Lateral diffusion of membrane proteins

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 R-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 μm2 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-Delbru¨ck model. At higher protein-to lipid ratios (up to 3000 μm-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 ∼25,000 μm-2) and use of diffusion measurements for protein geometry (size, oligomerization) determinations are discussed.