title: Proteasome regulates the delivery of LDL receptor-related protein into the degradation pathway.
abstract: The low-density lipoprotein receptor (LDLR)-related protein (LRP) is a multiligand endocytic receptor that has broad cellular and physiological functions. Previous studies have shown that both tyrosine-based and di-leucine motifs within the LRP cytoplasmic tail are responsible for mediating its rapid endocytosis. Little is known, however, about the mechanism by which LRP is targeted for degradation. By examining both endogenous full-length and a minireceptor form of LRP, we found that proteasomal inhibitors, MG132 and lactacystin, prolong the cellular half-life of LRP. The presence of proteasomal inhibitors also significantly increased the level of LRP at the cell surface, suggesting that the delivery of LRP to the degradation pathway was blocked at a compartment from which recycling of the receptor to the cell surface still occurred. Immunoelectron microscopy analyses demonstrated a proteasomal inhibitor-dependent reduction in LRP minireceptor within both limiting membrane and internal vesicles of the multivesicular bodies, which are compartments that lead to receptor degradation. In contrast to the growth hormone receptor, we found that the initial endocytosis of LRP minireceptor does not require a functional ubiquitin-proteasome system. Finally, using truncated cytoplasmic mutants of LRP minireceptors, we found that a region of 19 amino acids within the LRP tail is required for proteasomal regulation. Taken together our results provide strong evidence that the cellular turnover of a cargo receptor, i.e., LRP, is regulated by the proteasomal system, suggesting a broader function of the proteasome in regulating the trafficking of receptors into the degradation pathway.
authors.affiliation: Department of Pediatrics, Washington University School of Medicine, CB 8208, St. Louis Children's Hospital, Missouri 63110, USA.
authors.forename: Lora
authors.initials: L
authors.lastname: Melman
authors.forename: Hans J
authors.initials: HJ
authors.lastname: Geuze
authors.forename: Yonghe
authors.initials: Y
authors.lastname: Li
authors.forename: Lynn M
authors.initials: LM
authors.lastname: McCormick
authors.forename: Peter
authors.initials: P
authors.lastname: Van Kerkhof
authors.forename: Ger J
authors.initials: GJ
authors.lastname: Strous
authors.forename: Alan L
authors.initials: AL
authors.lastname: Schwartz
authors.forename: Guojun
authors.initials: G
authors.lastname: Bu
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D015153:Blotting, Western; D016466:CHO Cells; D006224:Cricetinae; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D004705:Endocytosis; D005434:Flow Cytometry; D006801:Humans; D007700:Kinetics; D007976:Leupeptins / Q000494:pharmacology; D008024:Ligands; D008077:Lipoproteins, LDL / Q000378:metabolism; D026503:Low Density Lipoprotein Receptor-Related Protein-1 / Q000737:chemistry / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D016253:Microscopy, Immunoelectron; D009097:Multienzyme Complexes / Q000378:metabolism*; D011233:Precipitin Tests; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D021381:Protein Transport; D011994:Recombinant Proteins / Q000378:metabolism; D013997:Time Factors; D014407:Tumor Cells, Cultured
chemical_list: D015853:Cysteine Proteinase Inhibitors; D007976:Leupeptins; D008024:Ligands; D008077:Lipoproteins, LDL; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D009097:Multienzyme Complexes; D011994:Recombinant Proteins; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde