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MAP-1 is a mitochondrial effector of Bax.

ABSTRACT: Apoptotic stimuli induce conformational changes in Bax and trigger its translocation from cytosol to mitochondria. Upon assembling into the mitochondrial membrane, Bax initiates a death program through a series of events, culminating in the release of apoptogenic factors such as cytochrome c. Although it is known that Bax is one of the key factors for integrating multiple death signals, the mechanism by which Bax functions in mitochondria remains controversial. We have previously identified modulator of apoptosis-1 (MAP-1) as a Bax-associating protein, but its functional relationship with Bax in contributing to apoptosis regulation remains to be established. In this study, we show that MAP-1 is a critical mitochondrial effector of Bax. MAP-1 is a mitochondria-enriched protein that associates with Bax only upon apoptotic induction, which coincides with the release of cytochrome c from mitochondria. Small interfering RNAs that diminish MAP-1 levels in mammalian cell lines confer selective inhibition of Bax-mediated apoptosis. Mammalian cells with stable expression of MAP-1 small interfering RNAs are resistant to multiple apoptotic stimuli in triggering apoptotic death as well as in inducing conformation change and translocation of Bax. Similar to Bax-deficient cells, MAP-1-deficient cells exhibit aggressive anchorage-independent growth. Remarkably, recombinant Bax- or tBid-mediated release of cytochrome c from isolated mitochondria is significantly compromised in the MAP-1 knockdown cells. We propose that MAP-1 is a direct mitochondrial target of Bax.

SUBMITTER: Tan KO

PROVIDER: S-EPMC1239892 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BackgroundEvidence indicates that Bax functions as a "lipidic" pore to regulate mitochondrial outer membrane permeabilization (MOMP), the apoptosis commitment step, through unknown membrane elements. Here we show mitochondrial ceramide elevation facilitates MOMP-mediated cytochrome c release in HeLa cells by generating a previously-unrecognized mitochondrial ceramide-rich macrodomain (MCRM), which we visualize and isolate, into which Bax integrates.Methodology/principal findingsMCRMs, virtually non-existent in resting cells, form upon irradiation coupled to ceramide synthase-mediated ceramide elevation, optimizing Bax insertion/oligomerization and MOMP. MCRMs are detected by confocal microscopy in intact HeLa cells and isolated biophysically as a light membrane fraction from HeLa cell lysates. Inhibiting ceramide generation using a well-defined natural ceramide synthase inhibitor, Fumonisin B1, prevented radiation-induced Bax insertion, oligomerization and MOMP. MCRM deconstruction using purified mouse hepatic mitochondria revealed ceramide alone is non-apoptogenic. Rather Bax integrates into MCRMs, oligomerizing therein, conferring 1-2 log enhanced cytochrome c release. Consistent with this mechanism, MCRM Bax isolates as high molecular weight "pore-forming" oligomers, while non-MCRM membrane contains exclusively MOMP-incompatible monomeric Bax.Conclusions/significanceOur recent studies in the C. elegans germline indicate that mitochondrial ceramide generation is obligate for radiation-induced apoptosis, although a mechanism for ceramide action was not delineated. Here we demonstrate that ceramide, generated in the mitochondrial outer membrane of mammalian cells upon irradiation, forms a platform into which Bax inserts, oligomerizes and functionalizes as a pore. We posit conceptualization of ceramide as a membrane-based stress calibrator, driving membrane macrodomain organization, which in mitochondria regulates intensity of Bax-induced MOMP, and is pharmacologically tractable in vitro and in vivo.

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MAP-1 is a mitochondrial effector of Bax.

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