The secrets of the Bcl-2 family

Abstract

The Bcl-2 family of proteins is formed by pro- and antiapoptotic members. Together they regulate the permeabilization of the mitochondrial outer membrane, a key step in apoptosis. Their complex network of interactions both in the cytosol and on mitochondria determines the fate of the cell. In the past 2 decades, the members of the family have been identified and classified according to their function. Several competing models have been proposed to explain how the Blc-2 proteins orchestrate apoptosis signaling. However, basic aspects of the action of these proteins remain elusive. This review is focused on the biophysical mechanisms that are relevant for their action in apoptosis and on the challenging gaps in our knowledge that necessitate further exploration to finally understand how the Bcl-2 family regulates apoptosis.

Figure 1

Model for lipid pore induced by Bax and Bak. Once Bax and Bak insert extensively into the MOM and oligomerize, they induce the formation of a toroidal (or lipid) pore. Such pores are thought to be formed by lipids and proteins. In order to avoid the exposure of the hydrophobic acyl tails to the aqueous solvent, the lipids at the pore edge reorganize to form a non-bilayer structure with a very high curvature, which also has an energetic cost. Bax and Bak stabilize the pore by decreasing the energy of the pore rim. According to current models, helices 5 and 6 are inserted into the membrane and are involved in pore formation, and helix 9 is also membrane inserted^,

Figure 2

Models for inhibition of MOM permeabilization by the antiapoptotic Bcl-2 proteins. (a) Modes 1 and 2 of inhibition proposed by Llambi et al. In mode 1, prosurvival Bcl-2 proteins inhibit the BH3-only activators, which cannot further induce the activation of the effectors Bax and Bak. In mode 2, the prosurvival Bcl-2s bind and inhibit directly the effectors. Mode 1 is less efficient than mode 2 to inhibit apoptosis. (b) According to the ‘embedded together' model, the prosurvival Bcl-2 proteins bind to Bax and Bak in their inserted form at the MOM and form non-productive heterodimers for further oligomerization and MOM permeabilization. (c) Bax and Bcl-xL constantly translocate to the mitochondria, where they form a complex and retrotranslocate back to the cytosol. This keeps Bax mainly cytosolic in healthy cells

Figure 3

Complexity of the Bcl-2 interaction network. The scheme shows the interactions between different Bcl-2 family members reported in the literature. Modes of action are shown in different colors: blue lines indicate binding, black connectors show reaction and green arrows indicate activation, while red cross lines mean inhibition. Interaction map generated for human Bcl-2 proteins with STRING 9.0 (http://string-db.org). The following nomenclature was used: BBC3 for Puma, Bcl2L11 for Bim, Bcl2L1 for Bcl-xL, Bcl2L2 for Bcl-w, Bcl2L3 for Mcl-2, Bcl2A1 for Bfl-1/A1, PMAIP1 for Noxa, BECN1 for Beclin1 and Bcl2L10 for Bcl-B/Boo/Diva

Figure 4

Fluorescence correlation spectroscopy to quantify protein/protein interactions. (a) FCS measures the fluorescence fluctuations due to the diffusion of individual molecules through the confocal volume of the microscope. To measure protein/protein interactions in membranes, the confocal volume (blue) is placed on the membrane plane. (b) If the proteins of interest (green and red balls in a) form a complex, they diffuse simultaneously in and out of the focal volume and the fluorescence spikes they provoke appear simultaneously in the detection channels. (c) This co-diffusion is detected and the cross-correlation analysis of the signal allows the quantification of the percentage of proteins that are in complex out of the total pool of proteins of interest. If the two proteins form a complex, the cross-correlation curve has positive amplitude at lag time 0 (solid line), which does not happen if they do not interact (dashed line)