Bcl-2 Homodimerization Involves Two Distinct Binding Surfaces, a Topographic Arrangement That Provides an Effective Mechanism for Bcl-2 to Capture Activated Bax

Abstract

The homo-and heterodimerization of Bcl-2 family proteins is important for transduction and integration of apoptotic signals and control of the permeability of mitochondria and endoplasmic reticulum membranes. Here we mapped the interface of the Bcl-2 homodimer in a cell-free system using site-specific photocross-linking. Bcl-2 homodimer-specific photoadducts were detected from 11 of 17 sites studied. When modeled into the structure of Bcl-2 core, the interface is composed of two distinct surfaces: an acceptor surface that includes the hydrophobic groove made by helices 2 and 8 and the loop connecting helices 4 and 5 and a donor surface that is made by helices 1-4 and the loop connecting helices 2 and 3. The two binding surfaces are on separate faces of the three-dimensional structure, explaining the formation of Bcl-2 homodimers, homo-oligomers, and Bcl-2/ Bax hetero-oligomers. We show that in vitro the Bcl-2 dimer can still interact with activated Bax as a larger oligomer. However, formation of a Bax/Bcl-2 heterodimer is favored, since this interaction inhibits Bcl-2 homodimerization. Our data support a simple model mechanism by which Bcl-2 interacts with activated Bax during apoptosis in an effective manner to neutralize the proapoptotic activity of Bax.Bcl-2 family proteins are key regulators of apoptosis. These proteins share sequence homology in Bcl-2 homology (BH) 1 domains and function to promote or prevent apoptosis. Antiapoptotic proteins such as Bcl-2 and Bcl-x L show homology in four BH domains (BH1 to -4). Proapoptotic proteins can be grouped into "multidomain" and "BH3-only" subfamilies. Multidomain proapoptotic proteins such as Bax and Bak display homology in BH domains 1-3, whereas BH3-only proteins such as Bid and Bim are similar structurally to multidomain family members, but sequence similarity is limited to only the BH3 domain. The current model for how Bcl-2 family proteins regulate apoptosis involves three sequential processes: (i) BH3-only proteins are activated by various death signals; (ii) the active BH3-only proteins then either activate multidomain proapoptotic proteins or inhibit antiapoptotic proteins or both; and (iii) unless inhibited by antiapoptotic Bcl-2 proteins, activated multidomain proapoptotic proteins form oligomers in the mitochondrial outer membrane that release proapoptotic proteins such as cytochrome c and Smac/DIABLO from the mitochondrial intermembrane space. The released proteins trigger activation of the caspases and nucleases that eventually dismantle the cell (1-4).

Embedded together: The life and death consequences of interaction of the Bcl-2 family with membranes

“…However, the embedded together model would also predict that allosteric regulators of the transitions between conformational states represent attractive "drugable" targets that may offer greater specificity. Mapping the surfaces of these conformation-specific domains in the physiologically relevant context of membranes is therefore an important priority [32,53,82].…”

Docosahexaenoic acid provides protective mechanism in bilaterally MPTP-lesioned rat model of Parkinson's disease

“…Phosphorylation inactivates Bcl-2, thus promoting apoptosis, possibly by releasing Bax from Bcl-2/Bax dimers [62][63][64]. The Bcl-2/Bax heterodimer is the active component for death protection [65,66]. In response to apoptotic stimulation, Bax can be released from Bcl-2/Bax dimers and act as the channels for either ions or proteins [64,67].…”

Bcl-2-related survival proteins

“…66,70,71 Bax pore activity appears to require dimeric and tetrameric associations, and similar considerations probably apply to antiapoptotic proteins. An X-ray crystallographic structure of Bcl-x L homodimers was recently published, with an unexpected three-dimensional domain-swapping mechanism of dimerization.…”