Paper BI-ThM4
Anisotropic Diffusion in Nanopatterned Supported Lipid Bilayers

Thursday, October 18, 2007, 9:00 am, Room 609

Membrane-associated proteins have a central role in cell-cell adhesion and communication, mediated in part by the ability of membrane proteins to diffuse along the cell surface. Many of these proteins exhibit long-range (tens of micrometers) diffusion coefficients that are orders of magnitude smaller than that expected for membrane components. Moreover, other experiments suggest that over short (submicrometer) distances, the diffusive properties of these proteins more closely resemble that of membrane lipids. To capture this anomalous diffusion in a controllable, in vitro model, glass-supported lipid bilayers were patterned with nanoscale barriers of chromium and/or titanium, creating periodic barriers that mimic the spacing of cytoskeletal elements in cells (which underlie several models of anomalous diffusion). Specifically, these barriers consisted of 50-nm wide, parallel barriers spaced at 125 and 250 nm intervals. Gaps in these barriers, measuring 30-50 nm and spaced at 500 nm intervals, were introduced to allow a limited amount of long-range diffusion across the barriers. Long-range diffusion coefficients of Texas-Red-DHPE, in a background of vesicles of Egg PC, were measured using an image-based, fluorescence recovery after photobleach approach. The long-range diffusion coefficient of lipids parallel to the barriers was similar to that on non-patterned glass for both types of metals and all geometries. In contrast, long-range diffusion perpendicular to the barriers was decreased by as much as a factor of ten, dependent on the pattern geometry. Barrier spacing, rather than gap size, was the major determinant of long-range diffusion. Barrier material had an additional influence. On surface with chromium lines, photobleach recovery agreed with a model of diffusion along a perforated surface, suggesting that this material forms perfect barriers to lipid diffusion. On surfaces patterned with titanium, diffusion across the barriers was consistently higher than predicted by the model; furthermore, lipids exhibited limited diffusion across barriers with no gaps. These results suggest a more complex interaction between the supported lipid bilayer and the substrates. The nature of this interaction is currently under investigation. In summary, we describe a controllable, nanopatterned supported lipid bilayer model that captures the complex patterns of membrane protein diffusion, which have immediate use in the study of cell-cell communication.