Dissociation of Bak α1 helix from the core and latch domains is required for apoptosis

During apoptosis, Bak permeabilizes mitochondria after undergoing major conformational changes, including poorly defined N-terminal changes. Here, we characterize those changes using 11 antibodies that were epitope mapped using peptide arrays and mutagenesis. After Bak activation by Bid, epitopes throughout the alpha 1 helix are exposed indicating complete dissociation of alpha 1 from alpha 2 in the core and from alpha 6-alpha 8 in the latch. Moreover, disulfide tethering of a1 to alpha 2 or alpha 6 blocks cytochrome c release, suggesting that alpha 1 dissociation is required for further conformational changes during apoptosis. Assaying epitope exposure when alpha 1 is tethered shows that Bid triggers alpha 2 movement, followed by alpha 1 dissociation. However, alpha 2 reaches its final position only after alpha 1 dissociates from the latch. Thus, alpha 1 dissociation is a key step in unfolding Bak into three major components, the N terminus, the core (alpha 2-alpha 5) and the latch (alpha 6-alpha 8).