Insights into p53-Dependent Apoptotic Signaling and Cell Fate vis-a-vis Functional Cooperation among BCL-xL, Cytoplasmic p53, and PUMA

Following DNA damage, nuclear p53 induces the expression of PUMA (p53 upregulated modulator of apoptosis), a BH3‑only protein that binds and inhibits the anti‑apoptotic BCL‑2 repertoire, including BCL‑xL. Structural investigations of PUMA and the BCL‑xL×PUMA BH3 domain complex by X‑ray crystallography and nuclear magnetic resonance (NMR) spectroscopy reveal a novel, PUMA‑induced, domain‑swapped dimerization of BCL‑xL that requires a π‑stacking interaction between PUMA W71 and BCL‑xL H113. PUMA is an intrinsically disordered protein, but upon interaction with BCL‑xL, PUMA W71 and the PUMA BH3 domain residues fold into an alpha helix and subtly remodel BCL‑xL to trigger its dimerization. Wild type PUMA or a PUMA mutant incapable of promoting BCL‑xL dimerization (PUMA W71A) equivalently inhibit the anti‑apoptotic BCL‑2 repertoire to sensitize for death receptor‑activated apoptosis, but only wild type PUMA promotes p53‑dependent DNA damage‑induced apoptosis. Biochemical and cellular data demonstrate that PUMA‑mediated structural remodeling and dimerization of BCL‑xL modulates its affinity for cytosolic p53, providing a detailed mechanism of BCL‑xL, cytosolic p53, and PUMA functional cooperation. Our data suggest that within the BCL‑2 family, ligand binding‑induced, domain‑swapped dimerization is a critical control point to increase signal transduction complexity within the apoptotic pathways