Quantifying the Diffusion of Membrane Proteins and Peptides in Black Lipid Membranes with 2-Focus Fluorescence Correlation Spectroscopy

Abstract

Protein diffusion in lipid membranes is a key aspect of many cellular signaling processes. To quantitatively describe protein diffusion in membranes, several competing theoretical models have been proposed. Among these, the Saffman-Delbrück model is the most famous. This model predicts a logarithmic dependence of a protein's diffusion coefficient on its inverse hydrodynamic radius (D ∝ ln 1/R) for small radius values. For large radius values, it converges toward a D ∝ 1/R scaling. Recently, however, experimental data indicate a Stokes-Einstein-like behavior (D ∝ 1/R) of membrane protein diffusion at small protein radii. In this study, we investigate protein diffusion in black lipid membranes using dual-focus fluorescence correlation spectroscopy. This technique yields highly accurate diffusion coefficients for lipid and protein diffusion in membranes. We find that despite its simplicity, the Saffman-Delbrück model is able to describe protein diffusion extremely well and a Stokes-Einstein-like behavior can be ruled out.

Strong influence of periodic boundary conditions on lateral diffusion in lipid bilayer membranes

“…Experimental measurements of L sd for lipid bilayer systems are typically in the 100 nm to micron range. 24,25,[38][39][40][41][42] In coarse-grained models, some details of lipid and solvent structure are lost and there is no reason to expect close correspondence to experimental numbers. For example, in the coarse-grained MARTINI force field, 5 η m has been determined to be 1.2 × 10 −8 P cm for DPPC bilayers, with water viscosity η f = 7 × 10 −3 P. 9 Within MARTINI simulations, we expect L sd ≈ 8.6 nm-much smaller than the experimental range.…”

“…29 Testing these system size effects in simulations may provide a validation of the Saffman-Delbrück assumptions, without having to deal with the complications arising from varying protein sizes, as done experimentally. 40,44,71 Even if SaffmanDelbruck predictions are not fully accurate in the description of experiment, as claimed by some, 71,72 this does not mean that hydrodynamic flows are irrelevant to the self-diffusion of proteins and/or lipids in the bilayer. For example, even within the context of non-continuum "free area" theories for lipid diffusion, hydrodynamic traction from the surrounding water may still influence diffusivities.…”

“…9 (b) Errors in diffusion coefficients appropriate for all-atom simulations, with viscosities set to match that of Ref. 40, η mono = 7.5 × 10 −8 P cm, η f = 0.0096 P (L sd = 78 nm), and interleaflet friction b = 10 7 P/cm (a representative physical value; see Ref. 34 and references within).…”

Biomembrane mimics and their roles in anti-bacterial drug discovery

“…The first model developed of this kind is a black lipid membrane (BLM) [25,26], where lipid bilayer is formed within an aperture on a thin layer of hydrophobic material such as Teflon. Black lipid membrane has been employed to study membrane properties (electrical potential [27], structure [28] and function [29]), lipid-protein interaction [30], peptides or protein diffusion across membrane [31]. Application of micro-black lipid membrane technology had led to development of high throughput drug permeability assay system PAMPA (Parallel Artificial Membrane Permeation Assay) [32].…”

Molecular dynamics study of rhodamine 6G diffusion atn-decane–water interfaces

“…Important biological examples include diffusion-limited processes of lipids [4,5] and membrane-embedded proteins [6] that play important roles in signal transduction at cell interfaces [7].…”