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The pro-apoptotic BH3-only protein Bim interacts with components of the translocase of the outer mitochondrial membrane (TOM).

ABSTRACT: The pro-apoptotic Bcl-2-family protein Bim belongs to the BH3-only proteins known as initiators of apoptosis. Recent data show that Bim is constitutively inserted in the outer mitochondrial membrane via a C-terminal transmembrane anchor from where it can activate the effector of cytochrome c-release, Bax. To identify regulators of Bim-activity, we conducted a search for proteins interacting with Bim at mitochondria. We found an interaction of Bim with Tom70, Tom20 and more weakly with Tom40, all components of the Translocase of the Outer Membrane (TOM). In vitro import assays performed on tryptically digested yeast mitochondria showed reduced Bim insertion into the outer mitochondrial membrane (OMM) indicating that protein receptors may be involved in the import process. However, RNAi against components of TOM (Tom40, Tom70, Tom22 or Tom20) by siRNA, individually or in combination, did not consistently change the amount of Bim on HeLa mitochondria, either at steady state or upon de novo-induction. In support of this, the individual or combined knock-downs of TOM receptors also failed to alter the susceptibility of HeLa cells to Bim-induced apoptosis. In isolated yeast mitochondria, lack of Tom70 or the TOM-components Tom20 or Tom22 alone did not affect the import of Bim into the outer mitochondrial membrane. In yeast, expression of Bim can sensitize the cells to Bax-dependent killing. This sensitization was unaffected by the absence of Tom70 or by an experimental reduction in Tom40. Although thus the physiological role of the Bim-TOM-interaction remains unclear, TOM complex components do not seem to be essential for Bim insertion into the OMM. Nevertheless, this association should be noted and considered when the regulation of Bim in other cells and situations is investigated.

SUBMITTER: Frank DO

PROVIDER: S-EPMC4398440 | biostudies-literature | 2015

REPOSITORIES: biostudies-literature

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The pro-apoptotic BH3-only protein Bim interacts with components of the translocase of the outer mitochondrial membrane (TOM).

Frank Daniel O DO Dengjel Jörn J Wilfling Florian F Kozjak-Pavlovic Vera V Häcker Georg G Weber Arnim A

PloS one 20150415 4

The pro-apoptotic Bcl-2-family protein Bim belongs to the BH3-only proteins known as initiators of apoptosis. Recent data show that Bim is constitutively inserted in the outer mitochondrial membrane via a C-terminal transmembrane anchor from where it can activate the effector of cytochrome c-release, Bax. To identify regulators of Bim-activity, we conducted a search for proteins interacting with Bim at mitochondria. We found an interaction of Bim with Tom70, Tom20 and more weakly with Tom40, all ...[more]

PMID: 25875815

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Project description:Mitochondria originated from α-proteobacterial endosymbionts, and the endosymbiont-to-organelle transition is tightly linked to the establishment of protein import pathways. The initial import of most proteins is mediated by the translocase of the outer membrane (TOM). Although TOM is common to all forms of mitochondria, recent studies have shown diversity of TOM subunits between main eukaryotic lineages. However, experimental knowledge is currently limited to a few model organisms. Here, we analysed the TvTOM complex in hydrogenosomes, an anaerobic form of mitochondria found in the excavate Trichomonas vaginalis. Using the pore-forming TvTom40-2 as bait, we immunoprecipitated the high molecular weight TvTOM complex both under crosslinking and native conditions and performed label-free quantitative mass spectrometry (LFQ-MS) to identify the components of the TvTOM complex. Identification of the components of the hydrogenosomal TOM complex To identify interaction partners for TvTom40-2 in the TOM complex, we performed co-immunoprecipitations (coIPs) of HA-tagged TvTom40-2 under protein crosslinking and under native conditions, and the eluted proteins were analyzed using label-free quantitative (LFQ) mass spectrometry. CoIPs using anti-HA antibody (Ab) were performed with hydrogenosomes isolated from both the strain expressing HA-tagged TvTom40-2 and the wild type strain, as a negative control. The analysis revealed 50 and 36 proteins that were enriched with HA-tagged TvTom40-2 under crosslinking and native conditions respectively. As TOM proteins are embedded in the hydrogenosomal outer membrane, we searched for proteins with TMDs in the datasets using TMHMM and found 19 and 13 proteins for crosslinking and native coIPs respectively. The intersection between the two datasets and the hydrogenosomal membrane proteome (Rada et al 2011) contained five TvTom40 isoforms, two TA proteins named Homp36 and Homp19 (Homp, Hydrogenosomal outer membrane protein), Sam50 and its paralogue Sam50p, and Hmp35. In addition, the intersection between the coIP dataset under crosslinking conditions and the membrane proteome contained two more TA proteins, Homp38 and Homp46. Based on these results, we selected Homp36 which was previously shown to reside in the hydrogenosomal outer membrane (Rada et al 2011) for the reciprocal coIPs. Proteins enriched in the HA-tagged Homp36 coIPs under crosslinking conditions included multiple isoforms of TvTom40, Sam50, Hmp35, Homp38, Homp46 and Homp19, and multiple isoforms of TvTom40, Sam50 and Hmp35 were enriched under native conditions. Altogether, the coIP and MS data indicated four TA candidate proteins, Homp19, Homp36, Homp38 and Homp46, and two β-barrel proteins, Sam50 and Hmp35. InterProScan predicted that Homp36, Homp38 and Homp46 carry an N-terminal Hsp20-like chaperone domain, three TPR-like domains and a C-terminal transmembrane helix. This domain architecture resembles the recently reported ATOM69 in T. brucei (Mani et al 2015). Indeed, HHpred searches using Homp36 and Homp38 as queries against the T. brucei proteome revealed ATOM69 as the first hit, with E values of 4.9e-17 and 2.3e-11 respectively. HHpred searches with Homp46 recognized various proteins with TPR domains, whereas no significant homology was observed for Homp19.

Project description:In this pre-clinical in vitro study conducted in estrogen receptor positive (ER+) breast cancer cells, we have characterized the effects of insulin-like growth factor I (IGF-1) on the cytostatic and cytotoxic action of antiestrogen treatment when used as a single agent or in combination with the antiprogestin mifepristone (MIF). Our goal was to identify new molecular targets to improve the efficacy of hormonal therapy in breast cancer patients that have a poor response to hormonal therapy, in part, due to high circulating levels of unbound insulinIGF-1.IGF-1-mediated effects on cytostasis and apoptotic cell death were determined with cell counts conducted in the presence and absence of trypan blue; enzyme-linked immunosorbent assays to determine the intracellular levels of cleaved cytokeratin 18, a marker of epithelial cancer cell apoptosis; and immunoblot analysis to determine the levels of cleaved poly-ADP ribose polymerase (PARP) and lamin A that result from caspase-dependent apoptosis. Cytotoxicity was further characterized by determination of the levels of reactive oxygen species (ROS) and the percent of mitochondrial membrane depolarization in cell populations treated with the different hormones in the presence and absence of IGF-1. Small molecule inhibitors of the dual-specificity protein kinase MEK1, MEK1 siRNA, Bim siRNA, and vectors overexpressing MEK1 wild type and mutant, dominant negative cDNA were used to identify key IGF-1 downstream prosurvival effectors.IGF-1, at physiologically relevant levels, blocked the cytotoxic action(s) of the antiestrogens 4-hydroxytamoxifen (4-OHT) and tamoxifen (TAM) when used as single agents or in combination with the antiprogestin MIF. The antiapoptotic action of IGF-1 was mediated primarily through the action of MEK1. MEK1 expression reduced the levels of ROS and mitochondrial membrane depolarization induced by the hormonal treatments via a mechanism that involved the phosphorylation and proteasomal turnover of the proapoptotic BH3-only Bcl-2 family member Bim. Importantly, small-molecule inhibitors of MEK1 circumvented the prosurvival action of IGF-1 by restoring Bim to levels that more effectively mediated apoptosis in ER+ breast cancer cells.his study provides strong support for the use of MEK1 inhibitors in combination with hormonal therapy to effectively affect cytostasis and activate a Bim-dependent apoptotic pathway in ER+ breast cancer cells. We discuss that MEK1 blockade may be a particularly effective treatment for women with high circulating levels of IGF-1, which have been correlated to a poor prognosis.

Dataset Information

The pro-apoptotic BH3-only protein Bim interacts with components of the translocase of the outer mitochondrial membrane (TOM).

Publications

The pro-apoptotic BH3-only protein Bim interacts with components of the translocase of the outer mitochondrial membrane (TOM).

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