Stepwise activation of BAX and BAK by tBID, BIM, and PUMA initiates mitochondrial apoptosis

Abstract

While activation of BAX/BAK by BH3-only molecules (BH3s) is essential for mitochondrial apoptosis, the underlying mechanisms remain unsettled. Here we demonstrate that BAX undergoes stepwise structural reorganization leading to mitochondrial targeting and homo-oligomerization. The alpha1 helix of BAX keeps the alpha9 helix engaged in the dimerization pocket, rendering BAX as a monomer in cytosol. The activator BH3s, tBID/BIM/PUMA, attack and expose the alpha1 helix of BAX, resulting in secondary disengagement of the alpha9 helix and thereby mitochondrial insertion. Activator BH3s remain associated with the N-terminally exposed BAX through the BH1 domain to drive homo-oligomerization. BAK, an integral mitochondrial membrane protein, has bypassed the first activation step, explaining why its killing kinetics are faster than those of BAX. Furthermore, death signals initiated at ER induce BIM and PUMA to activate mitochondrial apoptosis. Accordingly, deficiency of Bim/Puma impedes ER stress-induced BAX/BAK activation and apoptosis. Our study provides mechanistic insights regarding the spatiotemporal execution of BAX/BAK-governed cell death.

Figure 1. Mitochondrial targeting and homo-oligomerization are two separable, essential steps of BAX activation

(A) C-terminal α9 helix targets BAX to the mitochondria. Fluorescence microscopy of Bax/Bak DKO MEFs reconstituted with GFP-BAX or GFP-BAXΔC before or after treatment with staurosporine (12 hr) or etoposide (15 hr). (B) A single amino acid substitution at the BH3 domain or α9 helix of BAX constitutively targets BAX to the mitochondria. Fluorescence microscopy of DKO MEFs stably reconstituted with GFP-BAX S184V or L63E followed by retroviral transduction of DsRed-Mito. (C) Mitochondrially localized BAX is not constitutively active. DKO MEFs reconstituted with wt or mutant BAX were treated with staurosporine (18 hr), etoposide (18 hr), tunicamycin (TC, 30 hr), or thapsigargin (TG, 30 hr) to induce apoptosis. (D) BAX S184V displays faster proapoptotic kinetics than wt BAX. DKO MEFs reconstituted with wt BAX or BAX S184V were treated with etoposide or tunicamycin for the indicated time. Asterisk, P<0.05. (E) Mitochondrially localized BAX mutants or activated BAX expose the N-terminal α1 helix. DKO MEFs reconstituted with wt BAX before or after treatment with etoposide for 15 hr, or reconstituted with indicated BAX mutants were lysed in 1% CHAPS and then immunoprecipitated with the 6A7 antibody. Immunoprecipitates were analyzed by anti-BAX (N20) immunoblots. (F) BH3 domain is required for the homo-oligomerization of BAX. Mitochondria isolated from DKO MEFs reconstituted with BAX S184V or BAX L63E were incubated with recombinant tBID (1 ng/μl) and solubilized in 2 % CHAPS buffer. Protein lysates (200 μg) were subjected to Superdex 200 (HR 10/30) gel filtration chromatography and fractions were analyzed by anti-BAX immunoblots. Data shown in (C) and (D) are mean ± s.d. from three independent experiments. Cell death was quantified by Annexin-V.

Figure 2. Characterization of homo-oligomerization and mitochondrial targeting of BAX

(A) BH1 and BH3 domain mutants of BAX fail to undergo homo-oligomerization in response to tBID. Mitochondria isolated from DKO MEFs reconstituted with wt or mutant BAX were treated with recombinant tBID (1 ng/μl) and solubilized in 2 % CHAPS buffer. Protein lysates (200 μg) were subjected to Superdex 200 (HR 10/30) gel filtration chromatography and fractions were analyzed by anti-BAX immunoblots. (B and C) BH1 and BH3 domain mutants of BAX fail to trigger apoptosis. DKO MEFs reconstituted with wt or mutant BAX were treated with staurosporine (18 hr), etoposide (18 hr), tunicamycin (30 hr) or thapsigargin (30 hr) to induce apoptosis. Cell death was quantified by Annexin-V. Data shown are mean ± s.d. from three independent experiments. (D) S184V mutation fully targets BAX G108E but only partially targets BAX G67R to the mitochondria. Fluorescence microscopy of DKO MEFs reconstituted with the indicated GFP-tagged BAX mutants. (E) The N-terminal exposure of BAX correlates with mitochondrial targeting. DKO MEFs reconstituted with wt BAX before or after treatment with etoposide (15 hr), or reconstituted with indicated BAX mutants were lysed in 1% CHAPS and then immunoprecipitated with the 6A7 antibody. Immunoprecipitates were analyzed by anti-BAX (N20) immunoblots. (F) BH1 domain is required for the homo-oligomerization of BAX. Mitochondria isolated from DKO MEFs reconstituted with BAX S184V or BAX G108E/S184V were incubated with recombinant tBID for 30 min and then treated with BMH crosslinker. The BAX oligomers were detected by an anti-BAX immunoblot.

Figure 3. BH1 and BH3 domains of BAX are required for its activation

(A) BH1 and BH3 domain mutants of BAX fail to translocate to mitochondria upon apoptotic signals. Fluorescence microscopy of DKO MEFs reconstituted with GFP-tagged wt or mutant BAX before or after treatment with staurosporine (12 hr) or etoposide (15 hr). (B) BH1 and BH3 domain mutants of BAX fail to expose the α1 helix in response to DNA damage or tBID. DKO MEFs reconstituted with wt or mutant BAX were untreated, treated with etoposide, or transduced with tBID by retrovirus. Cells lysed in 1 % CHAPS were subject to the 6A7 immunoprecipitation, followed by an anti-BAX (N20) immunoblot. (C) BH1 and BH3 domain mutants of BAX fail to insert into the MOM in response to tBID. Mitochondria isolated from DKO MEFs reconstituted wt or mutant BAX were mock treated or treated with IVTT tBID, followed by alkaline extraction. The alkali-sensitive supernatant (S) and alkali-resistant pellet (P) fractions were analyzed by anti-BAX immunoblots. The numbers shown denote the percent of BAX quantified by densitometry. (D) BIM and PUMA induce the mitochondrial insertion of BAX. Mitochondria isolated from DKO MEFs reconstituted with wt BAX were mock treated or treated with IVTT BIM or PUMA, followed by alkali extraction. The alkali-sensitive supernatant (S) and alkali-resistant pellet (P) fractions were analyzed by anti-BAX immunoblots. The numbers shown denote the percent of BAX quantified by densitometry.

Figure 4. Activation of BAX can be dissected into two sequential steps, mitochondrial targeting and homo-oligomerization, both of which require activator BH3s

(A) The α1 helix of BAX keeps the α9 helix engaged in the dimerization pocket. Radiolabeled IVTT HA-tagged BAXΔC or HA-tagged BAXΔNΔC in combination with GST-α9 or GST-α9 S184V were subjected to anti-HA immunoprecipitation in 1 % CHAPS. Immunoprecipitates and pre-IP input were analyzed by Nu-PAGE and autoradiography. Asterisk denotes the degradation products. (B) The L63E mutation in BAX disrupts the binding of the α1 helix to the rest of the protein, resulting in N-terminal exposure and mitochondrial targeting. Radiolabeled IVTT HA₃-tagged BAX α1 helix in combination with BAXΔN wt, L63E or G67R were subjected to anti-HA immunoprecipitation in 1 % CHAPS. Immunoprecipitates and pre-IP input were analyzed by Nu-PAGE and autoradiography. (C) The S184V mutation in BAX destabilizes the binding of the α1 helix to the rest of the protein. Radiolabeled IVTT HA₃-tagged BAX α1 helix in combination with BAXΔN wt or S184V were subjected to anti-HA immunoprecipitation in 1 % CHAPS. Immunoprecipitates and pre-IP input were analyzed by Nu-PAGE and autoradiography. (D) tBID, BIM, and PUMA bind to the α1 helix of BAX. Radiolabeled IVTT HA₃-tagged BAX α1 helix in combination with tBID, BIM, or PUMA were subjected to anti-HA immunoprecipitation in 1 % CHAPS. Immunoprecipitates and pre-IP input were analyzed by Nu-PAGE and autoradiography. (E) tBID, BIM, and PUMA, but not BAD, induce the N-terminal exposure of BAX and remain associated with the N-terminally exposed BAX. Radiolabeled IVTT BAX incubated with tBID, BIM, PUMA, or BAD were immunoprecipitated with the 6A7 antibody in 1 % CHAPS. Immunoprecipitates and pre-IP input were analyzed by Nu-PAGE and autoradiography. (F) tBID, BIM, and PUMA induce the homo-oligomerization of BAX S184V. Mitochondria isolated from DKO MEFs reconstituted with BAX S184V were incubated with IVTT tBID, BIM, or PUMA for 30 min and then treated with BMH crosslinker. The BAX oligomers were detected by an anti-BAX immunoblot. (G) BH1 domain is required for N-terminally exposed BAX to interact with tBID. Radiolabeled IVTT BAX L63E or BAX L63E/G108E incubated with tBID were immunoprecipitated with the 6A7 antibody in 1 % CHAPS. Immunoprecipitates and pre-IP input were analyzed by Nu-PAGE and autoradiography.

Figure 5. BH1 and BH3 domains are required for the homo-oligomerization and proapoptotic activity of BAK

(A) BH1 and BH3 domain mutants of BAK fail to undergo homo-oligomerization in response to tBID. Mitochondria isolated from DKO MEFs reconstituted with wt or mutant BAK were treated with recombinant tBID (1 ng/μl) and solubilized in 2 % CHAPS buffer. Protein lysates (200 μg) were subjected to Superdex 200 (HR 10/30) gel filtration chromatography and fractions were analyzed by anti-BAK immunoblots. (B) BH1 and BH3 domain mutants of BAK fail to form homo-dimers. Radiolabeled IVTT N-terminal HA₃-tagged wt or mutant BAK plus non-tagged counterparts were immunoprecipitated with anti-HA antibody. Immunoprecipitates and pre-IP input were analyzed by Nu-PAGE and autoradiography. Asterisk denotes degradation products. (C) BH1 and BH3 domain mutants of BAK fail to trigger apoptosis. DKO MEFs reconstituted with wt or mutant BAK were treated with staurosporine (18 hr), etoposide (18 hr), tunicamycin (30 hr) or thapsigargin (30 hr) to induce apoptosis. Cell death was quantified by Annexin-V. Data shown are mean ± s.d. from three independent experiments. (D) BH1 and BH3 domain mutants of BAK are localized at the mitochondria. Fluorescence microscopy of DKO MEFs stably reconstituted with YFP-tagged wt or mutant BAK followed by retroviral transduction of DsRed-Mito. (E) BH1 domain is required for BAK to interact with tBID. Radiolabeled IVTT BAK wt, BAK BH3 mt (L75E) or BH1 mt (W122A/G123E/R124A) incubated with tBID-HA were immunoprecipitated with anti-HA antibody. Immunoprecipitates and pre-IP input were analyzed by Nu-PAGE and autoradiography. (F) BAK constitutively exposes its N-terminal α1 helix. Mitochondria isolated from wt MEFs, mock treated or treated with tBID, were lysed in 1% CHAPS and then immunoprecipitated with the α1 helix specific anti-BAK antibody (NT). Immunoprecipitates and pre-IP input were analyzed by anti-BAK (G23) immunoblots. Mitochondria isolated from DKO MEFs serve as a negative control. (G) BAK constitutively exposes its N-terminal α1 helix. 4T1 or N18 cells before or after staurosporine treatment were lysed in 1% CHAPS and then immunoprecipitated with the α1 helix specific anti-BAK antibody (NT). Immunoprecipitates and pre-IP input were analyzed by anti-BAK (G23) immunoblots.

Figure 6. BIM and PUMA activate mitochondrial BAK and BAX to execute apoptosis upon ER stress

(A) Schematic of the mitochondria- and ER-targeted BAK chimera mutants. (B) Mitochondria- but not ER-targeted BAK is required for apoptosis triggered by intrinsic death signals. DKO MEFs reconstituted with wt or mutant BAK were treated with staurosporine (9 hr), etoposide (18 hr), tunicamycin (30 hr) or thapsigargin (30 hr) to induce apoptosis. (C) BIM and PUMA are induced by ER stress. DKO MEFs untreated or treated with tunicamycin or thapsigargin for 18 hr were lysed and analyzed by anti-BIM and PUMA immunoblots. (D) BIM and PUMA are required for ER stress-induced apoptosis. Wild-type, Bim KO, Puma KO, or Bim/Puma DKO MEFs were treated with tunicamycin or thapsigargin for 30 hr. (E) Deficiency of Bim and Puma prevents ER stress-induced BAK homo-oligomerization. Wild-type or Bim/Puma DKO MEFs were untreated or treated with tunicamycin for 30 hr. Protein lysates were subjected to Superdex 200 (HR10/30) gel filtration chromatography. Fractions were analyzed by anti-BAK immunoblots. (F) Deficiency of Bim and Puma prevents ER stress-induced BAX homo-oligomerization. The blots shown in (E) were stripped and reprobed with anti-BAX antibody. Data shown in (B) and (D) are mean ± s.d. from three independent experiments. Cell death was quantified by Annexin-V.

Figure 7. Schematic depicts the model of activation of BAX and BAK driven by “activator” BH3s