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Fig. 1 Distribution of a circulating receptor on a locomoting cell. Shown in each part are an endocytic structure that contains a varying concentration of a recycling receptor (in brown) and the consequent concentration of this receptor on the cell surface. The expected distributions of: (A) a rapidly circulating receptor; (B) a more slowly circulating receptor; (C) a receptor that is neither concentrated nor depleted during endocytosis; (D) a receptor that is somewhat depleted from the endocytic cycle; (E) a receptor that is excluded from the endocytic cycle. The shapes of these gradients depend on the sites of endocytosis being randomly spread over the entire surface of the cell. In practice, the sites of endocytosis by clathrin-coated pits in tissue-culture cells are roughly randomly distributed, hence circulating receptors on moving cells are collected from all regions of the cell surface and transported to the cell front. The steepness of the gradient of a surface protein is expected to depend on its diffusion coefficient, the length of the cell and how long it resides on the cell surface compared with the time it takes for the cell to endocytose its entire surface. If the residence time is short, the surface protein will have a positive gradient (as in A); if it is very long, it will have a negative gradient (as in E). In either case, the steepness should be more accentuated in longer cells and by receptors that have lower diffusion coefficients.
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