Lateral Diffusion of Membrane Proteins

Abstract

We measured the lateral mobility of integral membrane proteins reconstituted in giant unilamellar vesicles (GUVs), using fluorescence correlation spectroscopy. Receptor, channel, and transporter proteins with 1-36 transmembrane segments (lateral radii ranging from 0.5 to 4 nm) and a alpha-helical peptide (radius of 0.5 nm) were fluorescently labeled and incorporated into GUVs. At low protein-to-lipid ratios (i.e., 10-100 proteins per microm(2) of membrane surface), the diffusion coefficient D displayed a weak dependence on the hydrodynamic radius (R) of the proteins [D scaled with ln(1/R)], consistent with the Saffman-Delbruck model. At higher protein-to lipid ratios (up to 3000 microm(-2)), the lateral diffusion coefficient of the molecules decreased linearly with increasing the protein concentration in the membrane. The implications of our findings for protein mobility in biological membranes (protein crowding of approximately 25,000 microm(-2)) and use of diffusion measurements for protein geometry (size, oligomerization) determinations are discussed.

A dynamically assembled cell wall synthesis machinery buffers cell growth

“…Our observed diffusion constant of PBP2 is substantially lower than measurements for transmembrane proteins of similar size (2-5 μm 2 /s) (17). Given the small correlation between MreB and PBP2 localization, it was possible that PBP2 diffusion is affected by its transient interaction with MreB or indirectly through an MreB-associated component such as MreC (Fig.…”

“…2B and Fig. S8), approximately to the levels expected based on other similarly sized transmembrane proteins (17). Importantly, this increase was substantially more than would be predicted from the Stokes-Einstein relation (D ∼ 1/R) based solely on the decrease in mass of PBP2 due to truncation, and is likely an underestimate, as molecules moving at that speed cannot be well localized with single-molecule fluorescence.…”

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

“…United-atom simulations lead to results intermediate between experiment and MARTINI. 44, and much larger numbers have also been reported. 45 L sd = 78 nm serves as a conservative estimate for the purposes of this paper; larger values of L sd exacerbate the errors associated with PBC.)…”

“…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.…”

“…(c) Errors in diffusion coefficients appropriate for all-atom simulations, with viscosities set to match that of Ref. 44, η mono = 1.63 × 10 −7 P cm, η f = 0.01 P (L sd = 163 nm), and interleaflet friction b = 10 7 P/cm. The object radius assumed is R = 0.5 nm in all plots, corresponding to a small peptide or lipid.…”