A tyrosine residue in the cytoplasmic domain of a class of cell surface receptors is necessary, but not sufficient, for internalization through coated pits. To identify the amino acid context enabling a tyrosine to serve as a signal for endocytosis, we mutated the short cytoplasmic domain of a mutant influenza virus hemagglutinin that is competent for internalization, HA-Y543, and determined the effect of each change on internalization. From these results and a comparison of sequences of other proteins recognized by coated pits, a "tyrosine internalization signal" was proposed. Site-directed mutagenesis was employed to insert complete, or incomplete "tyrosine internalization signals" into the cytoplasmic domain of a protein normally not endocytosed, human glycophorin A. Only the complete signal caused internalization of mutant glycophorins by coated pits. The signal is formed by a short amino acid sequence, with polar or basic residues preferred at certain positions on either side of the tyrosine. Amino acids, which in proteins of known structure are frequently found in turns, are clustered near the tyrosine on the side of the signal nearest the transmembrane domain.

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