Endophilin B1/Bif-1 Stimulates BAX Activation Independently from Its Capacity to Produce Large Scale Membrane Morphological Rearrangements

Abstract

Endophilin B1/BAX-interacting factor 1 (Bif-1) is a protein that cooperates with dynamin-like protein 1 (DLP1/Drp1) to maintain normal mitochondrial outer membrane (MOM) dynamics in healthy cells and also contributes to BAX-driven MOM permeabilization (MOMP), the irreversible commitment point to cell death for the majority of apoptotic stimuli. However, despite its importance, exactly how Bif-1 fulfils its proapoptotic role is unknown. Here, we demonstrate that the stimulatory effect of Bif-1 on BAX-driven MOMP and on BAX conformational activation observed in intact cells during apoptosis can be recapitulated in a simplified system consisting of purified proteins and MOM-like liposomes. In this reconstituted model system the N-BAR domain of Bif-1 reproduced the stimulatory effect of Bif-1 on functional BAX activation. This process was dependent on physical interaction between Bif-1 N-BAR and BAX as well as on the presence of the mitochondrion-specific lipid cardiolipin. Despite that Bif-1 N-BAR produced large scale morphological rearrangements in MOM-like liposomes, this phenomenon could be separated from functional BAX activation. Furthermore, DLP1 also caused global morphological changes in MOMlike liposomes, but DLP1 did not stimulate BAX-permeabilizing function in the absence or presence of Bif-1. Taken together, our findings not only provide direct evidence for a functional interplay between Bif-1, BAX, and cardiolipin during MOMP but also add significantly to the growing body of evidence indicating that components of the mitochondrial morphogenesis machinery possess proapoptotic functions that are independent from their recognized roles in normal mitochondrial dynamics.

GSK-3β promotes cell survival by modulating Bif-1-dependent autophagy and cell death

“…Most interestingly, Bif-1 was recently found to modulate autophagy by interacting with beclin-1-VPS34 complex through the UVRAG protein (Takahashi et al, 2007). Moreover, a recent study suggested that Bif-1 might act as a driving force for autophagosome formation (Etxebarria et al, 2009) (reviewed by Takahashi et al, 2009). In this study, we discovered that Bif-1 is required for GSK-3 inhibitor-induced autophagic response, necrosis and apoptosis.…”

The rheostat in the membrane: BCL-2 family proteins and apoptosis

“…99,[102][103][104] However, Kushnareva et al 95 found no evidence supporting a role for these proteins in Bax-induced permeabilization of isolated MOM vesicles. Chemical compounds (mDivi-1 analogs) that had been shown to inhibit Drp1's yeast ortholog, Dnm1, also inhibit MOMP in mammalian cells, 105 and some mDivi-1 analogs blocked BAX-induced pore formation in rat liver and Xenopus MOM vesicles.…”

Proapoptotic Bax and Bak Proteins Form Stable Protein-permeable Pores of Tunable Size

“…We gathered different lines of evidence supporting that active forms of Bax and Bak form toroidal pores of proteolipidic nature. On the one hand, we found that pore formation by Bax and Bak is linked to changes in the intrinsic monolayer curvature and line tension of the membrane, two physical properties known to be critical for toroidal pore formation (17,18,26,31,32). On the other hand, x-ray diffraction experiments demonstrated that a peptide derived from the central helix of Bax (Bax ␣5) forms pores in supported bilayers containing lipid molecules in their lumen (33), consistent with functional and structural information obtained with this peptide in a variety of model membrane systems (34 -36, 41-43).…”