PUMA binding induces partial unfolding within BCL-xL to disrupt p53 binding and promote apoptosis

Following DNA damage, nuclear p53 induces the expression of PUMA, a BH3-only protein that binds and inhibits the antiapoptotic Bcl-2 repertoire, including Bcl-xL. PUMA, unique among BH3-only proteins, disrupts the interaction between cytosolic p53 and Bcl-xL, allowing p53 to promote Apoptosis via direct activation of the Bcl-2 effector molecules Bax and Bak. Structural investigations using NMR spectroscopy and X-ray crystallography revealed that PUMA binding induced partial unfolding of two α-helices within Bcl-xL. Wild-type PUMA or a PUMA mutant incapable of causing binding-induced unfolding of Bcl-xL equivalently inhibited the antiapoptotic Bcl-2 repertoire to sensitize for death receptor-activated Apoptosis, but only wild-type PUMA promoted p53-dependent, DNA damage-induced Apoptosis. Our data suggest that PUMA-induced partial unfolding of Bcl-xL disrupts interactions between cytosolic p53 and Bcl-xL, releasing the bound p53 to initiate Apoptosis. We propose that regulated unfolding of Bcl-xL provides a mechanism to promote PUMA-dependent signaling within the apoptotic pathways.