A Supported Bilayer Model to Study the Effect of Membrane Composition on Bcl-2 Family Proteins

Abstract

Apoptosis is a self mediated form of cell death that plays a significant role in the growth, development, and well being of multicellular organisms. If this process fails, potentially harmful cells are able to multiply uncontrollably within the organism forming harmful structures such as tumours. Although there are many signals and processes involved that may trigger apoptosis, the Bcl-2 family of proteins find themselves at a bottle neck of many of these pathways, as well as at the point after which the cell is no longer able to abort committing suicide. Bax is such a protein, and upon insertion and oligomerization within the outer mitochondrial membrane of mitochondria, apoptotic factors (such as cytochrome c) are released into the cytoplasm of the cell sealing its fate. Lipid composition is known to affect the process of membrane permeabilization by Bcl-2 family proteins, where different lipid compositions can block the process at different steps. Here we have investigated whether these different effects could be related to structural membrane properties. Solid supported lipid bilayers and aligned bilayer stacks are a convenient model system to study membranes structural properties. Not only do they allow for a rigid macroscopic sample size, they also offer a well defined geometry exploitable through techniques such as atomic force microscopy as well as x-ray and neutron scattering. Through Fourier analysis, we are able to utilize x-ray reflectivity data to analyze the electron density profile perpendicular to the plane of the membranes with sub-Angstrom resolution. We have characterized the structure of a membrane with a lipid composition mimicking that of mitochondrial membranes, and that of a series of membranes slightly differing from this original composition, and known to block the Bcl-2 family proteins pore formation process.