In this talk, Gidalevitz will present highlights of two projects where cholesterol plays a pivotal, albeit not obvious role. One project looks at mechanisms of membrane curvature generation, while another one explores the protective role of membrane cholesterol against toxins. Both projects employ synchrotron X-ray scattering to resolve the interplay between cholesterol, lipids, proteins, and drugs.
Cellular membranes exhibit a broad spectrum of curvatures depending on the function they perform. Constituent and peripheral molecules can define membrane curvature in several ways: lipids of different molecular shape can distribute inhomogeneously within the bilayer, peripheral proteins with curved shapes can scaffold the membrane, or proteins can embed amphipathic domains shallowly into the lipid matrix. At the same time, the distribution of membrane constituent molecules and activity of bending proteins depend on curvature. The N-terminal fusion domain of glycoprotein gp41 is the only player of HIV-1 virus which directly interacts with the lipid bilayer of the host cell membrane and bends it via the insertion mechanism during virus entry. Membrane bending rigidity depends on cholesterol, i.e. membranes with higher cholesterol content require more energy to bend. We have discovered that cholesterol regulates membrane penetration depth and occupied surface area of the gp41 fusion domain in model Langmuir monolayer systems and thus can control curvature in biological membranes.
Hydrophobic drugs and toxins must permeate the plasma membrane barrier to carry out their specific functions. Plasma membranes of eukaryotes are primarily composed of phospholipids, proteins, and sterols. Surface X-ray scattering on Langmuir lipid monolayers of characteristic phospholipid DPPC and cholesterol mixtures show that this relationship to membrane cholesterol extends to the molecular level. Our results suggest that even a 20% ratio of membrane cholesterol could have a protective role as it pertains to the uptake and permeation of hydrophobic toxins, such as superwarfarins. These findings indicate that cellular responses to toxins are regulated by cholesterol content. Indeed, we have shown that even a 20% ratio of membrane cholesterol could have a protective role as it pertains to the uptake and permeation of hydrophobic toxins, such as superwarfarins.