Localization of cell membrane components by surface diffusion into a "trap".

Randomly distributed cell membrane components may become localized toward a specific region of the surface as a results of cell-cell contact or the cell's exposure to extracellular ligands. The mechanism for this localization process is unknown. In the present study, we investigated the plausibility of a passive mechanisms, namely that a local region of the cell surface serves as a "trap" for diffusing membrane proteins. Based on a model of spherical cell with a single circular trap on the surface, we derived the equation describing the surface density distribution and the average lifetime of the trappable molecules in the trap-free region of the membrane. This surface-trap theory was then used to analyze our experimental finding on the rapid localization of muscle surface soybean agglutinin receptors induced by cell-cell contact in culture. THe result indicates that the rate of localization of these receptors toward the cell-cell contact site can be accounted for by assuming that the receptors possess a diffusion coefficient of about 2.5 X10(-9) cm2/s (range: 1.2-9.3X10(-9) cm2/s) before they are trapped at the contact site. Independent measurement of the rate of lateral diffusion of these receptors yielded a lateral diffusion coefficient of about 1.9 X 10(-9) cm2/s (range 1.2-2.7 X10(-9) cm2/s), a value within the range of that predicted by the rate of localization. We thus conclude that lateral diffusion of mobile membrane components toward a local trap is a plausible mechanism for their localization induced by local surface modulation.