Allosteric Regulation of BH3 Proteins in Bcl-xL Complexes Enables Switch-like Activation of Bax

Current models of apoptosis regulation by the Bcl-2 family of proteins postulate that heterodimeric interactions between family members determine whether Bax and Bak are activated to trigger cell death. Thus, the relative abundance and binding affinities between pro- and anti-apoptotic proteins determines the outcome of these interactions. Examination of these interactions using purified mitochondria and liposomes with full-length recombinant proteins revealed that Bcl-xL inhibits apoptosis as a higher-order complex that binds multiple BH3 proteins. Allosteric regulation of this complex by the BH3 sensitizer Bad confers switch-like activity to the indirect activation of Bax. The BH3 activator cBid sequestered by Bcl-xL complexes changes from an inactive to an active form while bound to a Bcl-xL complex only when Bad is also bound. Bcl-xL complexes enable Bad to function as a non-competitive inhibitor of Bcl-xL and allosterically activate cBid, dramatically enhancing the pro-apoptotic potency of Bad. [Display omitted] •Bcl-xL binds BH3 proteins predominantly as a higher-order complex•Bcl-xL complexes act as a scaffold that binds different BH3 proteins simultaneously•The sensitizer Bad allosterically activates BH3 proteins bound to Bcl-xL complexes•Activation of Bcl-xL-bound BH3 proteins enables switch-like activation of Bax The affinities of full-length Bcl-2 family proteins measured in assays containing mitochondria reveal that at physiological concentrations, the pro-apoptotic BH3 protein Bad does not displace sufficient activator BH3 proteins from anti-apoptotic Bcl-xL to activate the pro-apoptotic protein Bax and elicit apoptosis. Instead, Bad mediates allosteric activation of activator BH3 proteins that remain bound to Bcl-xL complexes, explaining how sensitizer BH3 proteins such as Bad promote switch-like activation of pro-apoptotic Bax and induce apoptosis under physiological conditions.