Fig. 3

Cell apoptosis regulated by Bcl-x isoforms. a. Three modes that had been proposed to explain how Bcl-xL regulate MOMP. Mode 0: Bcl-xL prevented the binding of apoptotic effectors Bax to mitochondrial outer membrane through retrotranslocating Bax from the mitochondria into cytosol constantly. Mode 1: Bcl-xL sequestered BH3-only activators (tBid) to prevent them from binding to and activating Bax. Mode 2: Bcl-xL directly bound to activated Bax to prevent its oligomerization and MOMP. b. Cell apoptosis pathways regulated by Bcl-xL and Bcl-xS