Lateral mobility of proteins in liquid membranes revisited

Abstract

The biological function of transmembrane proteins is closely related to their insertion, which has most often been studied through their lateral mobility. For >30 years, it has been thought that hardly any information on the size of the diffusing object can be extracted from such experiments. Indeed, the hydrodynamic model developed by Saffman and Delbrü ck predicts a weak, logarithmic dependence of the diffusion coefficient D with the radius R of the protein. Despite widespread use, its validity has never been thoroughly investigated. To check this model, we measured the diffusion coefficients of various peptides and transmembrane proteins, incorporated into giant unilamellar vesicles of 1-stearoyl-2-oleoylsn-glycero-3-phosphocholine (SOPC) or in model bilayers of tunable thickness. We show in this work that, for several integral proteins spanning a large range of sizes, the diffusion coefficient is strongly linked to the protein dimensions. A heuristic model results in a Stokes-like expression for D, (D ؔ 1͞R), which fits literature data as well as ours. Diffusion measurement is then a fast and fruitful method; it allows determining the oligomerization degree of proteins or studying lipid-protein and protein-protein interactions within bilayers.bilayers ͉ transmembrane proteins ͉ diffusion ͉ peptides ͉ sponge phase I n the hydrodynamic model of Saffman and Delbrück (1), transmembrane peptides and proteins are described as diffusing in a perfectly continuous medium, ignoring the finite size of the lipids. This model predicts that the diffusion coefficient D of a simple cylinder embedded in a thin sheet of fluid matching its height ( Fig. 1) is given byIn this expression, the adjustable parameters are by order of importance: the thickness h and viscosity m of the liquid membrane, the radius R of the diffusing cylinder, and the viscosity of the surrounding aqueous phase w . This result follows from solving the flow field in the membrane and in the surrounding fluid, assuming no-slip boundary conditions at the surface of the cylinder, which is considered as large compared with the bilayer components (i.e., R Ͼ h). Numerous biological studies, both in model systems (2-4) and living cells (5, 6), refer to this continuum approach (7). Because D depends only weakly on R, the characterization of protein or rafts radii is delicate (8); for example, increasing the radius from 10 to 100 Å changes the mobility by a mere 30% [for h ϭ 30 Å and m ϭ 10 poise (P; 1 P ϭ 0.1 Pa⅐s)].To check the applicability of the Saffman-Delbrück formula (Eq. 1), we have used fringe pattern photobleaching under the microscope (9) to measure precisely the self-diffusion of transmembrane peptides and proteins of well characterized dimensions. Results and DiscussionThe weight of the bilayer thickness, h, has never been investigated. Rather than using lipids of various lengths, we opted for a unique system where the bilayer thickness can be continuously tuned, leaving the bilayer viscosity constant.We use a phase of model bilayers made of nonionic ...

Dynamic label-free imaging of lipid nanodomains

“…The nature of the relation between object size and diffusion coefficient in a bilayer is important for predicting the motions of both lipid nanodomains and proteins (29,34). In this work, we have shown a failure of the hydrodynamic model of Saffmann and Delbrück, and importantly we have the temporal and spatial resolution to test the scaling of diffusion coefficient with object radius and thus distinguish between different theoretical models of these processes (Fig.…”

A note on confined diffusion

“…Notice however that σ d (s) can also be obtained by direct Laplace transform of the results (5), (10) and (24), though the method described in this article has to be extended in order to evaluate the corresponding series. The result is σ 1 (s) = 2D s 2 1 − 1 √ sτ tanh √ sτ (A.1)…”

“…In a typical experiment, trajectories of tracer particles are recorded and analysed in terms of position correlations or mean square displacement. From those datas, information on the size of the diffusing objects or on their interactions with the surrounding medium can then be extracted [10].…”

T cell adhesion mechanisms revealed by receptor lateral mobility

“…[34][35][36][37] For example, measurements of the two-dimensional affinity of receptors within a contact zone 38 not only show deviations from the threedimensional (solution-phase) interaction of the same receptors, 1,39,40 but also are consistent with conformational changes in the activated receptor leading to increased receptor affinity and increased interaction with cytoskeletal regulator proteins. 41 Receptor Clustering 18 From both models, we would predict that large receptor clusters would exhibit reduced lateral mobility. The formation of small clusters (e.g., dimers or trimers) might be difficult to discern in a live cell, however.…”