sodium ion. However, here we show that we can switch the antiport activity of the bacterial Na+/H+ antiporter NapA from being electrogenic to electroneutral by the mutation of a single lysine residue (K305). Electroneutral lysine mutants show similar ion affinities when driven by Delta pH, but no longer respond to either an electrochemical potential (psi) or could generate one when driven by ion gradients. We further show that the exchange activity of the human Na+/H+ exchanger NHA2 (SLC9B2) is electroneutral, despite harboring the two conserved aspartic acid residues found in NapA and other bacterial homologues. Consistently, the equivalent residue to K305 in human NHA2 has been replaced with arginine, which is a mutation that makes NapA electroneutral. We conclude that a transmembrane embedded lysine residue is essential for electrogenic transport in Na+/H+ antiporters.

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