NPXY, a sequence often found in cytoplasmic tails, is required for coated pit-mediated internalization of the low density lipoprotein receptor

Rapid internalization of the cell surface low density lipoprotein (LDL) receptor requires the first 22 amino acids of the cytoplasmic domain (residues 790-811), which must include an aromatic residue at position 807. In the human LDL receptor, this position is part of a tetrameric sequence, NPVY. A similar tetramer, NPXY (where X stands for any amino acid) is conserved in LDL receptors from six species (including Xenopus laevis) and in two members of the LDL receptor gene family, human LDL receptor-related protein and rat GP330. To determine whether the NPXY sequence is necessary for coated pit-mediated internalization, we used oligonucleotide-directed mutagenesis to substitute alanines for individual amino acids in the cytoplasmic tail of the human LDL receptor. Substitution of alanine for Asn⁸⁰⁴, Pro⁸⁰⁵, or Tyr⁸⁰⁷ (but not Val⁸⁰⁶) markedly reduced internalization. Only one other amino acid in the cytoplasmic 22-mer (Phe⁸⁰²) was important for internalization. A review of published data revealed NPXY sequences in cytoplasmic domains of at least 10 other cell surface proteins, including tyrosine kinase-linked receptors of the epidermal growth factor and insulin receptor family, the β-subunits of three integrin receptors, and the amyloid A4 precursor protein. This occurrence is much more frequent than would be expected by chance alone. The possibility of a conditional role for the NPXY sequence in ligand-independent internalization of these proteins is discussed.