Allosteric regulation of BH3-in-groove interactions by tail anchors of BCL-xL complexes limits BH3 mimetic antagonism.

BCL-xL promotes cell survival by binding BH3-only initiators through its hydrophobic groove. Combining resonance energy transfer assays and molecular dynamics simulations, we unravel that membrane anchoring of BCL-xL via its tail anchor selectively advantages binding to membrane-anchored PUMA initiator over BH3 mimetic ligands of the groove. This is due to the combined allosteric effect on BH3-in-groove binding of BCL-xL and PUMA tail anchors. Moreover, doubly anchored PUMA / BCL-xL complexes recruit endogenous BAX, which favors their antagonism by BH3 mimetics. BAX's tail anchor alone is sufficient to enhance BH3 mimetics-induced death in cells expressing PUMA / BCL-xL. Our work supports a model in which the survival function of BCL-xL is regulated by a complex interplay between its tail anchor and those of its interacting partners. This enables both resistance to pharmacological inhibitors and modulation by BAX, which functions as a crucial feedback disruptor of the BCL-xL network.