Biomedical subjects

Martin Ott

Publications and source records attributed to Martin Ott.

6 recordsLinked to original sources

PubMed2006

Mba1, a membrane-associated ribosome receptor in mitochondria.

The genome of mitochondria encodes a small number of very hydrophobic polypeptides that are inserted into the inner membrane in a cotranslational reaction. The molecular process by which mitochondrial ribosomes are recruited to the membrane is poorly understood. Here, we show that the inner membrane protein Mba1 binds to the large subunit of mitochondrial ribosomes. It thereby cooperates with the C-terminal ribosome-binding domain of Oxa1, which is a central component of the insertion machinery of the inner membrane. In the absence of both Mba1 and the C-terminus of Oxa1, mitochondrial translation products fail to be properly inserted into the inner membrane and serve as substrates of the matrix chaperone Hsp70. We propose that Mba1 functions as a ribosome receptor that cooperates with Oxa1 in the positioning of the ribosome exit site to the insertion machinery of the inner membrane.

Electron Transport Complex IV↗

PubMed2006

Sequential processing of a mitochondrial tandem protein: insights into protein import in Schizosaccharomyces pombe.

The sequencing of the genome of Schizosaccharomyces pombe revealed the presence of a number of genes encoding tandem proteins, some of which are mitochondrial components. One of these proteins (pre-Rsm22-Cox11) consists of a fusion of Rsm22, a component of the mitochondrial ribosome, and Cox11, a factor required for copper insertion into cytochrome oxidase. Since in Saccharomyces cerevisiae, Cox11 is physically attached to the mitochondrial ribosome, it was suggested that the tandem organization of Rsm22-Cox11 is used to covalently tie the mitochondrial ribosome to Cox11 in S. pombe. We report here that pre-Rsm22-Cox11 is matured in two subsequent processing events. First, the mitochondrial presequence is removed. At a later stage of the import process, the Rsm22 and Cox11 domains are separated by cleavage of the mitochondrial processing peptidase at an internal processing site. In vivo data obtained using a tagged version of pre-Rsm22-Cox11 confirmed the proteolytic separation of Cox11 from the Rsm22 domain. Hence, the tandem organization of pre-Rsm22-Cox11 does not give rise to a persistent fusion protein but rather might be used to increase the import efficiency of Cox11 and/or to coordinate expression levels of Rsm22 and Cox11 in S. pombe.

Alternative Splicing↗

PubMed2005

Evolution of mitochondrial oxa proteins from bacterial YidC. Inherited and acquired functions of a conserved protein insertion machinery.

Members of the Oxa1/YidC family are involved in the biogenesis of membrane proteins. In bacteria, YidC catalyzes the insertion and assembly of proteins of the inner membrane. Mitochondria of animals, fungi, and plants harbor two distant homologues of YidC, Oxa1 and Cox18/Oxa2. Oxa1 plays a pivotal role in the integration of mitochondrial translation products into the inner membrane of mitochondria. It contains a C-terminal ribosome-binding domain that physically interacts with mitochondrial ribosomes to facilitate the co-translational insertion of nascent membrane proteins. The molecular function of Cox18/Oxa2 is not well understood. Employing a functional complementation approach with mitochondria-targeted versions of YidC we show that YidC is able to functionally replace both Oxa1 and Cox18/Oxa2. However, to integrate mitochondrial translation products into the inner membrane of mitochondria, the ribosome-binding domain of Oxa1 has to be appended onto YidC. On the contrary, the fusion of the ribosome-binding domain onto YidC prevents its ability to complement COX18 mutants suggesting an indispensable post-translational activity of Cox18/Oxa2. Our observations suggest that during evolution of mitochondria from their bacterial ancestors the two descendents of YidC functionally segregated to perform two distinct activities, one co-translational and one post-translational.

Cross-Linking Reagents↗

PubMed2003

Ribosome binding to the Oxa1 complex facilitates co-translational protein insertion in mitochondria.

The Oxa1 translocase of the mitochondrial inner membrane facilitates the insertion of both mitochondrially and nuclear-encoded proteins from the matrix into the inner membrane. Most mitochondrially encoded proteins are hydrophobic membrane proteins which are integrated into the lipid bilayer during their synthesis on mitochondrial ribosomes. The molecular mechanism of this co-translational insertion process is unknown. Here we show that the matrix-exposed C-terminus of Oxa1 forms an alpha-helical domain that has the ability to bind to mitochondrial ribosomes. Deletion of this Oxa1 domain strongly diminished the efficiency of membrane insertion of subunit 2 of cytochrome oxidase, a mitochondrially encoded substrate of the Oxa1 translocase. This suggests that co-translational membrane insertion of mitochondrial translation products is facilitated by a physical interaction of translation complexes with the membrane-bound translocase.

Chromatography, Affinity↗

PubMed2003

Cardiolipin is not required for Bax-mediated cytochrome c release from yeast mitochondria.

Cardiolipin (CL) is an inner mitochondrial membrane phospholipid that contributes to optimal mitochondrial function and is gaining widespread attention in studies of mitochondria-mediated apoptosis. Divergent hypotheses describing the role of CL in cytochrome c release and apoptosis have evolved. We addressed this controversy directly by comparing the spontaneous- and Bax-mediated cytochrome c release from mitochondria isolated from two strains of Saccharomyces cerevisiae: one lacking CL-synthase and therefore CL (DeltaCRD1) and the other, its corresponding wild type (WT). We demonstrated by liquid chromatography-mass spectrometry that the main yeast CL species [(16:1)2(18:1)2] differs in fatty acid composition from mammalian CL [(18:2)4], and we verified the absence of the yeast CL species in the DeltaCRD1 strain. We also demonstrated that the mitochondrial association of Bax and the resulting cytochrome c release is not dependent on the CL content of the yeast mitochondrial membranes. Bax inserted equally into both WT and DeltaCRD1 mitochondrial membranes under conditions that lead to the release of cytochrome c from both strains of yeast mitochondria. Furthermore, using models of synthetic liposomes and isolated yeast mitochondria, we found that cytochrome c was bound more "loosely" to the CL-deficient systems compared with when CL is present. These data challenge recent studies implicating that CL is required for Bax-mediated pore formation leading to the release of proteins from the mitochondrial intermembrane space. In contrast, they support our recently proposed two-step mechanism of cytochrome c release, which suggests that CL is required for binding cytochrome c to the inner mitochondrial membrane.

Animals↗

PubMed2002

Cytochrome c release from mitochondria proceeds by a two-step process.

Cytochrome c is often released from mitochondria during the early stages of apoptosis, although the precise mechanisms regulating this event remain unclear. In this study, with isolated liver mitochondria, we demonstrate that cytochrome c release requires a two-step process. Because cytochrome c is present as loosely and tightly bound pools attached to the inner membrane by its association with cardiolipin, this interaction must first be disrupted to generate a soluble pool of this protein. Specifically, solubilization of cytochrome c involves a breaching of the electrostatic and/or hydrophobic affiliations that this protein usually maintains with cardiolipin. Once cytochrome c is solubilized, permeabilization of the outer mitochondrial membrane by Bax is sufficient to allow the extrusion of this protein into the extramitochondrial environment. Neither disrupting the interaction of cytochrome c with cardiolipin, nor permeabilizing the outer membrane with Bax, alone, is sufficient to trigger this protein's release. This mechanism also extends to conditions of mitochondrial permeability transition insofar as cytochrome c release is significantly depressed when the electrostatic interaction between cytochrome c and cardiolipin remains intact. Our results indicate that the release of cytochrome c involves a distinct two-step process that is undermined when either step is compromised.

Animals↗