THE EFFECT OF CHOLESTEROL ON THE STRUCTURE OF MITOCHONDRIAL LIKE LIPID BILAYERS: AN X-RAY STUDY

Abstract

<p>Apoptosis plays a key role in the regulation and development of healthy multicellular organisms throughout their lifetimes. The mitochondria play a key role in this cellular process, as it contains proapoptotic factors, which once released into the cytosol of the cell, results in the death of the cell. The Bcl-2 family of proteins play a key role in apoptosis, acting as the gateway between life and death of the cell. Proteins such as tBid and Bax act to permeabilize the mitochondrial outer membrane (MOM), releasing the proapoptotic factors into the cell’s cytosol. The interactions between these proteins and the mitochondrial outer membrane have yet to be fully understood. The lipid composition and cholesterol content of the membrane effectively inhibit or promote pore formation by Bax. Specifically, the addition of cholesterol into the membrane inhibits pore formation. This thesis attempts to further understand the effects cholesterol has on the structure of the MOM, and link those changes to the inhibited activity of Bax pore formation. MOM-like lipid bilayers were studied under varying temperatures and with the addition of cholesterol using x-ray reflectivity. Increasing temperatures from 10°C to 30°C resulted in bilayer thinning, as did decreasing cholesterol concentrations below 30%. From 10°C to 20°C, bilayer thickness showed a bell shaped profile, and changed to a linear decrease above about 20°C. This may assist Bax in pore formation, as it has also been observed to cause bilayer thinning. Increasing Cholesterol concentrations up to 30% resulted in little variation in bilayer thickness though hindrance of Bax pore formation is observed at content levels as low as 8%. Thus it is unlikely that bilayer thickening by cholesterol causes the inhibition of Bax pore formation. In addition, cholesterol was observed to increase the electron density of the core of the bilayer at concentration levels above 25%.</p>