NANOTUBE · Lateral diffusion in artificial lipid nanotubes

Lateral diffusion in artificial lipid nanotubes

The aim of the project was to understand how the morphology of the phospholipid membrane, and in particular tubular geometry and high curvature, affects the diffusion and trapping of lipids and membrane proteins in synaptic spines of neuronal cells. The project represents the first effort to experimentally elucidate the role of membrane shape in regulation of lateral diffusion in neuronal structures by using a model membrane system. This work is a part of the larger multidisciplinary project jointly implemented by neurobiologists, theoretical physicists and biophysicists. Our theoretical understanding of lipid and protein lateral mobility can be traced to the seminal work of Saffman and Delbrück, who predicted a logarithmic dependence of the protein diffusion coefficient (i) on the inverse of the size of the protein and (ii) on the "membrane size" for membranes of finite size [Saffman P, Delbrück M (1975) Proc Natl Acad Sci USA 72:3111-3113]. Although the experimental proof of the first prediction is a matter of debate, the second has not previously been thought to be experimentally accessible. Here, we construct just such a geometrically confined membrane by forming lipid bilayer nanotubes of controlled radii connected to giant liposomes. We followed the diffusion of individual molecules in the tubular membrane using single particle tracking of QDs coupled to lipids or voltage-gated potassium (KvAP) channels while changing the membrane tube radius from approximately 250 to approximately 10 nm. We found that both lipid and protein diffusion was slower in tubular membranes with smaller radii. The protein diffusion coefficient decreased as much as 5-fold compared to diffusion on the effectively flat membrane of the giant liposomes. Both lipid and protein diffusion data are consistent with the predictions of a hydrodynamic theory that extends the work of Saffman and Delbrück to cylindrical geometries. This study therefore provides strong experimental support for the ubiquitous Saffman-Delbrück theory and elucidates the role of membrane geometry and size in regulating lateral diffusion. The results of this research were published in [Domanov YA et al. (2011) Proc Natl Acad Sci USA 108:12605-12610]. This project combined an original approach, powerful techniques, expertise of the host laboratory, of the fellow and dynamic scientific environment at the host Institute. This formed a basis for cutting-edge research to the benefit of both the fellow and the Community. Successful completion of this project promoted the fellow's transition towards permanent researcher position in an industrial laboratory and thus facilitated his professional integration in France.

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