Biophysics of Asymmetric Membranes| Protocols and Revelations

• Main Author: Doktorova, Milka N.
• Language: EN
• Published: Weill Medical College of Cornell University 2018
• Subjects: Biophysics
• Online Access: http://pqdtopen.proquest.com/#viewpdf?dispub=10844615

<p> Lipid membranes enclose cells and organelles, and actively participate in cellular processes. Their many functional roles require tight regulation of properties including structure and dynamics. Cells achieve this by producing and dynamically tuning the concentration and organization of hundreds of structurally different types of lipid molecules in the various cellular membranes. The cell-bounding plasma membranes of eukaryotes in particular, exhibit an actively maintained asymmetric lipid distribution across their two leaflets. In addition to exposing certain types of lipids to the extracellular space or intracellular milieu, this specialized transbilayer lipid arrangement also affects the properties of the membrane itself and its interactions with proteins, in ways that are difficult to explore and thus not understood. To address this problem and enable further advancements in the field, we have developed both <i> in vitro</i> and <i>in silico</i> protocols for building asymmetric model membranes with finely controlled lipid compositions. These protocols allowed us to investigate the dynamics, energetics and structural consequences of interleaflet communication: with small-angle scattering we uncovered asymmetry-mediated changes in the lipid packing of individual leaflets in free-floating liposomes; with electron spin resonance we revealed the ensuing trends in lipid order; and nuclear magnetic resonance helped us bring new appreciation of the interplay between asymmetric bilayers and transmembrane protein inclusions. To interpret and better understand the experimental observations, we developed a new <i> in silico</i> protocol for constructing atomistic models of tension-free asymmetric bilayers and used it to simulate the experimentally measured membranes and validate the simulation conditions. By devising a novel computational framework for calculating the compressibility of individual bilayer leaflets, we analyzed the energetics of protein interaction with the asymmetric membranes and obtained an estimate of the elastic energy of mixing the two leaflets. Together with additional experimental and computational studies of symmetric membrane systems, the results revealed fascinating ways in which cells can mediate the functional diversity of their membranes. The new methods and protocols leading to these insights generate previously unattainable opportunities for dissecting and exploring membrane-mediated cellular processes.</p><p>