Lateral mobility of proteins in liquid membranes revisited

doi:10.1073/pnas.0511026103

Lateral mobility of proteins in liquid membranes revisited

*Laboratoire de Physique Statistique de l’Ecole Normale Supérieure, Unité Mixte de Recherche 8550, Centre National de la Recherche Scientifique–Université Paris 6, 24 Rue Lhomond, 75005 Paris, France;
^‡Synthèse et Structure de Molécules d’Intérêt Pharmacologique, Unité Mixte de Recherche 8638, Centre National de la Recherche Scientifique–Université Paris 5, 4 Avenue de l’Observatoire, 75006 Paris, France;
^§Department of Chemical Physics, The Weizmann Institute of Science, Rehovot 76100, Israel; and
^¶Department of Biochemistry and Molecular Genetics, University of Colorado, Denver, CO 80045

Communicated by James E. Rothman, Columbia University, New York, NY, December 21, 2005 (received for review December 10, 2005)

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 Delbrück 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-oleoyl-sn-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 ∝ 1/R), 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.

Footnotes

^†To whom correspondence should be addressed. gambin{at}lps.ens.fr
Author contributions: W.U. designed research; Y.G., R.L.-E., and M.R. performed research; E.S., M.G., and R.S.H. contributed new reagents/analytic tools; Y.G., N.S.G., and W.U. analyzed data; and Y.G. and W.U. wrote the paper.
Conflict of interest statement: No conflicts declared.
↵ ∥ Considering the peptide L₁₈ (d _p = h = 28 Å, R = 5.5 Å), the viscosity μ_m of the SOPC bilayer at 20°C is calculated at μ_m=33 P. The apparent radius of BR was then obtained from: $\text{[math]}$ R _BR = 880 × 5.5 = 4,840 Å.
↵ ** Peptides are separated by 20 Å because of the structure of streptavidin. The dimer thus forms an anisotropic object of minor axis 5.5 Å and major axis 15.5 Å. From the area covered: 11 × (5.5 + 20 + 5.5) = 341 Å², and using R = $\text{[math]}$ , we estimated at 10.5 ± 1 Å the radius of the assembly created.
Abbreviations:

Abbreviations:

BR,

bacteriorhodopsin;

C12E5,

penta-monododecylether;

GUV,

giant unilamellar vesicle;

P,

poise;

SOPC,

1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine.