Transferrin receptor and its recycling in HeLa cells.

The transferrin receptor is a 180 000-dalton protein which can be dissociated to two 90 000-dalton polypeptides under reducing conditions. It can be labelled by lactoperoxidase-catalysed iodination on the cell surface at 0 degree C. Trypsin digestion of labelled cells at 0 degree C can be used to degrade those receptors on the cell surface; they release a 70 000-dalton soluble fragment which binds to transferrin. When cells are labelled at 0 degree C, then warmed to 37 degrees C, the labelled receptors enter the cells and become trypsin resistant. These receptors enter the cells, probably via coated pits, with a half-life of approximately 5 min. Since there is about three times as much receptor inside cells as on the surface, this means that transit through the cell to the cell surface takes approximately 21 min, if all receptors are on the same cycling pathway.