A Rugged Energy Landscape Mechanism for Trapping of Transmembrane Receptors during Endocytosis

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Abstract
Efficient clathrin-mediated endocytosis of transmembrane receptors requires that clathrin-coated pits retain the receptors long enough to allow vesicle formation and internalization. In many cases, however, the receptors can exhibit mean lifetimes in coated pits much shorter than the lifetime of the pit at the plasma membrane. A rugged energy landscape for binding, which produces a broad distribution of residence times, ensures a significant probability of times much greater than the mean and would allow efficient trapping of proteins in these cases. We used fluorescence correlation spectroscopy and total internal reflection microscopy to measure the kinetics of movement of a C5a receptor−yellow fluorescent protein fusion in living cells. These experiments demonstrate that clusters of trapped receptors exhibit fluctuations in fluorescence intensity that vary in time scale over 2 orders of magnitude. Most of the variation in intensity is likely due to the motion of the receptors in the plane of the plasma membrane, although it is not possible to rule out a small contribution from motion orthogonal to the plane of the membrane. The broad time scale distribution of the intensity fluctuations is consistent with a rugged energy landscape mechanism for trapping of the receptors. This mechanism, which allows efficient trapping to coexist with rapid exchange, may also be relevant to other biological processes involving binding in heterogeneous chemical environments.