CPM Seminar

From obstacle-induced obstructed diffusion to hop diffusion across polymeric fences: Membrane organization and fluidity of polymer-tethered phospholipid bilayers

Christoph A. Naumann

Department of Chemistry
Indiana Univerisity-Purdue University Indianapolis

The heterogeneous structure of the plasma membrane and the resulting dynamical properties critically impact several important biological functions in cells including signal transduction and cell-cell recognition. Consequently, there has been much effort to derive biophysical principles that link the organization, dynamics, and functionality of membranes. Here we discuss recent diffusion studies on phospholipids and membrane proteins in polymer-tethered phospholipid bilayers using wide-field single molecule fluorescence microscopy. These studies show that polymer-tethered lipids act as diffusion obstacles and obstruct the lateral diffusion of dye-labeled phospholipids and membrane proteins, thus providing unprecedented quantitative insight into the obstacle-induced obstructed diffusion in biomembranes. Interestingly, polymer-tethered lipids also obstruct the lipid diffusion in the opposite (obstacle-free) leaflet of the bilayer. Our tracking experiments suggest that the observed coupling of obstructed lipid diffusion is caused by the polymer-induced bending/protrusion of the bilayer around tethering points. At medium to high tethering concentrations, micron-size corrals can be formed within the bilayer, which are separated by polymeric diffusion fences. Single molecule tracking data obtained from quantum dot-conjugated phospholipids verify the occurrence of hop diffusion, which has been observed on cell surfaces. Our model membrane system is important because it provides, for the first time, a cell-free experimental platform for the study of hop diffusion where important experimental parameters (e.g., the fence thickness and density) can be controlled quite accurately.