The objective of this work is to identify the elements of the human transferrin receptor that are involved in receptor internalization, intracellular sorting, and recycling. We have found that an aromatic side chain at position 20 on the cytoplasmic portion of the human transferrin receptor is required for efficient internalization. The wild-type human transferrin receptor has a tyrosine at this position. Replacement of the Tyr-20 with an aromatic amino acid does not alter the rate constant of internalization, whereas substitution with the nonaromatic amino acids serine, leucine, or cysteine reduces the internalization rate constant approximately three-fold. These results are consistent with similar studies of other receptor systems that have also documented the requirement for a tyrosine in rapid internalization. The amino terminus of the transferrin receptor is cytoplasmic, with the tyrosine 41 amino acids from the membrane. These two features distinguish the transferrin receptor from the other membrane proteins for which the role of tyrosine in internalization has been examined, because these proteins have the opposite polarity with respect to the membrane and because the tyrosines are located closer to the membrane (within 25 amino acids). The externalization rate for the recycling of the transferrin receptor is not altered by any of these substitutions, demonstrating that the aromatic amino acid internalization signal is not required for the efficient exocytosis of internalized receptor.
A soluble form of the human transferrin receptor is released by activated lymphocytes in vitro
