Coated pits act as molecular filters.

Two proteins--theta and H63 antigens--are found to be excluded from coated pits on fibroblast plasma membranes. Coated pits thus act as molecular filters, pinching off, into the cell, lipid vesicles containing a limited number of specific receptors but excluding other plasma membrane proteins. This constitutes part of a lipid flow cycle that was proposed earlier to explain the capping of crosslinked surface antigens.