Stepwise Activation of BAX and BAK by tBID, BIM, and PUMA Initiates Mitochondrial Apoptosis

Abstract

SUMMARY 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 α1 helix of BAX keeps the α9 helix engaged in the dimerization pocket, rendering BAX as a monomer in cytosol. The activator BH3s, tBID/BIM/PUMA, attack and expose the α1 helix of BAX, resulting in secondary disengagement of the α9 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 its faster killing kinetics than 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.

MiR-135a functions as a selective killer of malignant glioma

“…By transfecting miR-135a or silencing SMAD5, Survivin was downregulated significantly (Figure 4f). Interestingly, with simultaneously silencing STAT6 and SMAD5, the potent proapoptotic factor Bax (Kim et al, 2009) was remarkably upregulated just as miR-135a transfection (Figure 4f). Taken together, these results suggest that miR-135a induces apoptosis of malignant glioma cells by targeting various genes including STAT6, SMAD5 and BMPR2 synergistically, thus affecting the survival pathway of mitochondria.…”

Bcl-xL phosphorylation at Ser49 by polo kinase 3 during cell cycle progression and checkpoints

“…Bax insertion and oligomerization into membranes require activation, i.e. structural reorganization by a BH3-only activating protein, events that lead to outer mitochondrial membrane permeabilization; in contrast, the ability of anti-apoptotic proteins, such as Bcl-xL, to trap and inhibit these BH3-only activating proteins, prevents membrane permeabilization [17][18][19][20].…”

“…Structural similarities between Bcl-xL, particularly its α5-and α6-helices and the pore-forming domains of some bacterial toxins that act as channels for either ions or proteins, suggest that Bcl-2 members could function by constituting pores in intracellular organelles, including mitochondria [5,[17][18][19][20][21]. Whether these channel activities function by themselves, or in association with other megachannels, such as components of mitochondrial permeability transition pores, or others, is still not completely elucidated [3,4,19,[22][23][24][25][26][27][28][29][30][31].…”

Mitochondrial outer membrane permeabilization: a focus on the role of mitochondrial membrane structural organization

“…In particular, this model proposes that the mitochondrial membrane induces the conformational changes in the Bcl-2 proteins necessary for pore formation. This model has been widely used and places the membrane as the pivotal point for the activity of all Bcl-2 family proteins including the proteinprotein and protein-membrane interactions required for apoptosis Kim et al 2009;Lee et al 2016;Zong et al 2001). Finally, a unified model has been described ( Fig.…”