The distribution of circulating receptors on COS 7 cells.

Rapidly circulating receptors on the surfaces of migrating or spreading cells are often concentrated toward the cells' leading edges. This polar distribution is the principal evidence that circulating membrane is returned to the surface of these cells at their fronts, where it is proposed to assist the forward extension of the cell. However, when low-density lipoprotein (LDL) receptors are transiently expressed in COS cells-cells which actively spread-their distribution is random, indicating that COS cells do not behave like other motile cells. This anomaly is examined here. I find that COS cell lines expressing either transferrin or LDL receptors do, in fact, usually have polar distributions of both receptors. The transient expression of LDL receptors interferes, in an unknown way, with the polar properties of COS cells. Furthermore, it is shown that whether receptors are located toward the cell's edge or are randomly distributed depends both on the ability of the receptor to circulate and on the state of the particular cell at that time.