AN INTERNALIZATION MOTIF IS CREATED IN THE CYTOPLASMIC DOMAIN OF THE TRANSFERRIN RECEPTOR BY SUBSTITUTION OF A TYROSINE AT THE FIRST POSITION OF A PREDICTED TIGHT TURN

Receptors are internalized from the plasma membrane at similar to 10 t imes the rate of bulk membrane. The pre dominant model for the motif t hat promotes rapid internalization proposes a requirement for a tyrosi ne located in the first position of a tight turn. In this report we sh ow that an internalization motif can be created de novo by substitutin g a tyrosine for the first or last residues of a tetrapeptide GDNS (re sidues 31-34) that is predicted to form a tight turn within the cytopl asmic domain of the human transferrin receptor. These substitutions re store wild-type levels of internalization to transferrin receptors tha t are poorly internalized due to missense mutations in the native inte rnalization motif. The introduction of a tyrosine at the first or last position of the GDNS tetrapeptide in a transferrin receptor containin g an unmodified wild-type internalization motif significantly increase s the internalization rate above that of the wild-type receptor. Our r esults indicate that a functional novel internalization motif can be c reated by placing specific aromatic amino acids within the overall str ucture of an existing beta-turn in a cytoplasmic domain of a receptor.