Transferrin receptors promote the formation of clathrin lattices.

Gold conjugates have been used to quantitate human transferrin receptors (hTfnRs) on transfected chick embryo fibroblasts. No relationship could be found between the number of hTfnRs and the number of clathrin-coated pits. However, hTfnRs are also associated with flat clathrin lattices that lie outside invaginated pits. With increasing levels of receptor expression, the density of hTfnRs within flat lattices increases, and at the highest levels of expression the total area of flat lattice increases up to 3-fold. These results show that increased receptor numbers can promote clathrin lattice growth and suggest that the recruitment of receptors like hTfnRs is an essential step in lattice construction. We conclude that the process of invagination, which gives rise to coated pits, is regulated separately.