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Stepwise activation of the pro-apoptotic protein Bid at mitochondrial membranes


Caspase-8-cleaved Bid (cBid) associates with mitochondria and promotes the activation of BAX, leading to mitochondria outer membrane permeabilization (MOMP) and apoptosis. However, current structural models of cBid are largely based on studies using membrane vesicles and detergent micelles. Here we employ spin-label ESR and site-directed PEGylation methods to identify conformations of cBid at real mitochondrial membranes, revealing stepwise mechanisms in the activation process. Upon the binding of cBid to mitochondria, its structure is reorganized to expose the BH3 domain while leaving the structural integrity only slightly altered. The mitochondria-bound cBid is in association with Mtch2 and it remains in the primed state until interacting with BAX. The interaction subsequently triggers the fragmentation of cBid, causes large conformational changes, and promotes BAX-mediated MOMP. Our results reveal structural differences of cBid between mitochondria and other lipid-like environments and, moreover, highlight the role of the membrane binding in modifying cBid structure and assisting the inactive-to-active transition in function.

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Fig. 1: Bid structure and study strategy.
Fig. 2: DEER data of cBid.
Fig. 3: DEER data of tBid.
Fig. 4: PEG-labeling study of α3.
Fig. 5: PEG-labeling study of α6–α8.
Fig. 6: Interaction between cBid and BAX.


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We thank the Research Instrument Center of Taiwan located at NTHU for the ESR/DEER measurements.


All of the authors were supported by grants from the Ministry of Science and Technology of Taiwan (108-2113-M-007-029) and the Frontier Research Center on Fundamental and Applied Sciences of Matters at NTHU.

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CLH and YWC designed the experiments and analyzed the data. CLH, HHC, and SWL performed all the experiments. CLH and YWC wrote the paper. All authors discussed the results and commented on the paper.

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Correspondence to Yun-Wei Chiang.

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Hung, CL., Chang, HH., Lee, S.W. et al. Stepwise activation of the pro-apoptotic protein Bid at mitochondrial membranes. Cell Death Differ 28, 1910–1925 (2021).

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