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 ...

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Serum albumin is the most abundant protein in the circulatory system. The ability of albumins to undergo a reversible conformational transition, observed with changes in pH, is conserved in distantly related species, suggesting for it a major physiological role possibly related to the transport of small molecules including drugs. We have followed changes of bovine serum albumin (BSA) in volume by densimetry and in adiabatic compressibility during its conformational transition from pH 7-2, using ultrasound measurements. In parallel, circular dichroism was measured. The volume and adiabatic compressibility decrease from pH 4 to 2. The change in ellipticity shows a decrease over the same pH range from 70% to 40% of its alpha-helix content. Sorbitol, at concentrations from 0 to 2 M, led to the progressive restoration of BSA volume and compressibility values, as well as a substantial recovery of its original alpha-helix content. This finding implies that the compressibility variation observed reflects the conformational changes during the transition. The mutual interactions of the mechanical properties and structural features of BSA reported here are important in biotechnology for research in material sciences and for the design and the development of new, tailor-made drug carriers.