Mechanistic differences in the membrane activity of Bax and Bcl-xL correlate with their opposing roles in apoptosis

Abstract

Based on their membrane-permeabilizing activity in vitro, it has been proposed that Bax-like proteins induce cytochrome c release during apoptosis via pore formation. However, antiapoptotic Bcl-2 proteins, which inhibit cytochrome c release, also display pore activity in model membranes. As a consequence, a unified description that aligns the pore activity of the Bcl-2 proteins with their apoptotic function is missing. Here, we studied the mechanism of membrane binding, oligomerization, and permeabilization by pro- and antiapoptotic Bcl-2 members at the single-vesicle level. We found that proapoptotic Bax forms large, stable pores via an all-or-none mechanism that can release cytochrome c. In contrast, antiapoptotic Bcl-xL induces transient permeability alterations in pure lipid membranes that have no consequences for the mitochondrial outer membrane but inhibit Bax membrane insertion. These differences in pore activity correlate with a distinct oligomeric state of Bax and Bcl-xL in membranes and can be reproduced in isolated mitochondria. Based on our findings, we propose new models for the mechanisms of action of Bax and Bcl-xL that relate their membrane activity to their opposing roles in apoptosis and beyond.

Figure 1

Bax and Bcl-xL binding to GUVs depends on cBid and CL. (A and B) Confocal images and radial profiles of individual GUVs made of PC/CL 80:20 (mol/mol) after 60 min incubation at room temperature, showing Bcl-xL_R binding to GUVs in the absence and presence of cBid_G (A) and cBid_G binding to GUVs in the absence and presence of Bcl-xL_R (B). Scale bar, 25 μM. (C–F) Bcl-2 proteins Bax_G (C), cBid_R (D), Bcl-xl (E), and cBid_G (F) binding to GUVs with different lipid compositions. M indicates a lipid mixture mimicking a MOM containing 6% CL (mol/mol).

Figure 2

Bax and Bcl-xL follow different mechanisms of membrane permeabilization. (A) The internalization of cyt c-Al488 into the lumen of the individual vesicles indicates membrane permeabilization. Green GUVs composed of PC 0.05% DiO and red GUVs of PC/CL 8:2 0.05% DiD. Images were taken 90 min after mixing the components. Scale bar, 75 μm. (B) Fraction of nonpermeabilized GUVs in the presence of Bcl-2 proteins. Gray bars correspond to PC GUVs, red bars to GUVs containing 20% CL, and blue bars to a mixture mimicking the MOM. (C) Distribution of degree of filling of the GUVs after 90 min incubation with cBid/Bax (red bars) and cBid/Bcl-xL (blue bars). GUVs were composed of PC/CL 8:2 0.05% DiD. In each of three experiments, a minimum of 250 vesicles were analyzed. (D and E) Time-lapse images of the permeabilization of GUVs (grayscale) to cyt c-al488 (green) induced by cBid/Bax (D) or cBid/Bcl-xL (E). Scale bar, 25 μm. (F and G) Filling kinetics measured by the increase of fluorescence intensity inside of individual GUVs (different colors) incubated with cBid/Bax (F) or cBid/Bcl-xL (G). Normalized data are shown in dots and fitting curves in lines. (H and I) Estimated radius of the initial (H) and relaxed (I) total pore area of individual GUVs after incubation with cBid/Bax (red) or cBid/ Bcl-xL (blue). Outliers are shown in gray. Protein concentration was 10 nM for cBid, 20 nM for Bax,, and 50 nM for Bcl-xL.

Figure 3

Unlike Bcl-xL, Bax induces stable pores in lipid membranes. (A and B) GUVs in the presence of cBid and Bax or Bcl-xL after 2 h incubation. PC/CL 8:2 0.05% DiI GUVs (second panel) and Al488 (first panel) were added simultaneously with the proteins, and Al633 (third panel) was added after the 90-min incubation. Merged images are shown in the fourth panel. (C–E) Filling degree of individual, permeabilized GUVs to Al488 (green bars) and Al633 (red bars) after incubation in the presence of cBid/Bax (C), cBid/Bcl-xL (D), or without proteins (E). (F) Fraction of GUVs with stable pores (>50% filling with both Al488 and Al633) from five independent experiments. Error bars represent the SD. Unpaired two-tailed t-test, p value < 0.001.

Figure 4

Bax, but not Bcl-xL, oligomerizes in the membrane of GUVs. (A and B) Confocal images, auto- (orange and green), and cross-correlation (blue) curves from two-focus scanning FCCS measurements in the membranes of GUVs using Bax_G/Bax_R (A) or Bcl-xL_G/Bcl-xL_R (B) in the presence of cBid. Straight lines correspond to fitted curves and dotted lines to the raw data. Scale bar, 10 μm. (C) Percent complex between Bax and Bcl-xL molecules in individual GUVs. Unpaired two-tail t-test, p value < 0.001.

Figure 5

Bax stably permeabilizes isolated yeast mitochondria. (A) Fluorescence spectra of pelleted yeast mitochondria incubated with DexR and buffer (black); 50 nM cBid and 100 nM Bax (red); or 50 nM cBid and 200nM Bcl-xL (blue). (B) Mean ± SD of the fluorescence intensity in the pellet fraction at 667 nm (from five independent experiments). (C–G) Confocal images of isolated yeast mitochondria after incubation with buffer (C), Bcl-xL/cBid (D), or Bax/cBid in the presence of Dex_G (added at time 0) (E–G) and Dex_R (C–F) or Bcl-xL_R added after 45 min incubation (G). In F agarose was added at a temperature suitable to fast gel hardening >1 min, so that the dextrans in the medium are diluted and no equilibration with the dextrans inside the mitochondria can take place. Images in C–G are (left to right) the transmission image, Dex_G (green), Dex_R or Bcl-xL_R (red), and a merge of the red and green channels. Protein concentrations were 50 nM cBid, 100 nM Bax, 200 nM Bcl-xL, or 50 nM Bcl-xL_R (in G). Scale bar, 5 μm.

Figure 6

Models for membrane activity of Bax and Bcl-xL. (A) The membrane tension associated with Bax binding to the accessible membrane leaflet is locally elevated by oligomerization. This increases the efficiency of pore opening, which is then stabilized by a decrease in line tension promoted by Bax. Bax oligomerization most likely also increases the line-tension attenuation and thus, pore stability. (B) Upon membrane binding, Bcl-xL also increases the membrane tension, although less efficiently. When the membrane tension is sufficiently high, a pore opens. As the membrane tension is relieved, and since Bcl-xL does not attenuate the line tension, the pore will close.