title: Caenorhabditis elegans reveals a FxNPxY-independent low-density lipoprotein receptor internalization mechanism mediated by epsin1.
abstract: Low-density lipoprotein receptor (LDLR) internalization clears cholesterol-laden LDL particles from circulation in humans. Defects in clathrin-dependent LDLR endocytosis promote elevated serum cholesterol levels and can lead to atherosclerosis. However, our understanding of the mechanisms that control LDLR uptake remains incomplete. To identify factors critical to LDLR uptake, we pursued a genome-wide RNA interference screen using Caenorhabditis elegans LRP-1/megalin as a model for LDLR transport. In doing so, we discovered an unanticipated requirement for the clathrin-binding endocytic adaptor epsin1 in LDLR endocytosis. Epsin1 depletion reduced LDLR internalization rates in mammalian cells, similar to the reduction observed following clathrin depletion. Genetic and biochemical analyses of epsin in C. elegans and mammalian cells uncovered a requirement for the ubiquitin-interaction motif (UIM) as critical for receptor transport. As the epsin UIM promotes the internalization of some ubiquitinated receptors, we predicted LDLR ubiquitination as necessary for endocytosis. However, engineered ubiquitination-impaired LDLR mutants showed modest internalization defects that were further enhanced with epsin1 depletion, demonstrating epsin1-mediated LDLR endocytosis is independent of receptor ubiquitination. Finally, we provide evidence that epsin1-mediated LDLR uptake occurs independently of either of the two documented internalization motifs (FxNPxY or HIC) encoded within the LDLR cytoplasmic tail, indicating an additional internalization mechanism for LDLR.
authors.affiliation: Department of Genetics, Cell Biology, and Development and the Developmental Biology Center, University of Minnesota, Minneapolis, MN 55455, USA.
authors.forename: Yuan-Lin
authors.initials: YL
authors.lastname: Kang
authors.forename: John
authors.initials: J
authors.lastname: Yochem
authors.forename: Leslie
authors.initials: L
authors.lastname: Bell
authors.forename: Erika B
authors.initials: EB
authors.lastname: Sorensen
authors.forename: Lihsia
authors.initials: L
authors.lastname: Chen
authors.forename: Sean D
authors.initials: SD
authors.lastname: Conner
mesh_terms: D033942:Adaptor Proteins, Vesicular Transport / Q000235:genetics* / Q000378:metabolism; D020816:Amino Acid Motifs; D019943:Amino Acid Substitution; D000818:Animals; D017173:Caenorhabditis elegans / Q000235:genetics / Q000378:metabolism*; D029742:Caenorhabditis elegans Proteins / Q000235:genetics / Q000378:metabolism; D004705:Endocytosis; D055785:Gene Knockdown Techniques; D006367:HeLa Cells; D006801:Humans; D026503:Low Density Lipoprotein Receptor-Related Protein-1 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D055550:Protein Stability; D021381:Protein Transport; D034622:RNA Interference; D054875:Ubiquitination
chemical_list: D033942:Adaptor Proteins, Vesicular Transport; D029742:Caenorhabditis elegans Proteins; C510381:DAB-1 protein, C elegans; D026503:Low Density Lipoprotein Receptor-Related Protein-1; C114154:epsin