Abstract
It has been widely accepted that mitochondria-dependent apoptosis initiates when select BH3-only proteins (BID, BIM, etc.) directly engage and allosterically activate effector proteins BAX/BAK. Here, through reconstitution of cells lacking all eight pro-apoptotic BH3-only proteins, we demonstrate that all BH3-only proteins primarily target the anti-apoptotic BCL-2 proteins BCL-xL/MCL-1, whose simultaneous suppression enables membrane-mediated spontaneous activation of BAX/BAK. BH3-only proteins’ apoptotic activities correlate with affinities for BCL-xL/MCL-1 instead of abilities to directly activate BAX/BAK. Further, BID and BIM do not distinguish BAX from BAK or accelerate BAX/BAK activation following inactivation of BCL-xL/MCL-1. Remarkably, death ligand-induced apoptosis in cells lacking BH3-only proteins and MCL-1 is fully restored by BID mutants capable of neutralizing BCL-xL, but not direct activation of BAX/BAK. Taken together, our findings provide a “Membrane-mediated Permissive” model, in which the BH3-only proteins only indirectly activate BAX/BAK by neutralizing the anti-apoptotic BCL-2 proteins, and thus allowing BAX/BAK to undergo unimpeded, spontaneous activation in the mitochondrial outer membrane milieu, leading to apoptosis initiation.
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Acknowledgements
We are grateful for Keith Johnson and Robert Lewis for helpful discussion and advice. We are also grateful for the UNMC DNA sequencing and flow cytometry facilities for the excellent technical help. This work was supported by a pilot grant from the Nebraska Center for Cellular Signaling (P20-GM103489) and NIH Grants R03CA205496 and R01GM118437 to X.L.
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K.H., K.L.O, and X.L. designed the experiments and wrote the manuscript. K.H. and K.L.O. carried out all the experiments. J.L., W.Z., N.H., X.P., W.W., L.S, G.B., Z.L. and L.Z. created some of the cell lines and reagents.
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Huang, K., O’Neill, K.L., Li, J. et al. BH3-only proteins target BCL-xL/MCL-1, not BAX/BAK, to initiate apoptosis. Cell Res 29, 942–952 (2019). https://doi.org/10.1038/s41422-019-0231-y
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DOI: https://doi.org/10.1038/s41422-019-0231-y
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