Avoiding Mitochondrial Apoptosis by the Bcl-2-Driven Bax Oligomerization on Membrane Surfaces

TL;DR

This study reveals how Bcl-2 proteins prevent cell death by trapping Bax proteins on mitochondrial membranes, offering insights into cancer cell survival.

Contribution

The study identifies a novel mechanism of Bcl-2-mediated Bax sequestration involving heterodimerization and oligomerization.

Findings

Bcl-2 sequesters Bax through heterodimerization and Bax oligomerization on mitochondrial membranes.

This sequestration occurs in two steps: rapid heterodimer formation followed by slower Bax oligomerization.

The mechanism persists even in the presence of cardiolipin, a lipid that normally promotes Bax pore formation.

Abstract

The Bcl-2 family of proteins governs mitochondrial outer
membrane
(MOM) permeabilization, a critical step in apoptosis that is dysfunctional
in many cancers. Although cellular studies have long implicated direct
interactions between the pore-forming apoptotic Bax protein and its
opponent, the antiapoptotic Bcl-2 protein in apoptosis regulation,
the underlying basic principles behind this control remained unresolved.
To provide in-depth insight, we carried out a systematic biophysical
study in which we utilized neutron reflectometry (NR) and ATR-FTIR
to elucidate the molecular communication between those proteins in
and around the mitochondrial membrane environment. The spatial and
temporal changes across model MOM surfaces were resolved during the
interaction of Bax with Bcl-2. The NR-derived membrane surface Bax
distributions suggested that Bcl-2 mediated Bax sequestration through
both Bcl-2/Bax heterodimerization and Bax/Bax oligomerization. Kinetic
analysis revealed a two-step process: rapid formation of Bcl-2/Bax
heterodimers, followed by slower Bax oligomerization on these complexes.
Importantly, this sequestration mechanism was also observed in the
presence of cardiolipin, a lipid known to promote the formation of
an apoptotic pore by Bax in the absence of Bcl-2. These findings suggest
a fundamental mechanism by which cancer cells may evade apoptosis
by exploiting Bcl-2’s ability to neutralize Bax through structural
entrapment, even if excess Bax is present, either in response to treatment
or natural death signals.

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