The role of BH3-only protein Bim extends beyond inhibiting Bcl-2–like prosurvival proteins

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129

Bcl-2 family proteins regulate a critical step in apoptosis referred to as mitochondrial outer membrane permeabilization (MOMP). Members of a subgroup of the Bcl-2 family, known as the BH3-only proteins, activate pro-apoptotic effectors (Bax and Bak) to initiate MOMP. They do so by neutralizing pro-survival Bcl-2 proteins and/or directly activating Bax/ Bak. Bim and Bid are reported to be direct activators; however, here we show that BH3 peptides other than Bim and Bid exhibited various degrees of direct activation of the effector Bax or Bak, including Bmf and Noxa BH3s. In the absence of potent direct activators, such as Bim and Bid, we unmasked novel direct activator BH3 ligands capable of inducing effector-mediated cytochrome c release and liposome permeabilization, even when both Bcl-xL-and Mcl-1-type anti-apoptotic proteins were inhibited. The ability of these weaker direct activator BH3 peptides to cause MOMP correlated with that of the corresponding full-length proteins to induce apoptosis in the absence of Bim and Bid. We propose that, in certain contexts, direct activation by BH3-only proteins other than Bim and Bid may significantly contribute to MOMP and apoptosis.Regulation of mitochondrial outer membrane permeabilization (MOMP) 4 is one of the most critical steps in apoptosis pathways, as the release of apoptogenic proteins from the mitochondrial intermembrane space commits the cell to death, either by a caspase-dependent or -independent mechanism (1, 2). Bcl-2 family proteins regulate MOMP (3-5), and the molecular mechanisms underlying MOMP have been a focus of intensive studies. The Bcl-2 family comprises both pro-and anti-apoptotic members. The pro-apoptotic members are divided into multidomain (Bax, Bak, and perhaps Bok in a limited number of tissues) and BH3 domain-only members (Bim, Bid, Bad, Bik, Bmf, Hrk, Puma, Noxa etc.) (5). Loss-of-function studies have established that the multidomain pro-apoptotic members, Bax and Bak, are the effectors of MOMP (6, 7), and that the BH3-only proteins effect the activation of Bax and Bak. To initiate MOMP, Bax, and Bak change their conformation (8 -12) and form homo-oligomers in the membrane (11-16). These events are referred to as Bax/Bak (for Bax and/or Bak) activation. As a consequence of this activation, the membrane is permeabilized and apoptogenic molecules are released from the intermembrane space.The neutralization model postulates that Bax and Bak are activated, when they are liberated from anti-apoptotic Bcl-2 proteins by BH3-only proteins (17, 18), whereas the direct activation model posits that, in addition to inhibiting prosurvival proteins, select BH3-only proteins must directly activate Bax/Bak (19 -21). It has also been shown that certain BH3-only proteins displace activator BH3-only proteins (Bim, cleaved Bid or Puma in some instances) from anti-apoptotic proteins, which will then directly activate Bax/Bak. We investigated the membrane permeabilization by Bcl-2 family proteins in a defined liposome system, using BH3 peptides (13,22). It...

Section: Discussionmentioning

confidence: 97%

BH3 Domains other than Bim and Bid Can Directly Activate Bax/Bak

Wolf

Schafer

et al. 2011

145

129

“…Like other antiapoptotic Bcl-2 proteins, Mcl-1 prevents the permeabilization of the mitochondrial outer membrane, which requires the oligomerization of either Bax or Bak (38). Although it has been established that activation of Bax and Bak is mediated by certain BH3-only proteins, the exact mechanisms involved remain controversial (45)(46)(47). Two principal mechanisms have been proposed, focusing on the sequestration by antiapoptotic Bcl-2 proteins of either Bax and Bak or BH3-only proteins.…”

Section: Discussionmentioning

confidence: 99%

Regulation of Mitochondrial Apoptotic Events by p53-mediated Disruption of Complexes between Antiapoptotic Bcl-2 Members and Bim

Han

Goldstein

Hou

et al. 2010

Multiple mechanisms have been proposed for the mitochondrial function of p53 that are either dependent on or independent of its transcriptional activity. However, none of these mechanisms involves Bim functioning downstream of p53 mitochondrial translocation. Utilizing a p53 nuclear localization signal mutant, whose nuclear import is completely abrogated, we demonstrate that its apoptotic activity at the outer mitochondrial membrane, which involves conformational changes in Bax and Bak, is mediated by Bim. We further demonstrate an inverse correlation between the binding levels of p53 and Bim to Mcl-1. Thus, enhanced binding of p53 to Mcl-1 involves the disruption of existing complexes between Mcl-1 and Bim. We propose that mitochondrial p53 functions as a Bim derepressor by releasing Bim from sequestrating complexes with Mcl-1, Bcl-2, and Bcl-XL, and allowing its engagement in Bak/Bax activation.

“…The binding with Bcl-x L is likely mediated by an interaction between the BH3 region of Bak and the BH1-3 groove of Bcl-x L (19). During apoptosis Bad or another BH3-only protein may activate Bak by displacing Bak from the BH1-3 groove of Bcl-x L without directly binding to Bak (16,20,21). The freed Bak can then oligomerize via the two interfaces described above.…”

Section: Mitochondrial Outer Membrane Permeabilization (Momp)mentioning

confidence: 99%

Bax Forms an Oligomer via Separate, Yet Interdependent, Surfaces

Zhang

Zhu

Lapolla

et al. 2010

Interactions of Bcl-2 family proteins regulate permeability of the mitochondrial outer membrane and apoptosis. In particular, Bax forms an oligomer that permeabilizes the membrane. To map the interface of the Bax oligomer we used Triton X-100 as a membrane surrogate and performed site-specific photocross-linking. Bax-specific adducts were formed through photo-reactive probes at multiple sites that can be grouped into two surfaces. The first surface overlaps with the BH1-3 groove formed by Bcl-2 Homology motif 1, 2, and 3; the second surface is a rear pocket located on the opposite side of the protein from the BH1-3 groove. Further cross-linking experiments using Bax BH3 peptides and mutants demonstrated that the two surfaces interact with their counterparts in neighboring proteins to form two separated interfaces and that interaction at the BH1-3 groove primes the rear pocket for further interaction. Therefore, Bax oligomerization proceeds through a series of interactions that occur at separate, yet allosterically, coupled interfaces.