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Differential Functions of Members of the Low Density Lipoprotein Receptor Family Suggested by Their Distinct Endocytosis Rates

2001; Elsevier BV; Volume: 276; Issue: 21 Linguagem: Inglês

10.1074/jbc.m101589200

ISSN

1083-351X

Autores

Yonghe Li, Wenyan Lü, María‐Paz Marzolo, Guojun Bu,

Tópico(s)

Retinal Diseases and Treatments

Resumo

The low density lipoprotein receptor (LDLR) family is composed of a class of cell surface endocytic receptors that recognize extracellular ligands and internalize them for degradation by lysosomes. In addition to LDLR, mammalian members of this family include the LDLR-related protein (LRP), the very low density lipoprotein receptor (VLDLR), the apolipoprotein E receptor-2 (apoER2), and megalin. Herein we have analyzed the endocytic functions of the cytoplasmic tails of these receptors using LRP minireceptors, its chimeric receptor constructs, and full-length VLDLR and apoER2 stably expressed in LRP-null Chinese hamster ovary cells. We find that the initial endocytosis rates mediated by different cytoplasmic tails are significantly different, with half-times of ligand internalization ranging from less than 30 s to more than 8 min. The tail of LRP mediates the highest rate of endocytosis, whereas those of the VLDLR and apoER2 exhibit least endocytosis function. Compared with the tail of LRP, the tails of the LDLR and megalin display significantly lower levels of endocytosis rates. Ligand degradation analyses strongly support differential endocytosis rates initiated by these receptors. Interestingly apoER2, which has recently been shown to mediate intracellular signal transduction, exhibited the lowest level of ligand degradation efficiency. These results thus suggest that the endocytic functions of members of the LDLR family are distinct and that certain receptors in this family may play their main roles in areas other than receptor-mediated endocytosis. The low density lipoprotein receptor (LDLR) family is composed of a class of cell surface endocytic receptors that recognize extracellular ligands and internalize them for degradation by lysosomes. In addition to LDLR, mammalian members of this family include the LDLR-related protein (LRP), the very low density lipoprotein receptor (VLDLR), the apolipoprotein E receptor-2 (apoER2), and megalin. Herein we have analyzed the endocytic functions of the cytoplasmic tails of these receptors using LRP minireceptors, its chimeric receptor constructs, and full-length VLDLR and apoER2 stably expressed in LRP-null Chinese hamster ovary cells. We find that the initial endocytosis rates mediated by different cytoplasmic tails are significantly different, with half-times of ligand internalization ranging from less than 30 s to more than 8 min. The tail of LRP mediates the highest rate of endocytosis, whereas those of the VLDLR and apoER2 exhibit least endocytosis function. Compared with the tail of LRP, the tails of the LDLR and megalin display significantly lower levels of endocytosis rates. Ligand degradation analyses strongly support differential endocytosis rates initiated by these receptors. Interestingly apoER2, which has recently been shown to mediate intracellular signal transduction, exhibited the lowest level of ligand degradation efficiency. These results thus suggest that the endocytic functions of members of the LDLR family are distinct and that certain receptors in this family may play their main roles in areas other than receptor-mediated endocytosis. low density lipoprotein LDL receptor α2-macroglobulin apolipoprotein E receptor-2 Chinese hamster ovary LDL receptor-related protein receptor-associated protein very low density lipoprotein VLDL receptor polymerase chain reaction hemagglutinin polyacrylamide gel electrophoresis endoplasmic reticulum. The low density lipoprotein receptor (LDLR)1 family includes five members in mammals: the LDLR itself, the apolipoprotein E receptor 2 (apoER2), the very low density lipoprotein receptor (VLDLR), the LDLR-related protein (LRP), and megalin (1Krieger M. Herz J. Annu. Rev. Biochem. 1994; 63: 601-637Crossref PubMed Scopus (1057) Google Scholar, 2Strickland D.K. Kounnas M.Z. Argraves W.S. FASEB. J. 1995; 9: 890-898Crossref PubMed Scopus (248) Google Scholar, 3Hussain M.M. Strickland D.K. Bakillah A. Annu. Rev. Nutr. 1999; 19: 141-172Crossref PubMed Scopus (311) Google Scholar). LDLR, VLDLR, and apoER2 have molecular masses of ∼130 kDa, whereas LRP and megalin are significantly larger with molecular masses of ∼600 kDa. There are several structural modules that are present in each member of the LDLR family. These modules include 1) ligand-binding repeats of ∼40 amino acids that include six cysteine residues forming three disulfide bonds; 2) epidermal growth factor precursor repeats, which also contain six cysteines residues each; and 3) modules of ∼50 amino acids with a consensus tetrapeptide, Tyr-Trp-Thr-Asp (YWTD). In addition to these extracellular modules, each of these receptors also contains a single transmembrane domain and a relatively short cytoplasmic tail with potential endocytosis signals (1Krieger M. Herz J. Annu. Rev. Biochem. 1994; 63: 601-637Crossref PubMed Scopus (1057) Google Scholar, 2Strickland D.K. Kounnas M.Z. Argraves W.S. FASEB. J. 1995; 9: 890-898Crossref PubMed Scopus (248) Google Scholar, 3Hussain M.M. Strickland D.K. Bakillah A. Annu. Rev. Nutr. 1999; 19: 141-172Crossref PubMed Scopus (311) Google Scholar, 4Chen W.J. Goldstein J.L. Brown M.S. J. Biol. Chem. 1990; 265: 3116-3123Abstract Full Text PDF PubMed Google Scholar, 5Li Y. Marzolo M.P. van Kerkhof P. Strous G.J. Bu G. J. Biol. Chem. 2000; 275: 17187-17194Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar). A cluster of several complement-type ligand binding repeats constitutes a ligand-binding domain, and differential clustering of these repeats within a domain may impart specificity with respect to ligand recognition (3Hussain M.M. Strickland D.K. Bakillah A. Annu. Rev. Nutr. 1999; 19: 141-172Crossref PubMed Scopus (311) Google Scholar). The epidermal growth factor precursor homology domains and YWTD repeats are necessary for the dissociation of ligands from the receptor in endosomes (6Davis C.G. Goldstein J.L. Sudhof T.C. Anderson R.G. Russell D.W. Brown M.S. Nature. 1987; 326: 760-765Crossref PubMed Scopus (304) Google Scholar, 7Mikhailenko I. Considine W. Argraves K.M. Loukinov D. Hyman B.T. Strickland D.K. J. Cell Sci. 1999; 112: 3269-3281Crossref PubMed Google Scholar). Ligand interactions with all members of the LDLR family can be antagonized by a receptor-associated protein (RAP), a unique ligand frequently used as a tool in the study of ligand-receptor interaction (8Bu G. Curr. Opin. Lipidol. 1998; 9: 149-155Crossref PubMed Scopus (66) Google Scholar). RAP also functions intracellularly as a molecular chaperone to facilitate receptor folding and trafficking within the early secretory pathway (8Bu G. Curr. Opin. Lipidol. 1998; 9: 149-155Crossref PubMed Scopus (66) Google Scholar).Traditionally, all members of the LDLR family have been regarded as cell surface endocytosis receptors that function in delivering their ligands to lysosomes for degradation (1Krieger M. Herz J. Annu. Rev. Biochem. 1994; 63: 601-637Crossref PubMed Scopus (1057) Google Scholar, 2Strickland D.K. Kounnas M.Z. Argraves W.S. FASEB. J. 1995; 9: 890-898Crossref PubMed Scopus (248) Google Scholar, 3Hussain M.M. Strickland D.K. Bakillah A. Annu. Rev. Nutr. 1999; 19: 141-172Crossref PubMed Scopus (311) Google Scholar). However, recent studies have revealed new roles for these receptors in signal transduction (9Herz J. Gotthardt M. Willnow T.E. Curr. Opin. Lipidol. 2000; 11: 161-166Crossref PubMed Scopus (65) Google Scholar). A set of cytoplasmic adaptor and scaffold proteins containing protein interaction domain or PSD-95/DLG/Z0-1 domains have been shown to bind to the cytoplasmic tails of members of the LDLR family (10Trommsdorff M. Borg J.P. Margolis B. Herz J. J. Biol. Chem. 1998; 273: 33556-33560Abstract Full Text Full Text PDF PubMed Scopus (487) Google Scholar, 11Trommsdorff M. Gotthardt M. Hiesberger T. Shelton J. Stockinger W. Nimpf J. Hammer R.E. Richardson J.A. Herz J. Cell. 1999; 97: 689-701Abstract Full Text Full Text PDF PubMed Scopus (1077) Google Scholar, 12Howell B.W. Lanier L.M. Frank R. Gertler F.B. Cooper J.A Mol. Cell. Biol. 1999; 19: 5179-5188Crossref PubMed Scopus (332) Google Scholar, 13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar, 14Stockinger W Brandes C. Fasching D. Hermann M. Gotthardt M. Herz J. Schneider W.J. Nimpf J. J. Biol. Chem. 2000; 275: 25625-25632Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar, 15Oleinikov A.V. Zhao J. Makker S.P. Biochem. J. 2000; 347: 613-621Crossref PubMed Scopus (131) Google Scholar). These new findings suggest that members of the LDLR family may participate in several signal transduction pathways including the regulation of mitogen-activated protein kinases, cell adhesion, vesicle trafficking, neurotransmission, and neuronal migration (9Herz J. Gotthardt M. Willnow T.E. Curr. Opin. Lipidol. 2000; 11: 161-166Crossref PubMed Scopus (65) Google Scholar). Cellular signaling through this class of receptors may be regulated by receptor endocytosis (13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar). For example, binding of the adaptor protein Dab1 (Disabled-1) to the cytoplasmic domain of the LDLR impedes its interaction with the endocytic machinery (13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar).The relatively short cytoplasmic tails of LDLR, VLDLR, apoER2, LRP, and megalin contain 50, 54, 115, 100, and 209 amino acid residues, respectively (16Yamamoto T. Davis C.G. Brown M.S. Schneider W.J. Casey M.L. Goldstein J.L. Russell D.W. Cell. 1984; 39: 27-38Abstract Full Text PDF PubMed Scopus (970) Google Scholar, 17Herz J. Hamann U. Rogne S. Myklebost O. Gausepohl H. K. K., Stanley H.H. EMBO J. 1988; 7: 4119-4127Crossref PubMed Scopus (736) Google Scholar, 18Sakai J. Hoshino A. Takahashi S. Miura Y. Ishii H. Suzuki H. Kawarabayasi Y. Yamamoto T. J. Biol. Chem. 1994; 269: 2173-2182Abstract Full Text PDF PubMed Google Scholar, 19Kim D.H. Iijima H. Goto K. Sakai J. Ishii H. Kim H.J. Suzuki H. Kondo H. Saeki S. Yamamoto T. J. Biol. Chem. 1996; 271: 8373-8380Abstract Full Text Full Text PDF PubMed Scopus (348) Google Scholar, 20Hjalm G. Murray E. Crumley G. Harazim W. Lundgren S. Onyango I. Ek B. Larsson M. Juhlin C. Hellman P. Davis H. Akerstrom G. Rask L. Morse B. Eur. J. Biochem. 1996; 239: 132-137Crossref PubMed Scopus (135) Google Scholar). A common characteristic of the LDLR family members is that at least one copy of the NPXY sequence is found within their cytoplasmic tails. For LDLR, this NPXY motif serves as a signal for receptor endocytosis through coated pits (4Chen W.J. Goldstein J.L. Brown M.S. J. Biol. Chem. 1990; 265: 3116-3123Abstract Full Text PDF PubMed Google Scholar). However, we recently reported that the YXXL motif, but not the two NPXY sequences, within the cytoplasmic tail of LRP serves as the dominant signal for receptor-mediated endocytosis (5Li Y. Marzolo M.P. van Kerkhof P. Strous G.J. Bu G. J. Biol. Chem. 2000; 275: 17187-17194Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar). We also demonstrated that the distal dileucine motif and a serine phosphorylation within the LRP tail contribute to receptor endocytosis (5Li Y. Marzolo M.P. van Kerkhof P. Strous G.J. Bu G. J. Biol. Chem. 2000; 275: 17187-17194Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar, 21Li Y. van Kerkhof P. Marzolo M.P. Strous G.J. Bu G. Mol. Cell Biol. 2001; 21: 1185-1195Crossref PubMed Scopus (65) Google Scholar). These results suggest that each member of the LDLR family may utilize different potential signal(s) within their cytoplasmic tails for receptor-mediated endocytosis.In the present study, we have directly compared the endocytic functions of the members of the LDLR family using LRP minireceptors, its chimeric receptor constructs, and full-length VLDLR and apoER2 stably expressed in LRP-null Chinese hamster ovary (CHO) cells. We find that the initial endocytosis rates mediated by the cytoplasmic tails of the LDLR family members differ significantly, suggesting that members of the LDLR family may play their main roles in either receptor-mediated endocytosis or signal transduction.DISCUSSIONAll members of LDLR family are recognized as cell surface endocytisis receptors. However, the endocytosis rates mediated by these receptors are unclear. In the present study, we provide direct evidence that there are distinct differences in endocytic functions among the tails of the LDLR family members. We found that the tail of LRP supports the most efficient endocytosis, whereas LDLR tail and megalin tail display reduced endocytosis rates compared with LRP tail, while VLDLR and apoER2 exhibit minimal endocytosis function.It is not surprising that among all members of the LDLR family examined, the LRP tail supports receptor endocytosis to the greatest degree. LRP belongs to the class of receptors that undergo constitutive endocytosis in the presence or absence of ligand. This feature may be determined by the constant exposure of its endocytosis signals and is highlighted by its cell surface distribution concentrated within clathrin-coated pits (5Li Y. Marzolo M.P. van Kerkhof P. Strous G.J. Bu G. J. Biol. Chem. 2000; 275: 17187-17194Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar, 38Bu G. Maksymovitch E.A. Geuze H. Schwartz A.L. J. Biol. Chem. 1994; 269: 29874-29882Abstract Full Text PDF PubMed Google Scholar). The tail of LRP consists of 100 amino acid residues and contains multiple potential endocytosis signals including two NPXY, one YXXL, and two dileucine motifs. Our recent studies indicate that the YXXL motif within the cytoplasmic tail of LRP serves as the dominant signal for LRP endocytosis. Furthermore, the distal dileucine motif and a serine phosphorylation in its cytoplasmic tail also contributes to the endocytosis of LRP (21Li Y. van Kerkhof P. Marzolo M.P. Strous G.J. Bu G. Mol. Cell Biol. 2001; 21: 1185-1195Crossref PubMed Scopus (65) Google Scholar). At present, it has been reported that LRP can bind and internalize over 20 structurally and functionally distinct ligands. In this report, we have demonstrated that LRP possesses extremely fast endocytosis rate, which is consistent with its major function as a clearance receptor.In the present study, we found that the endocytosis rates mediated by mLRP4-LDLR, mLRP4-megalin, VLDLR, and apER2 are significantly lower than that of mLRP4T100. One common characteristic of the LDLR family members is that each contains at least one copy of the NPXY sequence within the cytoplasmic tail. The tails of LDLR, VLDLR, and apoER2 contains one copy of the NPXY sequence, whereas the megalin tail contains two copies of the NPXY sequence. The NPXY motif in the LDLR has been shown to serve as a signal for receptor endocytosis through coated pits (4Chen W.J. Goldstein J.L. Brown M.S. J. Biol. Chem. 1990; 265: 3116-3123Abstract Full Text PDF PubMed Google Scholar). Thus, it is possible that the NPXY motifs within the tails of VLDLR, apoER2, and megalin also function as their dominant endocytosis signals.In contrast to other family members, we found that the tails of VLDLR and apoER2 support limited endocytosis. Recently, it has been demonstrated that VLDLR and apoER2 serve as obligate components in Reelin/Dab1-mediated neuronal migration (10Trommsdorff M. Borg J.P. Margolis B. Herz J. J. Biol. Chem. 1998; 273: 33556-33560Abstract Full Text Full Text PDF PubMed Scopus (487) Google Scholar, 11Trommsdorff M. Gotthardt M. Hiesberger T. Shelton J. Stockinger W. Nimpf J. Hammer R.E. Richardson J.A. Herz J. Cell. 1999; 97: 689-701Abstract Full Text Full Text PDF PubMed Scopus (1077) Google Scholar, 39D'Arcangelo G. Homayouni R. Keshvara L. Rice D.S. Sheldon M. Curran T. Neuron. 1999; 24: 471-479Abstract Full Text Full Text PDF PubMed Scopus (682) Google Scholar, 40Hiesberger T. Trommsdorff M. Howell B.W. Goffinet A. Mumby M.C. Cooper J.A. Herz J. Neuron. 1999; 24: 481-489Abstract Full Text Full Text PDF PubMed Scopus (781) Google Scholar). Mice that lack the genes for both VLDLR and apoER2 demonstrate a neurological and neuroanatomical phenotype that is indistinguishable from that seen in animals deficient in either Reelin or Dab1 (11Trommsdorff M. Gotthardt M. Hiesberger T. Shelton J. Stockinger W. Nimpf J. Hammer R.E. Richardson J.A. Herz J. Cell. 1999; 97: 689-701Abstract Full Text Full Text PDF PubMed Scopus (1077) Google Scholar). Thus, signal transduction is likely the main function of VLDLR and apoER2.It is worthy of note that compared with other members of the LDLR family, apoER2 exhibits extremely low level of ligand degradation efficiency. This observation is in agreement with previous report by Sun and Soutar (37Sun X.M. Soutar A.K. Eur. J. Biochem. 1999; 262: 230-239Crossref PubMed Scopus (36) Google Scholar), who found that β-VLDL was poorly degraded by apoER2 or its spliced variants compared with LDLR. The mechanistic basis thereof is not presently known. apoER2 shows high homology to both the LDLR and VLDLR, including the positions of the exon/intron boundaries of the genes (41Kim D.H. Magoori K. Inoue T.R. Mao C.C. Kim H.J. Suzuki H. Fujita T. Endo Y. Saeki S. Yamamoto T.T. J. Biol. Chem. 1997; 272: 8498-8504Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar). The most notable difference between the tail of apoER2 and that of LDLR or VLDLR is the presence of an additional 59 amino acid residues encoded by an additional exon (19Kim D.H. Iijima H. Goto K. Sakai J. Ishii H. Kim H.J. Suzuki H. Kondo H. Saeki S. Yamamoto T. J. Biol. Chem. 1996; 271: 8373-8380Abstract Full Text Full Text PDF PubMed Scopus (348) Google Scholar). This domain has been recently reported to interact with two members of the JUK-interacting protein family, JIP-1 and JIP-2 (JUK-interacting protein1 and 2). These molecules belong to a group of mitogen-activated protein kinase scaffolding proteins (13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar, 14Stockinger W Brandes C. Fasching D. Hermann M. Gotthardt M. Herz J. Schneider W.J. Nimpf J. J. Biol. Chem. 2000; 275: 25625-25632Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar). The interaction with JIPs is specific for apoER2, because neither LDLR nor VLDLR bind (14Stockinger W Brandes C. Fasching D. Hermann M. Gotthardt M. Herz J. Schneider W.J. Nimpf J. J. Biol. Chem. 2000; 275: 25625-25632Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar). Taken together, these observations suggest apoER2 is a unique cell surface receptor within the LDLR family with its main function likely in mediating signal transduction.Recent studies have suggested new roles for LDLR family members as transducers of extracellular signals (9Herz J. Gotthardt M. Willnow T.E. Curr. Opin. Lipidol. 2000; 11: 161-166Crossref PubMed Scopus (65) Google Scholar). For example, several cytoplasmic adaptor and scaffold proteins containing protein interacting domain or PSD-95/DLG/Z0-1 domains, including Dab1, Dab2, FE65, JIP-1, JIP-2, PSD-95, CAPON, and SEMCAP-1, bind to the cytoplasmic tails of members of the LDLR family (10Trommsdorff M. Borg J.P. Margolis B. Herz J. J. Biol. Chem. 1998; 273: 33556-33560Abstract Full Text Full Text PDF PubMed Scopus (487) Google Scholar, 11Trommsdorff M. Gotthardt M. Hiesberger T. Shelton J. Stockinger W. Nimpf J. Hammer R.E. Richardson J.A. Herz J. Cell. 1999; 97: 689-701Abstract Full Text Full Text PDF PubMed Scopus (1077) Google Scholar, 12Howell B.W. Lanier L.M. Frank R. Gertler F.B. Cooper J.A Mol. Cell. Biol. 1999; 19: 5179-5188Crossref PubMed Scopus (332) Google Scholar, 13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar, 14Stockinger W Brandes C. Fasching D. Hermann M. Gotthardt M. Herz J. Schneider W.J. Nimpf J. J. Biol. Chem. 2000; 275: 25625-25632Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar, 15Oleinikov A.V. Zhao J. Makker S.P. Biochem. J. 2000; 347: 613-621Crossref PubMed Scopus (131) Google Scholar). Thus, the present studies extend our understanding of the differences in the biological activities of members of the LDLR family. Rapid receptor internalization mediated by receptors such as LRP may result in rapid desensitization of signals initiated by receptor ligation, whereas slower endocytosis mediated by the VLDLR and apoER2 may allow for more sustained signal transduction upon ligand binding. The low density lipoprotein receptor (LDLR)1 family includes five members in mammals: the LDLR itself, the apolipoprotein E receptor 2 (apoER2), the very low density lipoprotein receptor (VLDLR), the LDLR-related protein (LRP), and megalin (1Krieger M. Herz J. Annu. Rev. Biochem. 1994; 63: 601-637Crossref PubMed Scopus (1057) Google Scholar, 2Strickland D.K. Kounnas M.Z. Argraves W.S. FASEB. J. 1995; 9: 890-898Crossref PubMed Scopus (248) Google Scholar, 3Hussain M.M. Strickland D.K. Bakillah A. Annu. Rev. Nutr. 1999; 19: 141-172Crossref PubMed Scopus (311) Google Scholar). LDLR, VLDLR, and apoER2 have molecular masses of ∼130 kDa, whereas LRP and megalin are significantly larger with molecular masses of ∼600 kDa. There are several structural modules that are present in each member of the LDLR family. These modules include 1) ligand-binding repeats of ∼40 amino acids that include six cysteine residues forming three disulfide bonds; 2) epidermal growth factor precursor repeats, which also contain six cysteines residues each; and 3) modules of ∼50 amino acids with a consensus tetrapeptide, Tyr-Trp-Thr-Asp (YWTD). In addition to these extracellular modules, each of these receptors also contains a single transmembrane domain and a relatively short cytoplasmic tail with potential endocytosis signals (1Krieger M. Herz J. Annu. Rev. Biochem. 1994; 63: 601-637Crossref PubMed Scopus (1057) Google Scholar, 2Strickland D.K. Kounnas M.Z. Argraves W.S. FASEB. J. 1995; 9: 890-898Crossref PubMed Scopus (248) Google Scholar, 3Hussain M.M. Strickland D.K. Bakillah A. Annu. Rev. Nutr. 1999; 19: 141-172Crossref PubMed Scopus (311) Google Scholar, 4Chen W.J. Goldstein J.L. Brown M.S. J. Biol. Chem. 1990; 265: 3116-3123Abstract Full Text PDF PubMed Google Scholar, 5Li Y. Marzolo M.P. van Kerkhof P. Strous G.J. Bu G. J. Biol. Chem. 2000; 275: 17187-17194Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar). A cluster of several complement-type ligand binding repeats constitutes a ligand-binding domain, and differential clustering of these repeats within a domain may impart specificity with respect to ligand recognition (3Hussain M.M. Strickland D.K. Bakillah A. Annu. Rev. Nutr. 1999; 19: 141-172Crossref PubMed Scopus (311) Google Scholar). The epidermal growth factor precursor homology domains and YWTD repeats are necessary for the dissociation of ligands from the receptor in endosomes (6Davis C.G. Goldstein J.L. Sudhof T.C. Anderson R.G. Russell D.W. Brown M.S. Nature. 1987; 326: 760-765Crossref PubMed Scopus (304) Google Scholar, 7Mikhailenko I. Considine W. Argraves K.M. Loukinov D. Hyman B.T. Strickland D.K. J. Cell Sci. 1999; 112: 3269-3281Crossref PubMed Google Scholar). Ligand interactions with all members of the LDLR family can be antagonized by a receptor-associated protein (RAP), a unique ligand frequently used as a tool in the study of ligand-receptor interaction (8Bu G. Curr. Opin. Lipidol. 1998; 9: 149-155Crossref PubMed Scopus (66) Google Scholar). RAP also functions intracellularly as a molecular chaperone to facilitate receptor folding and trafficking within the early secretory pathway (8Bu G. Curr. Opin. Lipidol. 1998; 9: 149-155Crossref PubMed Scopus (66) Google Scholar). Traditionally, all members of the LDLR family have been regarded as cell surface endocytosis receptors that function in delivering their ligands to lysosomes for degradation (1Krieger M. Herz J. Annu. Rev. Biochem. 1994; 63: 601-637Crossref PubMed Scopus (1057) Google Scholar, 2Strickland D.K. Kounnas M.Z. Argraves W.S. FASEB. J. 1995; 9: 890-898Crossref PubMed Scopus (248) Google Scholar, 3Hussain M.M. Strickland D.K. Bakillah A. Annu. Rev. Nutr. 1999; 19: 141-172Crossref PubMed Scopus (311) Google Scholar). However, recent studies have revealed new roles for these receptors in signal transduction (9Herz J. Gotthardt M. Willnow T.E. Curr. Opin. Lipidol. 2000; 11: 161-166Crossref PubMed Scopus (65) Google Scholar). A set of cytoplasmic adaptor and scaffold proteins containing protein interaction domain or PSD-95/DLG/Z0-1 domains have been shown to bind to the cytoplasmic tails of members of the LDLR family (10Trommsdorff M. Borg J.P. Margolis B. Herz J. J. Biol. Chem. 1998; 273: 33556-33560Abstract Full Text Full Text PDF PubMed Scopus (487) Google Scholar, 11Trommsdorff M. Gotthardt M. Hiesberger T. Shelton J. Stockinger W. Nimpf J. Hammer R.E. Richardson J.A. Herz J. Cell. 1999; 97: 689-701Abstract Full Text Full Text PDF PubMed Scopus (1077) Google Scholar, 12Howell B.W. Lanier L.M. Frank R. Gertler F.B. Cooper J.A Mol. Cell. Biol. 1999; 19: 5179-5188Crossref PubMed Scopus (332) Google Scholar, 13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar, 14Stockinger W Brandes C. Fasching D. Hermann M. Gotthardt M. Herz J. Schneider W.J. Nimpf J. J. Biol. Chem. 2000; 275: 25625-25632Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar, 15Oleinikov A.V. Zhao J. Makker S.P. Biochem. J. 2000; 347: 613-621Crossref PubMed Scopus (131) Google Scholar). These new findings suggest that members of the LDLR family may participate in several signal transduction pathways including the regulation of mitogen-activated protein kinases, cell adhesion, vesicle trafficking, neurotransmission, and neuronal migration (9Herz J. Gotthardt M. Willnow T.E. Curr. Opin. Lipidol. 2000; 11: 161-166Crossref PubMed Scopus (65) Google Scholar). Cellular signaling through this class of receptors may be regulated by receptor endocytosis (13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar). For example, binding of the adaptor protein Dab1 (Disabled-1) to the cytoplasmic domain of the LDLR impedes its interaction with the endocytic machinery (13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar). The relatively short cytoplasmic tails of LDLR, VLDLR, apoER2, LRP, and megalin contain 50, 54, 115, 100, and 209 amino acid residues, respectively (16Yamamoto T. Davis C.G. Brown M.S. Schneider W.J. Casey M.L. Goldstein J.L. Russell D.W. Cell. 1984; 39: 27-38Abstract Full Text PDF PubMed Scopus (970) Google Scholar, 17Herz J. Hamann U. Rogne S. Myklebost O. Gausepohl H. K. K., Stanley H.H. EMBO J. 1988; 7: 4119-4127Crossref PubMed Scopus (736) Google Scholar, 18Sakai J. Hoshino A. Takahashi S. Miura Y. Ishii H. Suzuki H. Kawarabayasi Y. Yamamoto T. J. Biol. Chem. 1994; 269: 2173-2182Abstract Full Text PDF PubMed Google Scholar, 19Kim D.H. Iijima H. Goto K. Sakai J. Ishii H. Kim H.J. Suzuki H. Kondo H. Saeki S. Yamamoto T. J. Biol. Chem. 1996; 271: 8373-8380Abstract Full Text Full Text PDF PubMed Scopus (348) Google Scholar, 20Hjalm G. Murray E. Crumley G. Harazim W. Lundgren S. Onyango I. Ek B. Larsson M. Juhlin C. Hellman P. Davis H. Akerstrom G. Rask L. Morse B. Eur. J. Biochem. 1996; 239: 132-137Crossref PubMed Scopus (135) Google Scholar). A common characteristic of the LDLR family members is that at least one copy of the NPXY sequence is found within their cytoplasmic tails. For LDLR, this NPXY motif serves as a signal for receptor endocytosis through coated pits (4Chen W.J. Goldstein J.L. Brown M.S. J. Biol. Chem. 1990; 265: 3116-3123Abstract Full Text PDF PubMed Google Scholar). However, we recently reported that the YXXL motif, but not the two NPXY sequences, within the cytoplasmic tail of LRP serves as the dominant signal for receptor-mediated endocytosis (5Li Y. Marzolo M.P. van Kerkhof P. Strous G.J. Bu G. J. Biol. Chem. 2000; 275: 17187-17194Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar). We also demonstrated that the distal dileucine motif and a serine phosphorylation within the LRP tail contribute to receptor endocytosis (5Li Y. Marzolo M.P. van Kerkhof P. Strous G.J. Bu G. J. Biol. Chem. 2000; 275: 17187-17194Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar, 21Li Y. van Kerkhof P. Marzolo M.P. Strous G.J. Bu G. Mol. Cell Biol. 2001; 21: 1185-1195Crossref PubMed Scopus (65) Google Scholar). These results suggest that each member of the LDLR family may utilize different potential signal(s) within their cytoplasmic tails for receptor-mediated endocytosis. In the present study, we have directly compared the endocytic functions of the members of the LDLR family using LRP minireceptors, its chimeric receptor constructs, and full-length VLDLR and apoER2 stably expressed in LRP-null Chinese hamster ovary (CHO) cells. We find that the initial endocytosis rates mediated by the cytoplasmic tails of the LDLR family members differ significantly, suggesting that members of the LDLR family may play their main roles in either receptor-mediated endocytosis or signal transduction. DISCUSSIONAll members of LDLR family are recognized as cell surface endocytisis receptors. However, the endocytosis rates mediated by these receptors are unclear. In the present study, we provide direct evidence that there are distinct differences in endocytic functions among the tails of the LDLR family members. We found that the tail of LRP supports the most efficient endocytosis, whereas LDLR tail and megalin tail display reduced endocytosis rates compared with LRP tail, while VLDLR and apoER2 exhibit minimal endocytosis function.It is not surprising that among all members of the LDLR family examined, the LRP tail supports receptor endocytosis to the greatest degree. LRP belongs to the class of receptors that undergo constitutive endocytosis in the presence or absence of ligand. This feature may be determined by the constant exposure of its endocytosis signals and is highlighted by its cell surface distribution concentrated within clathrin-coated pits (5Li Y. Marzolo M.P. van Kerkhof P. Strous G.J. Bu G. J. Biol. Chem. 2000; 275: 17187-17194Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar, 38Bu G. Maksymovitch E.A. Geuze H. Schwartz A.L. J. Biol. Chem. 1994; 269: 29874-29882Abstract Full Text PDF PubMed Google Scholar). The tail of LRP consists of 100 amino acid residues and contains multiple potential endocytosis signals including two NPXY, one YXXL, and two dileucine motifs. Our recent studies indicate that the YXXL motif within the cytoplasmic tail of LRP serves as the dominant signal for LRP endocytosis. Furthermore, the distal dileucine motif and a serine phosphorylation in its cytoplasmic tail also contributes to the endocytosis of LRP (21Li Y. van Kerkhof P. Marzolo M.P. Strous G.J. Bu G. Mol. Cell Biol. 2001; 21: 1185-1195Crossref PubMed Scopus (65) Google Scholar). At present, it has been reported that LRP can bind and internalize over 20 structurally and functionally distinct ligands. In this report, we have demonstrated that LRP possesses extremely fast endocytosis rate, which is consistent with its major function as a clearance receptor.In the present study, we found that the endocytosis rates mediated by mLRP4-LDLR, mLRP4-megalin, VLDLR, and apER2 are significantly lower than that of mLRP4T100. One common characteristic of the LDLR family members is that each contains at least one copy of the NPXY sequence within the cytoplasmic tail. The tails of LDLR, VLDLR, and apoER2 contains one copy of the NPXY sequence, whereas the megalin tail contains two copies of the NPXY sequence. The NPXY motif in the LDLR has been shown to serve as a signal for receptor endocytosis through coated pits (4Chen W.J. Goldstein J.L. Brown M.S. J. Biol. Chem. 1990; 265: 3116-3123Abstract Full Text PDF PubMed Google Scholar). Thus, it is possible that the NPXY motifs within the tails of VLDLR, apoER2, and megalin also function as their dominant endocytosis signals.In contrast to other family members, we found that the tails of VLDLR and apoER2 support limited endocytosis. Recently, it has been demonstrated that VLDLR and apoER2 serve as obligate components in Reelin/Dab1-mediated neuronal migration (10Trommsdorff M. Borg J.P. Margolis B. Herz J. J. Biol. Chem. 1998; 273: 33556-33560Abstract Full Text Full Text PDF PubMed Scopus (487) Google Scholar, 11Trommsdorff M. Gotthardt M. Hiesberger T. Shelton J. Stockinger W. Nimpf J. Hammer R.E. Richardson J.A. Herz J. Cell. 1999; 97: 689-701Abstract Full Text Full Text PDF PubMed Scopus (1077) Google Scholar, 39D'Arcangelo G. Homayouni R. Keshvara L. Rice D.S. Sheldon M. Curran T. Neuron. 1999; 24: 471-479Abstract Full Text Full Text PDF PubMed Scopus (682) Google Scholar, 40Hiesberger T. Trommsdorff M. Howell B.W. Goffinet A. Mumby M.C. Cooper J.A. Herz J. Neuron. 1999; 24: 481-489Abstract Full Text Full Text PDF PubMed Scopus (781) Google Scholar). Mice that lack the genes for both VLDLR and apoER2 demonstrate a neurological and neuroanatomical phenotype that is indistinguishable from that seen in animals deficient in either Reelin or Dab1 (11Trommsdorff M. Gotthardt M. Hiesberger T. Shelton J. Stockinger W. Nimpf J. Hammer R.E. Richardson J.A. Herz J. Cell. 1999; 97: 689-701Abstract Full Text Full Text PDF PubMed Scopus (1077) Google Scholar). Thus, signal transduction is likely the main function of VLDLR and apoER2.It is worthy of note that compared with other members of the LDLR family, apoER2 exhibits extremely low level of ligand degradation efficiency. This observation is in agreement with previous report by Sun and Soutar (37Sun X.M. Soutar A.K. Eur. J. Biochem. 1999; 262: 230-239Crossref PubMed Scopus (36) Google Scholar), who found that β-VLDL was poorly degraded by apoER2 or its spliced variants compared with LDLR. The mechanistic basis thereof is not presently known. apoER2 shows high homology to both the LDLR and VLDLR, including the positions of the exon/intron boundaries of the genes (41Kim D.H. Magoori K. Inoue T.R. Mao C.C. Kim H.J. Suzuki H. Fujita T. Endo Y. Saeki S. Yamamoto T.T. J. Biol. Chem. 1997; 272: 8498-8504Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar). The most notable difference between the tail of apoER2 and that of LDLR or VLDLR is the presence of an additional 59 amino acid residues encoded by an additional exon (19Kim D.H. Iijima H. Goto K. Sakai J. Ishii H. Kim H.J. Suzuki H. Kondo H. Saeki S. Yamamoto T. J. Biol. Chem. 1996; 271: 8373-8380Abstract Full Text Full Text PDF PubMed Scopus (348) Google Scholar). This domain has been recently reported to interact with two members of the JUK-interacting protein family, JIP-1 and JIP-2 (JUK-interacting protein1 and 2). These molecules belong to a group of mitogen-activated protein kinase scaffolding proteins (13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar, 14Stockinger W Brandes C. Fasching D. Hermann M. Gotthardt M. Herz J. Schneider W.J. Nimpf J. J. Biol. Chem. 2000; 275: 25625-25632Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar). The interaction with JIPs is specific for apoER2, because neither LDLR nor VLDLR bind (14Stockinger W Brandes C. Fasching D. Hermann M. Gotthardt M. Herz J. Schneider W.J. Nimpf J. J. Biol. Chem. 2000; 275: 25625-25632Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar). Taken together, these observations suggest apoER2 is a unique cell surface receptor within the LDLR family with its main function likely in mediating signal transduction.Recent studies have suggested new roles for LDLR family members as transducers of extracellular signals (9Herz J. Gotthardt M. Willnow T.E. Curr. Opin. Lipidol. 2000; 11: 161-166Crossref PubMed Scopus (65) Google Scholar). For example, several cytoplasmic adaptor and scaffold proteins containing protein interacting domain or PSD-95/DLG/Z0-1 domains, including Dab1, Dab2, FE65, JIP-1, JIP-2, PSD-95, CAPON, and SEMCAP-1, bind to the cytoplasmic tails of members of the LDLR family (10Trommsdorff M. Borg J.P. Margolis B. Herz J. J. Biol. Chem. 1998; 273: 33556-33560Abstract Full Text Full Text PDF PubMed Scopus (487) Google Scholar, 11Trommsdorff M. Gotthardt M. Hiesberger T. Shelton J. Stockinger W. Nimpf J. Hammer R.E. Richardson J.A. Herz J. Cell. 1999; 97: 689-701Abstract Full Text Full Text PDF PubMed Scopus (1077) Google Scholar, 12Howell B.W. Lanier L.M. Frank R. Gertler F.B. Cooper J.A Mol. Cell. Biol. 1999; 19: 5179-5188Crossref PubMed Scopus (332) Google Scholar, 13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar, 14Stockinger W Brandes C. Fasching D. Hermann M. Gotthardt M. Herz J. Schneider W.J. Nimpf J. J. Biol. Chem. 2000; 275: 25625-25632Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar, 15Oleinikov A.V. Zhao J. Makker S.P. Biochem. J. 2000; 347: 613-621Crossref PubMed Scopus (131) Google Scholar). Thus, the present studies extend our understanding of the differences in the biological activities of members of the LDLR family. Rapid receptor internalization mediated by receptors such as LRP may result in rapid desensitization of signals initiated by receptor ligation, whereas slower endocytosis mediated by the VLDLR and apoER2 may allow for more sustained signal transduction upon ligand binding. All members of LDLR family are recognized as cell surface endocytisis receptors. However, the endocytosis rates mediated by these receptors are unclear. In the present study, we provide direct evidence that there are distinct differences in endocytic functions among the tails of the LDLR family members. We found that the tail of LRP supports the most efficient endocytosis, whereas LDLR tail and megalin tail display reduced endocytosis rates compared with LRP tail, while VLDLR and apoER2 exhibit minimal endocytosis function. It is not surprising that among all members of the LDLR family examined, the LRP tail supports receptor endocytosis to the greatest degree. LRP belongs to the class of receptors that undergo constitutive endocytosis in the presence or absence of ligand. This feature may be determined by the constant exposure of its endocytosis signals and is highlighted by its cell surface distribution concentrated within clathrin-coated pits (5Li Y. Marzolo M.P. van Kerkhof P. Strous G.J. Bu G. J. Biol. Chem. 2000; 275: 17187-17194Abstract Full Text Full Text PDF PubMed Scopus (188) Google Scholar, 38Bu G. Maksymovitch E.A. Geuze H. Schwartz A.L. J. Biol. Chem. 1994; 269: 29874-29882Abstract Full Text PDF PubMed Google Scholar). The tail of LRP consists of 100 amino acid residues and contains multiple potential endocytosis signals including two NPXY, one YXXL, and two dileucine motifs. Our recent studies indicate that the YXXL motif within the cytoplasmic tail of LRP serves as the dominant signal for LRP endocytosis. Furthermore, the distal dileucine motif and a serine phosphorylation in its cytoplasmic tail also contributes to the endocytosis of LRP (21Li Y. van Kerkhof P. Marzolo M.P. Strous G.J. Bu G. Mol. Cell Biol. 2001; 21: 1185-1195Crossref PubMed Scopus (65) Google Scholar). At present, it has been reported that LRP can bind and internalize over 20 structurally and functionally distinct ligands. In this report, we have demonstrated that LRP possesses extremely fast endocytosis rate, which is consistent with its major function as a clearance receptor. In the present study, we found that the endocytosis rates mediated by mLRP4-LDLR, mLRP4-megalin, VLDLR, and apER2 are significantly lower than that of mLRP4T100. One common characteristic of the LDLR family members is that each contains at least one copy of the NPXY sequence within the cytoplasmic tail. The tails of LDLR, VLDLR, and apoER2 contains one copy of the NPXY sequence, whereas the megalin tail contains two copies of the NPXY sequence. The NPXY motif in the LDLR has been shown to serve as a signal for receptor endocytosis through coated pits (4Chen W.J. Goldstein J.L. Brown M.S. J. Biol. Chem. 1990; 265: 3116-3123Abstract Full Text PDF PubMed Google Scholar). Thus, it is possible that the NPXY motifs within the tails of VLDLR, apoER2, and megalin also function as their dominant endocytosis signals. In contrast to other family members, we found that the tails of VLDLR and apoER2 support limited endocytosis. Recently, it has been demonstrated that VLDLR and apoER2 serve as obligate components in Reelin/Dab1-mediated neuronal migration (10Trommsdorff M. Borg J.P. Margolis B. Herz J. J. Biol. Chem. 1998; 273: 33556-33560Abstract Full Text Full Text PDF PubMed Scopus (487) Google Scholar, 11Trommsdorff M. Gotthardt M. Hiesberger T. Shelton J. Stockinger W. Nimpf J. Hammer R.E. Richardson J.A. Herz J. Cell. 1999; 97: 689-701Abstract Full Text Full Text PDF PubMed Scopus (1077) Google Scholar, 39D'Arcangelo G. Homayouni R. Keshvara L. Rice D.S. Sheldon M. Curran T. Neuron. 1999; 24: 471-479Abstract Full Text Full Text PDF PubMed Scopus (682) Google Scholar, 40Hiesberger T. Trommsdorff M. Howell B.W. Goffinet A. Mumby M.C. Cooper J.A. Herz J. Neuron. 1999; 24: 481-489Abstract Full Text Full Text PDF PubMed Scopus (781) Google Scholar). Mice that lack the genes for both VLDLR and apoER2 demonstrate a neurological and neuroanatomical phenotype that is indistinguishable from that seen in animals deficient in either Reelin or Dab1 (11Trommsdorff M. Gotthardt M. Hiesberger T. Shelton J. Stockinger W. Nimpf J. Hammer R.E. Richardson J.A. Herz J. Cell. 1999; 97: 689-701Abstract Full Text Full Text PDF PubMed Scopus (1077) Google Scholar). Thus, signal transduction is likely the main function of VLDLR and apoER2. It is worthy of note that compared with other members of the LDLR family, apoER2 exhibits extremely low level of ligand degradation efficiency. This observation is in agreement with previous report by Sun and Soutar (37Sun X.M. Soutar A.K. Eur. J. Biochem. 1999; 262: 230-239Crossref PubMed Scopus (36) Google Scholar), who found that β-VLDL was poorly degraded by apoER2 or its spliced variants compared with LDLR. The mechanistic basis thereof is not presently known. apoER2 shows high homology to both the LDLR and VLDLR, including the positions of the exon/intron boundaries of the genes (41Kim D.H. Magoori K. Inoue T.R. Mao C.C. Kim H.J. Suzuki H. Fujita T. Endo Y. Saeki S. Yamamoto T.T. J. Biol. Chem. 1997; 272: 8498-8504Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar). The most notable difference between the tail of apoER2 and that of LDLR or VLDLR is the presence of an additional 59 amino acid residues encoded by an additional exon (19Kim D.H. Iijima H. Goto K. Sakai J. Ishii H. Kim H.J. Suzuki H. Kondo H. Saeki S. Yamamoto T. J. Biol. Chem. 1996; 271: 8373-8380Abstract Full Text Full Text PDF PubMed Scopus (348) Google Scholar). This domain has been recently reported to interact with two members of the JUK-interacting protein family, JIP-1 and JIP-2 (JUK-interacting protein1 and 2). These molecules belong to a group of mitogen-activated protein kinase scaffolding proteins (13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar, 14Stockinger W Brandes C. Fasching D. Hermann M. Gotthardt M. Herz J. Schneider W.J. Nimpf J. J. Biol. Chem. 2000; 275: 25625-25632Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar). The interaction with JIPs is specific for apoER2, because neither LDLR nor VLDLR bind (14Stockinger W Brandes C. Fasching D. Hermann M. Gotthardt M. Herz J. Schneider W.J. Nimpf J. J. Biol. Chem. 2000; 275: 25625-25632Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar). Taken together, these observations suggest apoER2 is a unique cell surface receptor within the LDLR family with its main function likely in mediating signal transduction. Recent studies have suggested new roles for LDLR family members as transducers of extracellular signals (9Herz J. Gotthardt M. Willnow T.E. Curr. Opin. Lipidol. 2000; 11: 161-166Crossref PubMed Scopus (65) Google Scholar). For example, several cytoplasmic adaptor and scaffold proteins containing protein interacting domain or PSD-95/DLG/Z0-1 domains, including Dab1, Dab2, FE65, JIP-1, JIP-2, PSD-95, CAPON, and SEMCAP-1, bind to the cytoplasmic tails of members of the LDLR family (10Trommsdorff M. Borg J.P. Margolis B. Herz J. J. Biol. Chem. 1998; 273: 33556-33560Abstract Full Text Full Text PDF PubMed Scopus (487) Google Scholar, 11Trommsdorff M. Gotthardt M. Hiesberger T. Shelton J. Stockinger W. Nimpf J. Hammer R.E. Richardson J.A. Herz J. Cell. 1999; 97: 689-701Abstract Full Text Full Text PDF PubMed Scopus (1077) Google Scholar, 12Howell B.W. Lanier L.M. Frank R. Gertler F.B. Cooper J.A Mol. Cell. Biol. 1999; 19: 5179-5188Crossref PubMed Scopus (332) Google Scholar, 13Gotthardt M. Trommsdorff M. Nevitt M.F. Shelton J. Richardson J.A. Stockinger W. Nimpf J. Herz J. J. Biol. Chem. 2000; 275: 25616-25624Abstract Full Text Full Text PDF PubMed Scopus (393) Google Scholar, 14Stockinger W Brandes C. Fasching D. Hermann M. Gotthardt M. Herz J. Schneider W.J. Nimpf J. J. Biol. Chem. 2000; 275: 25625-25632Abstract Full Text Full Text PDF PubMed Scopus (214) Google Scholar, 15Oleinikov A.V. Zhao J. Makker S.P. Biochem. J. 2000; 347: 613-621Crossref PubMed Scopus (131) Google Scholar). Thus, the present studies extend our understanding of the differences in the biological activities of members of the LDLR family. Rapid receptor internalization mediated by receptors such as LRP may result in rapid desensitization of signals initiated by receptor ligation, whereas slower endocytosis mediated by the VLDLR and apoER2 may allow for more sustained signal transduction upon ligand binding. We are grateful to Alan Schwartz for critical readings and suggestions on the manuscript. We also thank Tokuo Yamamoto (Tohoku University Gene Research Center) for providing the cDNAs of VLDLR and apoER2, James S. Owen (University College London, London) and Johannes Nimpf (University of Vienna, Austria) for providing the anti-apoER2 antibodies, and David FitzGerald (NIH) for providing the LRP-null CHO cell line.

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