BFL1 Modulates Apoptosis at the Membrane Level through a Bifunctional And Multimodal Mechanism Showing Key Differences With BCLXL

Abstract

BFL1 is a relatively understudied member of the BCL2 protein family which has been implicated in the pathogenesis andchemoresistance of a variety of human cancers, including hematological malignancies and solid tumours. BFL1 is generallyconsidered to have an antiapoptotic function, although its precise mode of action remains unclear. By quantitativelyanalyzing BFL1 action in synthetic membrane models and in cells, we found that BFL1 inhibits apoptosis through threedistinct mechanisms which are similar but not identical to those of BCLXL, the paradigmatic antiapoptotic BCL2 familyprotein. Strikingly, alterations in lipid composition during apoptosis activate a prodeath function of BFL1 that is based onnoncanonical oligomerization of the protein and breaching of the permeability barrier of the outer mitochondrial membrane(OMM). This lipid-triggered prodeath function of BFL1 is absent in BCLXL and also differs from that of the apoptoticeffector BAX, which sets it apart from other BCL2 family members. Ourfindings support a new model in which BFL1modulates apoptosis through a bifunctional and multimodal mode of action that is distinctly regulated by OMM lipidscompared to BCLXL.