Towards a More Accurate Model of the Plasma Membrane: Influence of Cholesterol on Phase Behavior and Nonrandom mixing of Membrane Components

Cornell University, Ithaca NY

Investigators

Abstract

The outer membrane, the place where a living cell is in contact with the outside world, is a complex structure. This project will result in a new picture of animal cell outer membranes, which will increase understanding of how a cell responds to events such as binding of small molecule hormones, contacts with other cells or organisms friendly or unfriendly, and even viral infection. The complexity of this membrane includes the large number of different components, both lipid and protein, as well as the membrane's layered structure, with each half of the bilayer being different, yet in communication with the other half. One goal of these studies is to measure how each half of the bilayer communicates with the other half. Overall, this research will focus on both the behavior of individual lipids and proteins, as well as on the behavior of large collections of lipids and proteins, termed "phase domains". This project aims to develop simplified models of the cell membrane, using well-defined bilayer mixtures of lipids and peptides to mimic the behaviors of live cell membranes. An important aspect of these studies will be to train undergraduates, graduate students, and postdocs in the specialized methods that are so useful in membrane research. This project will result in a new picture of animal cell plasma membranes as having patterned morphology, possibly down to the scale of tens of nanometers. This new view is likely to replace the current, less well-defined view of ordered rafts floating in a sea of disordered lipids. The project objectives are to: (i) develop a competing interactions model with interfacial line tension competing with dipolar repulsion and curvature energies, enabling prediction of phase patterns at the tens of nanometer scale; (ii) measure line tension and simulate line tension to enable predictions at the nanometer scale; (iii) measure nanodomain size for lipid mixtures that are good models of animal cell plasma membranes; (iv) discover how membrane proteins respond to and modify the lipid phase patterns; and (v) discover how leaflets influence each other in asymmetric lipid bilayer vesicles. The competing interactions model will be implemented by Monte Carlo simulations of competing fields. Molecular dynamics simulations will use GROMACS, both United Atom or Coarse Grain. Size of nanodomain pattern features will be measured by analysis of small angle neutron scattering. Phase morphology and line tension at the macro scale will be determined via fluorescence microscopy imaging of giant unilamellar vesicles (GUVs). A major objective of this project will be to train undergraduates, graduate students, and postdocs in the specialized computational and experimental methods that are so useful in membrane research.