Bax-type Apoptotic Proteins Porate Pure Lipid Bilayers through a Mechanism Sensitive to Intrinsic Monolayer Curvature

Abstract

During apoptosis, Bax-type proteins permeabilize the outer mitochondrial membrane to release intermembrane apoptogenic factors into the cytosol via a poorly understood mechanism. We have proposed that Bax and ⌬N76Bcl-x L (the Bax-like cleavage fragment of Bcl-x L ) function by forming pores that are at least partially composed of lipids (lipidic pore formation). Since the membrane monolayer must bend during lipidic pore formation, we here explore the effect of intrinsic membrane monolayer curvature on pore formation. Nonlamellar lipids with positive intrinsic curvature such as lysophospholipids promoted membrane permeabilization, whereas nonlamellar lipids with negative intrinsic curvature such as diacylglycerol and phosphatidylethanolamine inhibited membrane permeabilization. The differential effects of nonlamellar lipids on membrane permeabilization were not correlated with lipid-induced changes in membrane binding or insertion of Bax or ⌬N76Bcl-x L . Altogether, these results are consistent with a model whereby Bax-type proteins change the bending propensity of the membrane to form pores comprised at least in part of lipids in a structure of net positive monolayer curvature.

Methods for the assessment of mitochondrial membrane permeabilization in apoptosis

“…Bak) to promote the formation of large homoor hetero-multimeric channels, through which IMS proteins are released [40][41][42][43]. (2) Pro-apoptotic factors may act directly on the lipidic component of the OM, or at the lipidprotein interface, thus favoring the formation of pores though which IMS proteins may exit to the cytosol [29,[44][45][46]. (3) Activated pro-apoptotic members of the Bcl-2 family may interact with components of the permeability transition pore complex (PTPC), thus favoring (or de-inhibiting) the permeability transition (PT) of the IM, that in turn leads to the physical rupture of the OM [47][48][49].…”

Mitochondrial dynamics in the regulation of neuronal cell death

“…One may envisage, therefore, a model in which the fission machinery is required for apoptosis and is recruited during apoptosis signalling to induce cell death by as of yet unidentified mechanisms. One possibility is that mitochondrial fission may participate in cell death by modifying the lipid microenvironment or deform lipid bilayers for Bax/Bak induced mitochondrial permeabilization [74,75]. While the pro-apoptotic role for mitochondrial fragmentation remains unclear, all of these studies support an important role of fission proteins in the execution of cell death.…”

Mitochondrial factors with dual roles in death and survival

“…Their activation is triggered by a subset of the BH3-only proteins, the best known of which is Bid (Li et al, 1998;Luo et al, 1998). The ability of Bid to activate Bax was clearly demonstrated by adding recombinant proteins to isolated mitochondria or to pure lipid vesicles loaded with cytochrome c or other material (Basan˜ez et al, 1999(Basan˜ez et al, , 2002Hardwick and Polster, 2002;Kuwana et al, 2002). However, Bid also needs to be activated in cells by proteolytic cleavage or in vitro by detergent-induced conformational changes.…”