Effect of membrane flow on the capture of receptors by coated pits. Theoretical results.

Coated pits trap cell surface receptors and mediate their internalization. Once internalized, many receptors recycle back to the cell surface. When recycled receptors are inserted into the plasma membrane, they move until they are again trapped in coated pits. The mechanisms for moving receptors from their insertion sites to coated pits are unknown. Unaided diffusion as the transport mechanism is consistent with the observed kinetics of receptor recycling. Another candidate for the transport mechanism is convection. For receptors that recycle to random positions on the cell surface, or to restricted regions about coated pits, we assess the importance of convective flow in the transport of receptors to coated pits. First we consider local flows set up by the formation of coated pits and their transformation into coated vesicles. As coated pits form and round into coated vesicles, surrounding membrane is drawn inward, creating flows directed toward the coated pit centers. We show that unless the lifetime of a coated pit is very short, 10 s or less, such local flows have a negligible effect on the time it takes receptors to reach coated pits. We also show that they are unlikely to be the mechanism that keeps receptors that have reached coated pits trapped within coated pits until they are internalized. Finally we calculate the mean time tau for a diffusing receptor to reach a coated pit in the presence of membrane flow that is constant in magnitude and direction, as may occur on moving cells. We show that for typical membrane flow velocities, tau can be reduced significantly from its value in the absence of flow. For example, a velocity v = 2.8 micron/min cuts the mean transport time in half.