Functional and physical interaction between Bcl-X_L and a BH3-like domain in Beclin-1

The anti-apoptotic proteins Bcl-2 and Bcl-X_L bind and inhibit Beclin-1, an essential mediator of autophagy. Here, we demonstrate that this interaction involves a BH3 domain within Beclin-1 (residues 114-123). The physical interaction between Beclin-1 and Bcl-X_L is lost when the BH3 domain of Beclin-1 or the BH3 receptor domain of Bcl-X_L is mutated. Mutation of the BH3 domain of Beclin-1 or of the BH3 receptor domain of Bcl-X_L abolishes the Bcl-X_L-mediated inhibition of autophagy triggered by Beclin-1. The pharmacological BH3 mimetic ABT737 competitively inhibits the interaction between Beclin-1 and Bcl-2/Bcl-X _L, antagonizes autophagy inhibition by Bcl-2/Bcl-X_L and hence stimulates autophagy. Knockout or knockdown of the BH3-only protein Bad reduces starvation-induced autophagy, whereas Bad overexpression induces autophagy in human cells. Gain-of-function mutation of the sole BH3-only protein from Caenorhabditis elegans, EGL-1, induces autophagy, while deletion of EGL-1 compromises starvation-induced autophagy. These results reveal a novel autophagy-stimulatory function of BH3-only proteins beyond their established role as apoptosis inducers. BH3-only proteins and pharmacological BH3 mimetics induce autophagy by competitively disrupting the interaction between Beclin-1 and Bcl-2 or Bcl-X_L.