Bcl-XL Inhibits Membrane Permeabilization by Competing with Bax

Abstract

Although Bcl-XL and Bax are structurally similar, activated Bax forms large oligomers that permeabilize the outer mitochondrial membrane, thereby committing cells to apoptosis, whereas Bcl-XL inhibits this process. Two different models of Bcl-XL function have been proposed. In one, Bcl-XL binds to an activator, thereby preventing Bax activation. In the other, Bcl-XL binds directly to activated Bax. It has been difficult to sort out which interaction is important in cells, as all three proteins are present simultaneously. We examined the mechanism of Bax activation by tBid and its inhibition by Bcl-XL using full-length recombinant proteins and measuring permeabilization of liposomes and mitochondria in vitro. Our results demonstrate that Bcl-XL and Bax are functionally similar. Neither protein bound to membranes alone. However, the addition of tBid recruited molar excesses of either protein to membranes, indicating that tBid activates both pro- and antiapoptotic members of the Bcl-2 family. Bcl-XL competes with Bax for the activation of soluble, monomeric Bax through interaction with membranes, tBid, or t-Bid-activated Bax, thereby inhibiting Bax binding to membranes, oligomerization, and membrane permeabilization. Experiments in which individual interactions were abolished by mutagenesis indicate that both Bcl-XL–tBid and Bcl-XL–Bax binding contribute to the antiapoptotic function of Bcl-XL. By out-competing Bax for the interactions leading to membrane permeabilization, Bcl-XL ties up both tBid and Bax in nonproductive interactions and inhibits Bax binding to membranes. We propose that because Bcl-XL does not oligomerize it functions like a dominant-negative Bax in the membrane permeabilization process.

Molecular Mechanism Underlying Hesperetin-induced Apoptosis by in silico Analysis and in Prostate Cancer PC-3 Cells

“…In one, BCL XL binds to an activator (tBID), thereby preventing BAX activation. In the second, BCL XL binds directly to the activated BAX resulting in preventing oligomerization (Billen et al, 2008). RT-PCR and Western blot assays revealed that hesperetin significantly down-regulated the expression of BCL XL mRNA and protein in PC-3 cells.…”

BH3-only proteins and their roles in programmed cell death

“…Following that sort of logic, David Andrews recently proposed the 'embedding together' model, which tries to incorporate elements from both the direct and indirect activation model to form a new hypothesis in which the pro-survival proteins promote survival both by inhibiting the BH3-only proteins (direct model) as well as Bax/Bak (indirect model) (Leber et al, 2007). In this model, membrane permeabilization does not occur until Bax/Bak insert multiple sequences into the mitochondrial outer membrane lipid bilayer (Leber et al, 2007;Billen et al, 2008). Bax undergoes a conformational change upon membrane interaction and then a second change triggered by truncated Bid or other activators.…”

“…This activated integral form of Bax then recruits more cytosolic Bax in the process of homo-oligomerization. Pro-survival proteins, such as Bcl-xL, are proposed to perform two functions: sequestering the activators and also neutralizing the membrane-bound Bax to prevent them from homo-oligomerizing (functioning as a dominant-negative Bax molecule) (Billen et al, 2008). More work on these hypotheses will for sure test their merit.…”

PACS-2 mediates the ATM and NF-κB-dependent induction of anti-apoptotic Bcl-xL in response to DNA damage

“…3,4 Notably, Bcl-xL binds proapoptotic Bax and Bak, which prevents these proapoptotic Bcl-2 members from forming pores that induce mitochondria outer membrane permeabilization (MOMP). [5][6][7] In response to DNA damage, the tumor suppressor p53 promotes apoptosis by inducing expression of Puma, which sequesters Bcl-xL, thereby promoting Bax/Bak-mediated MOMP. 8,9 The prototypical NF-κB transcription factor, p65/p50, is tightly regulated by the inhibitory protein IκBα, which sequesters this heterodimer in the cytoplasm by masking the p65 nuclear localization signal (NLS).…”