Translocation of a Bak C-Terminus Mutant from Cytosol to Mitochondria to Mediate Cytochrome c Release: Implications for Bak and Bax Apoptotic Function

One of two proapoptotic Bcl-2 proteins, Bak or Bax, is required to permeabilize the mitochondrial outer membrane during apoptosis. While Bax is mostly cytosolic and translocates to mitochondria following an apoptotic stimulus, Bak is constitutively integrated within the outer membrane. Membrane anchorage occurs via a C-terminal transmembrane domain that has been studied in Bax but not in Bak, therefore what governs their distinct subcellular distribution is uncertain. In addition, whether the distinct subcellular distributions of Bak and Bax contributes to their differential regulation during apoptosis remains unclear.To gain insight into Bak and Bax targeting to mitochondria, elements of the Bak C-terminus were mutated, or swapped with those of Bax. Truncation of the C-terminal six residues (C-segment) or substitution of three basic residues within the C-segment destabilized Bak. Replacing the Bak C-segment with that from Bax rescued stability and function, but unexpectedly resulted in a semi-cytosolic protein, termed Bak/BaxCS. When in the cytosol, both Bax and Bak/BaxCS sequestered their hydrophobic transmembrane domains in their hydrophobic surface groove. Upon apoptotic signalling, Bak/BaxCS translocated to the mitochondrial outer membrane, inserted its transmembrane domain, oligomerized, and released cytochrome c. Despite this Bax-like subcellular distribution, Bak/BaxCS retained Bak-like regulation following targeting of Mcl-1.Residues in the C-segment of Bak and of Bax contribute to their distinct subcellular localizations. That a semi-cytosolic form of Bak, Bak/BaxCS, could translocate to mitochondria and release cytochrome c indicates that Bak and Bax share a conserved mode of activation. In addition, the differential regulation of Bak and Bax by Mcl-1 is predominantly independent of the initial subcellular localizations of Bak and Bax

Induction of Noxa-Mediated Apoptosis by Modified Vaccinia Virus Ankara Depends on Viral Recognition by Cytosolic Helicases, Leading to IRF-3/IFN-β-Dependent Induction of Pro-Apoptotic Noxa

Viral infection is a stimulus for apoptosis, and in order to sustain viral replication many viruses are known to carry genes encoding apoptosis inhibitors. F1L, encoded by the orthopoxvirus modified vaccinia virus Ankara (MVA) has a Bcl-2-like structure. An MVA mutant lacking F1L (MVAΔF1L) induces apoptosis, indicating that MVA infection activates and F1L functions to inhibit the apoptotic pathway. In this study we investigated the events leading to apoptosis upon infection by MVAΔF1L. Apoptosis largely proceeded through the pro-apoptotic Bcl-2 family protein Bak with some contribution from Bax. Of the family of pro-apoptotic BH3-only proteins, only the loss of Noxa provided substantial protection, while the loss of Bim had a minor effect. In mice, MVA preferentially infected macrophages and DCs in vivo. In both cell types wt MVA induced apoptosis albeit more weakly than MVAΔF1L. The loss of Noxa had a significant protective effect in macrophages, DC and primary lymphocytes, and the combined loss of Bim and Noxa provided strong protection. Noxa protein was induced during infection, and the induction of Noxa protein and apoptosis induction required transcription factor IRF3 and type I interferon signalling. We further observed that helicases RIG-I and MDA5 and their signalling adapter MAVS contribute to Noxa induction and apoptosis in response to MVA infection. RNA isolated from MVA-infected cells induced Noxa expression and apoptosis when transfected in the absence of viral infection. We thus here describe a pathway leading from the detection of viral RNA during MVA infection by the cytosolic helicase-pathway, to the up-regulation of Noxa and apoptosis via IRF3 and type I IFN signalling

A TM:groove interaction forms in cytosolic Bax and in cytosolic Bak/BaxCS.

<p>(<b>A</b>) Schematic of Bax converting from a TM:groove to a TM:membrane conformation during apoptosis. Bax is shown as surface representation (<i>orange</i>) with the hydrophobic surface groove highlighted (<i>white</i>). The Bax TM domain is represented (<i>blue</i>) and the C-segment (KKMG) has the sidechains shown. Residues proposed to interact during TM:groove interaction are highlighted (S184 and D98; <i>red</i>). Images were generated in Pymol using the RCSB Protein Data Bank file 1F16 for Bax <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0031510#pone.0031510-Suzuki1" target="_blank">[38]</a>. (<b>B</b>) Mutations in both the TM domain and the groove alter mitochondrial targeting. Cytosolic and membrane fractions from <i>bak^−/−bax^−/−</i> MEFs expressing the indicated Bax or Bak/BaxCS cysteine mutants were immunoblotted for Bax, Bak/BaxCS, or HSP70. (<b>C</b>) A TM:groove interaction forms in cytosolic Bax and in cytosolic Bak/BaxCS. Cytosolic and membrane fractions from cells in (B) were incubated with oxidant (CuPhe) and electrophoresed under non-reducing conditions (<i>upper</i>) or reducing conditions (+2ME, <i>lower</i>) and immunoblotted for Bax or Bak/BaxCS. Intramolecular cysteine linkage (D98C:S184C in Bax; S117C:Q202C in Bak/BaxCS) results in faster migration under non-reducing conditions (<i>X-link</i>). (<b>D</b>) Etoposide treatment decreases TM:groove interaction in membrane-associated Bak/BaxCS. Cells expressing the Bak/BaxCS S117C/Q202C variant were incubated with or without etoposide for 24 h. Cytosolic and membrane fractions were incubated with CuPhe, electrophoresed under non-reducing conditions, and immunoblotted for Bak. (<b>E</b>) tBid treatment decreases the TM:groove interaction in membrane-associated Bak/BaxCS. Cells in (D) were permeabilized and incubated with or without tBid (100 nM). Cytosolic and membrane fractions were separated and then incubated with CuPhe, electrophoresed under non-reducing conditions, and immunoblotted for Bak. (<b>F</b>) The TM domain of Bak/BaxCS inserts into membranes following tBid. Permeabilized cells were treated with or without tBid as in (E), then incubated with or without IASD. Membrane fractions were electrophoresed under reducing conditions and immunoblotted for Bak. Results are representative of two or more independent experiments.</p

Bak regulation is independent of initial subcellular localization.

<p>(<b>A</b>) Schematic of Bak-mediated apoptosis initiated by Noxa-Mcl-1 signalling in MEFs. The four prosurvival proteins expressed in MEF (Mcl-1, Bcl-x_L, Bcl-w and Bcl-2) are depicted, together with their preferential binding of BH3-only proteins (Noxa, Bim and Bad) and of Bak and Bax <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0031510#pone.0031510-Willis1" target="_blank">[20]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0031510#pone.0031510-Chen1" target="_blank">[46]</a>. The Noxa-Mcl-1-Bak pathway to apoptosis is indicated (<i>bold</i>). (<b>B</b>) Expression of Bim_S^NOXA preferentially mediates apoptosis via either Bak or Bak/BaxCS. <i>bak^−/−bax^−/−</i>MEFs expressing the indicated Bak and Bax variants were retrovirally infected with Bim_S or with Bim_S containing the Bad or Noxa BH3 domains (Bim_S^BAD and Bim_S^NOXA). Percentage cell death at 36 h (normalized to the efficiency of infection) is expressed as the mean ± SEM of three independent experiments. Statistical significance for treatment when compared to Bak; **p<0.01.</p

Basic residues in the C-segment are necessary for Bak stability and function.

<p>(<b>A</b>) C-terminal sequence of Bak variants indicating which basic residues in the C-segment that were substituted with serine. (<b>B</b>) Substitution of basic residues in the C-segment reduces Bak proapoptotic function. <i>bak^−/−bax^−/−</i> MEFs expressing Bak, BakRRS, BakRSS or BakSSS were left untreated, or treated with UV or etoposide for 24 h. Percentage cell death is expressed as the mean ± SEM from three independent experiments. Statistical significance for treatment when compared to Bak; *p<0.05, **p<0.01. Upper panel is a western blot of cell lysates immunoblotted for Bak, and for β-actin as a loading control. (<b>C</b>) Substitution of basic residues in the C-segment destabilizes Bak. Cells from (B) were incubated with cycloheximide for up to 24 h and cell lysates immunoblotted for Bak, and for β-actin as a loading control. Note that due to low expression of BakSSS, 4-fold total protein was loaded onto the gels. (<b>D</b>) Substitution of basic residues does not prevent targeting to membranes. Cells from (B) were left untreated or treated with UV, separated into cytosolic and membrane fractions, and immunoblotted for Bak and for the cytosolic marker HSP70. Results are representative of two or more independent experiments.</p

The Bak C-segment is essential for stability, membrane targeting and proapoptotic function.

<p>(<b>A</b>) C-terminal sequence of Bak variants truncated at the C-terminus. The C-terminus (CT) contains a hydrophobic transmembrane (TM) domain and a hydrophilic C-segment (CS). (<b>B</b>) Truncation of the Bak C-terminus or C-segment blocks proapoptotic function. <i>bak^−/−bax^−/−</i>MEFs expressing Bak, BakΔCT and BakΔCS were left untreated, or treated with UV or etoposide for 24 h. Percentage cell death is expressed as the mean ± SEM from three independent experiments. Statistical significance for treatment when compared to Bak; ***p<0.001. Upper panel is a western blot of cell lysates immunoblotted for Bak, and for β-actin as a loading control. (<b>C</b>) Truncation of the C-segment reduces half-life. Cells from (B) were incubated with cycloheximide for up to 24 h and cell lysates immunoblotted for Bak, and for β-actin as a loading control. Note that due to low expression of BakΔCS, 4-fold total protein was loaded onto the gels. (<b>D</b>) Truncation of the Bak C-terminus prevents membrane targeting. Cells were left untreated or treated with UV, separated into cytosolic and membrane fractions, and immunoblotted for Bak and the cytosolic marker HSP70. (<b>E</b>) Bak lacking the C-terminus fails to undergo conformation change and oligomerization in response to UV. Cells treated as in (D) were exposed to oxidant (CuPhe), separated into cytosolic and membrane fractions, run on non-reducing SDS-PAGE and immunoblotted for Bak. M_X, non-activated intramolecular cross-linked monomer; M, non-crosslinked monomer; D, intermolecular crosslinked dimers. Results are representative of two or more independent experiments.</p

Bak/BaxCS translocates to mitochondria and following tBid releases cytochrome <i>c</i>.

<p>(<b>A</b>) Cytosolic and mitochondrial Bak/BaxCS both contribute to permeabilization of MEF mitochondria. Cytosol and membrane fractions from <i>bak^−/−bax^−/−</i> MEFs or from <i>bak^−/−bax^−/−</i> MEFs expressing either Bak or Bak/BaxCS were combined as shown, and incubated at 30°C in the presence of 100 nM tBid for 0, 30 or 60 mins, or without tBid for 60 mins (60-). Supernatant (<i>Cyt</i>) and membrane (<i>Memb</i>) fractions were immunoblotted for cytochrome <i>c</i> and for Bak. (<b>B</b>) Cytosolic Bak/BaxCS is sufficient to permeabilize mouse liver mitochondria. MEF cytosol fractions derived as in (A) were combined with mitochondria isolated from wild-type (<i>bak^+/+</i>) or <i>bak^−/−</i> mouse liver, and incubated at 37°C with or without 100 nM tBid for 60 min. Supernatant (<i>Cyt</i>) and membrane (<i>Memb</i>) fractions were immunoblotted for cytochrome <i>c</i> and for Bak. Note that Bak/BaxCS levels appear high compared to the endogenous mouse Bak, possibly due to different recognition by the anti-Bak antibody. (<b>C</b>) Mcl-1 is higher in MEF membranes than in mouse liver mitochondria. MEF membranes and mouse liver mitochondria at the concentrations used in (A) and (B) were examined for levels of Mcl-1, Bcl-x_L and VDAC1 by immunoblotting. Results are representative of two or more independent experiments.</p