Lipid asymmetry of a model mitochondrial outer membrane affects Bax-dependent permeabilization

The presence of an asymmetric distribution of lipids in biological membranes was first described ca. 50 years ago. While various studies had reported the role of loss of lipid asymmetry on signaling processes, its effect on membrane physical properties and membrane-protein interactions lacks further understanding. The recent description of new technologies for the preparation of asymmetric model membranes has helped to fill part of this gap. However, the major effort so far has been on plasma membrane models. Here we describe the preparation of liposomes mimicking the mitochondria outer membrane (MOM) in regard to its lipid composition and asymmetry. By employing the methyl-β-cyclodextrin-catalyzed lipid exchange technology and accurate quantification of lipid asymmetry with head group-specific probes we showed the successful preparation of a MOM model bearing a physiologically relevant lipid composition and asymmetry. In addition, by a direct comparison with its lipid symmetrical counterpart it is shown that asymmetric models were more resistant to tBid-promoted Bax-permeabilization, suggesting a role played by MOM lipid asymmetry on the mitochondria pathway of apoptosis. The barrier imposed by lipid asymmetry on membrane permeabilization was in part due to a decrease in the concentration of membrane-bound proteins, which was likely a consequence of the two mutually-dependent properties; i.e., the lower electrostatic surface potential and the higher molecular packing imposed by lipid asymmetry. It is proposed that MOM lipid asymmetry imparts different physical properties on the membrane and might add an additional component of regulation in intricate mitochondrial processes.