title: The complex of the insect LDL receptor homolog, lipophorin receptor, LpR, and its lipoprotein ligand does not dissociate under endosomal conditions.
abstract: The insect low-density lipoprotein (LDL) receptor (LDLR) homolog, lipophorin receptor (LpR), mediates endocytic uptake of the single insect lipoprotein, high-density lipophorin (HDLp), which is structurally related to LDL. However, in contrast to the fate of LDL, which is endocytosed by LDLR, we previously demonstrated that after endocytosis, HDLp is sorted to the endocytic recycling compartment and recycled for re-secretion in a transferrin-like manner. This means that the integrity of the complex between HDLp and LpR is retained under endosomal conditions. Therefore, in this study, the ligand-binding and ligand-dissociation capacities of LpR were investigated by employing a new flow cytometric assay, using LDLR as a control. At pH 5.4, the LpR-HDLp complex remained stable, whereas that of LDLR and LDL dissociated. Hybrid HDLp-binding receptors, containing either the beta-propeller or both the beta-propeller and the hinge region of LDLR, appeared to be unable to release ligand at endosomal pH, revealing that the stability of the complex is imparted by the ligand-binding domain of LpR. The LpR-HDLp complex additionally appeared to be EDTA-resistant, excluding a low Ca(2+) concentration in the endosome as an alternative trigger for complex dissociation. From binding of HDLp to the above hybrid receptors, it was inferred that the stability upon EDTA treatment is confined to LDLR type A (LA) ligand-binding repeats 1-7. Additional (competition) binding experiments indicated that the binding site of LpR for HDLp most likely involves LA-2-7. It is therefore proposed that the remarkable stability of the LpR-HDLp complex is attributable to this binding site. Together, these data indicate that LpR and HDLp travel in complex to the endocytic recycling compartment, which constitutes a key determinant for ligand recycling by LpR.
authors.affiliation: Division of Endocrinology and Metabolism, Department of Biology, Institute of Biomembranes, Utrecht University, Utrecht, The Netherlands.
authors.forename: Sigrid D
authors.initials: SD
authors.lastname: Roosendaal
authors.forename: Jana
authors.initials: J
authors.lastname: Kerver
authors.forename: Maria
authors.initials: M
authors.lastname: Schipper
authors.forename: Kees W
authors.initials: KW
authors.lastname: Rodenburg
authors.forename: Dick J
authors.initials: DJ
authors.lastname: Van der Horst
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D004492:Edetic Acid / Q000494:pharmacology; D004705:Endocytosis; D011992:Endosomes / Q000187:drug effects / Q000378:metabolism*; D004815:Epidermal Growth Factor / Q000378:metabolism; D005434:Flow Cytometry; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D008074:Lipoproteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D011485:Protein Binding; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics / Q000378:metabolism*; D011973:Receptors, LDL / Q000737:chemistry / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D012680:Sensitivity and Specificity; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: D008024:Ligands; D008074:Lipoproteins; D018160:Receptors, Cytoplasmic and Nuclear; D011973:Receptors, LDL; C432945:lipophorin receptor; D004815:Epidermal Growth Factor; D004492:Edetic Acid