Surface distribution and recycling of the low density lipoprotein receptor as visualized with antireceptor antibodies.

For more than 45 years biologists have known that cells internalize extracellular fluid and its contents by fluid-phase endocytosis, an inefficient process for the uptake of molecules that are present at low concentrations . More recently, animal cells have been found to express surface receptors that bind, and thereby concentrate, certain molecules so that they can be taken up in large amounts even when they are present at low concentrations outside the cell . Ligands that are internalized in this way include plasma transport proteins, hormones, and other macromolecules that fulfill nutritional and regulatory roles in cells . In many cases this process, called receptormediated endocytOsis, takes place in specialized regions of the plasma membrane called coated pits (reviewed in references 1 and 2) .