Structure Of Complexes Of Helix-5 From Bax With Lipid Membranes

Abstract

Bax is a proapoptotic protein implicated in the release of cell-death activating factors from the mitochondrial intermembrane space. Although the structure of the membrane-bound forms of Bax is unknown, it has been proposed to form proteolipidic pores. Studies with synthetic lipid vesicles have shown that fragments encompassing helix-5 of Bax retain a membrane permeabilization ability that is similar to that of the full-length protein. Here we report on the structure of peptide-membrane complexes formed by a Bax helix-5 peptide and lipid bilayers. The relative orientation of the peptide and the lipids are determined using site-specific infrared spectroscopy, assisted by isotopic labeling of backbone groups with the 13C=18O probe. The peptide is highly α-helical in all lipid membranes studied, and its orientation reveals different binding modes that depend on the bilayer phase state. In partially fluid POPC bilayers helix-5 of Bax lies almost parallel with respect to the membrane plane, most likely interacting at the level of the interface. However, in gel phase DMPC bilayers the peptide adopts a tilted orientation, which suggests a deeper insertion in the membrane. In turn infrared spectroscopy and X-ray diffraction data show that in some instances the Bax helix-5 peptide influences the phase transition properties of the lipids by increasing membrane fluidity. Taken together, these effects can be related with the membrane perturbation properties of the Bax helix-5 fragment and with its mechanism of action as a molecule inducing the formation of lipidic pores.