Weak dependence of mobility of membrane protein aggregates on aggregate size supports a viscous model of retardation of diffusion

Proteins in plasma membranes diffuse more slowly than proteins inserted int o artificial lipid bilayers, On a long-range scale (>250 nm), submembrane b arriers, or skeleton fences that hinder long-range diffusion and create con finement zones, have been described. Even within such confinement zones, ho wever, diffusion of proteins is much slower than predicted by the viscosity of the lipid. The cause of this slowing of diffusion on the micro scale ha s not been determined and is the focus of this paper. One way to approach t his question is to determine the dependence of particle motion on particle size. Some current models predict that the diffusion coefficient of a membr ane protein aggregate will depend strongly on its size, while others do not . We have measured the diffusion coefficients of membrane glycoprotein aggr egates linked together by concanavalin A molecules bound to beads of variou s sizes, and also the diffusion coefficients of individual concanavalin A b inding proteins. The measurements demonstrate at most a weak dependence of diffusion coefficient on aggregate size. This finding supports retardation by viscous effects, and is not consistent with models involving direct inte raction of diffusing proteins with cytoskeletal elements.