THE JOURNAL OF BIOLOGICAL CHEMISTRY
© 2000 by The American Society for Biochemistry and Molecular Biology, Inc.
Vol. 275, No. 22, Issue of June 2, pp. 17187–17194, 2000
Printed in U.S.A.
The YXXL Motif, but Not the Two NPXY Motifs, Serves as the
Dominant Endocytosis Signal for Low Density Lipoprotein
Receptor-related Protein*
Received for publication, January 19, 2000, and in revised form, March 21, 2000
Published, JBC Papers in Press, March 28, 2000, DOI 10.1074/jbc.M000490200
Yonghe Li‡, Maria Paz Marzolo§, Peter van Kerkhof¶, Ger J. Strous¶, and Guojun Bu‡i
From the ‡Departments of Pediatrics, and Cell Biology and Physiology, Washington University School of Medicine and St.
Louis Children’s Hospital, St. Louis, Missouri 63110, the §Department of Biology, University of Chile, Santiago, Chile,
and the ¶Department of Cell Biology, Utrecht University, Utrecht, The Netherlands
All members of the low density lipoprotein (LDL) receptor family contain at least one copy of the NPXY
sequence within their cytoplasmic tails. For the LDL
receptor, it has been demonstrated that the NPXY motif
serves as a signal for rapid endocytosis through coated
pits. Thus, it is generally believed that the NPXY sequences function as endocytosis signals for all the LDL
receptor family members. The primary aim of this study
is to define the endocytosis signal(s) within the cytoplasmic tail of LDL receptor-related protein (LRP). By using
LRP minireceptors, which mimic the function and trafficking of full-length endogenous LRP, we demonstrate
that the YXXL motif, but not the two NPXY motifs,
serves as the dominant signal for LRP endocytosis. We
also found that the distal di-leucine motif within the
LRP tail contributes to its endocytosis, and its function
is independent of the YXXL motif. Although the proximal NPXY motif and the proximal di-leucine motif each
play a limited role in LRP endocytosis in the context of
the full-length tail, these motifs were functional within
the truncated receptor tail. In addition, we show that
LRP minireceptor mutants defective in endocytosis signal(s) accumulate at the cell surface and are less efficient in delivery of ligand for degradation.
The low density lipoprotein (LDL)1 receptor-related protein
(LRP) is a member of the LDL receptor (LDLR) gene family,
which consists of at least six known cell surface receptors:
LDLR itself, LRP, the very low density lipoprotein receptor
(VLDLR), megalin/LRP-2, apolipoprotein E receptor-2
(apoER2)/LR8B, and LR11 (1– 4). LRP is synthesized as a 600kDa single-chain precursor, which undergoes post-translational proteolytic processing within the trans-Golgi compart-
* This work was supported by National Institutes of Health Grants
NS37525 and HL59150 (to G. B.) and Fondo Nacional de Ciencia y
Technologia Grant 1990600 and Departmento de Investigacion y Desarrollo Grant I007-98/2 (to M. P. M.) The costs of publication of this
article were defrayed in part by the payment of page charges. This
article must therefore be hereby marked “advertisement” in accordance
with 18 U.S.C. Section 1734 solely to indicate this fact.
i To whom correspondence should be addressed: Dept. of Pediatrics,
Washington University School of Medicine, Box 8116, One Children’s
Pl., St. Louis, MO 63110. Tel.: 314-454-2726; Fax: 314-454-2685; Email: bu@kids.wustl.edu.
1
The abbreviations used are: LDL, low density lipoprotein; CHO,
Chinese hamster ovary; LDLR, low density lipoprotein receptor; LRP,
low density lipoprotein receptor-related protein; RAP, receptor-associated protein; apoE, apolipoprotein E; apoER2, apolipoprotein E receptor-2; VLDLR, very low density lipoprotein receptor; scuPA, single
chain urokinase; PAGE, polyacrylamide gel electrophoresis; HA, hemagglutinin; BSA, bovine serum albumin; ER, endoplasmic reticulum.
This paper is available on line at http://www.jbc.org
This is an Open Access article under the CC BY license.
ment by the endopeptidase furin (5, 6). This post-translational
processing results in the formation of mature LRP as a noncovalently associated heterodimer, consisting of the extracellular
515-kDa chain and the transmembrane 85-kDa chain (5). The
515-kDa subunit contains all of the putative ligand-binding
domains consisting of 31 copies of the complement-type ligandbinding repeats arranged in four clusters of 2, 8, 10, and 11 (1,
2). The 85-kDa subunit contains a single transmembrane domain and the cytoplasmic tail. LRP is widely expressed in adult
tissues, most abundantly in the liver and the brain (1, 7, 8). To
date, more than 10 structurally and functionally distinct ligands have been identified, including a2-macroglobulin, apoE/
lipoproteins, lipoprotein lipase, tissue-type plasminogen activator, and urokinase (1, 2, 9). Ligand interactions with LRP
can be antagonized by a 39-kDa receptor-associated protein
(RAP). The recombinant form of RAP has been used extensively
in the study of ligand-receptor interactions. Under physiological conditions, however, RAP functions intracellularly as a
molecular chaperone for LRP and facilitates LRP folding and
trafficking within the secretory pathway (9). Increasing evidence has shown that LRP plays important roles in lipoprotein
remnant catabolism (10), protease regulation (11), cell migration (12, 13), neuronal process outgrowth (14), and the pathogenesis of Alzheimer’s disease (15, 16).
Cell surface receptors that traffic between the plasma membrane and endocytic compartments contain signals within their
cytoplasmic tails that allow their efficient recruitment into
endocytic vesicles. To date, four classes of endocytosis signals
have been identified that target surface proteins to clathrincoated vesicles. Tyrosine-based signals within NPXY or YXXØ
sequence (where X can be any amino acid and Ø is an amino
acid with a bulky hydrophobic group) were initially identified
in LDLR and transferrin receptor, respectively (17). In many
cases these signals are constitutively active, i.e. receptors undergo continuous rounds of endocytosis and recycling independent of ligand binding. The di-leucine motif is another well
known endocytosis motif that is present within many transmembrane cell surface proteins (18). The third type of endocytosis signal is serine phosphorylation. For several members of
the family of G protein-coupled receptors, ligand-induced phosphorylation of serine residues serves as a signal for receptor
endocytosis (19). In CD4 and CD3g, the di-leucine motif acts in
cooperation with a neighboring phosphorylated serine residue
to mediate endocytosis (20, 21). Finally, the attachment of
ubiquitin moieties has been recently identified as another
regulator of the endocytosis of several membrane receptors
(22, 23).
A common characteristic of the LDLR family members is
that they all contain at least one copy of the NPXY sequence
17187
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LRP Endocytosis Signals
within their cytoplasmic tails. For LDLR, it has been demonstrated that the NPXY motif serves as a signal for rapid endocytosis through coated pits (24). Therefore, it is generally believed that the NPXY sequences in these receptors
categorically serve as endocytosis signals. More recently, Herz
and colleagues (25–27) have demonstrated that two cytoplasmic adaptor proteins, mammalian Disabled-1 and FE65, interact with NPXY motifs in the cytoplasmic tails of LRP, LDLR,
VLDLR, and ApoER2, and that VLDLR and ApoER2 function
as obligate components in the Reelin/Disabled-mediated neuronal migration pathway. These data suggest that the NPXY
motifs within the tails of the LDLR family members may function not only as endocytosis signals but also as binding motifs
for cellular components, which are involved in signal transduction. The tail of LRP consists of 100 amino acid residues, and
contains multiple potential endocytosis motifs including two
NPXY motifs, one YXXØ motif, and two di-leucine motifs. In
this study, the five potential endocytosis signals are termed, in
their relationships to the transmembrane domain, as proximal
NPXY, proximal di-leucine, distal NPXY, YXXL, and distal
di-leucine (see Fig. 1). The primary aim of this study is to define
endocytosis signal(s) within the LRP tail. Here, we demonstrate that the YXXL motif, but not the two NPXY motifs,
serves as the dominant signal for LRP endocytosis. We also
show that the distal di-leucine motif contributes to LRP endocytosis, and its function is independent of the YXXL motif.
These data suggest that each member of the LDLR family may
utilize different potential signal(s) within their cytoplasmic
tails for receptor-mediated endocytosis.
EXPERIMENTAL PROCEDURES
Materials—Human recombinant RAP was expressed in a glutathione
S-transferase expression vector and isolated as described previously
(28). scuPA was kindly provided by G. F. Vovis of Collaborative Research (29). All tissue culture media, serum, and plasticware were from
Life Technologies, Inc. Non-enzymatic cell dissociation solution was
from Sigma. Monoclonal anti-HA antibody has been described before
(30). Goat anti-mouse IgFITC was from Becton Dickinson. Cy3-Goat
anti-mouse IgG was from Sigma. Quantum Simply Cellular microbead
standard was from Flow Cytometry Standards Corp., San Juan, Puerto
Rico. Peroxidase-labeled anti-mouse antibody and ECL system were
from Amersham Pharmacia Biotech. Immobilon-P transfer membrane
was from Millipore. Rainbow molecular weight markers were from
Bio-Rad. Carrier-free Na125I was purchased from NEN Life Science
Products.
Cell Culture and Transfection—The LRP-null Chinese hamster
ovary (CHO) cell line (kindly provided by David FitzGerald, National
Institutes of Health; see Ref. 31) was cultured in Ham’s F-12 medium as
described (31). Stable transfection into LRP-null CHO cells was
achieved by transfection of 30 mg of plasmid DNA in 10-cm dishes by
using a calcium phosphate precipitation method (32). Stable transfectants were selected using 700 mg/ml G418 and maintained with 350
mg/ml G418.
Construction of LRP Minireceptor and Site-directed Mutagenesis—
The construction of the membrane-containing minireceptor of LRP (see
Fig. 1, A and C) via polymerase chain reaction was performed essentially as described previously (30, 33). Site-directed mutagenesis was
carried out using the QuickChange mutagenesis kit (Stratagene) according to the manufacturer’s instructions. All oligonucleotides were
synthesized at the Washington University School of Medicine Protein
Chemistry Laboratory.
Western Blotting of LRP Minireceptors—Stably transfected CHO
cells were lysed with 0.5 ml of lysis buffer (phosphate-buffered saline
containing 1% Triton X-100 and 1 mM phenylmethylsulfonyl fluoride) at
4 °C for 30 min. Equal quantities of protein were subjected to SDSPAGE (6%) under reducing conditions. Following transfer to polyvinylidene difluoride membrane, successive incubations with anti-HA antibody and horseradish peroxidase-conjugated goat anti-mouse IgG were
carried out for 60 min at room temperature. The immunoreactive proteins were then detected using the ECL system. Films showing immunoreactive bands were scanned using a Kodak Digital Science DC120
Zoom digital camera and analyzed with Kodak Digital Science1D image
analysis software.
Flow Cytometric Analysis of Cell Surface LRP Minireceptors—For
cell surface LRP minireceptor analysis, living cells were used (34).
Briefly, CHO cells were detached by incubation with non-enzymatic cell
dissociation solution. Successive incubations with affinity-purified anti-HA IgG (25 mg/ml) and goat anti-mouse IgFITC were carried out at
4 °C for 45 min. Background fluorescence intensity was assessed in the
absence of primary monoclonal antibody. The antibody binding capacities were evaluated from the standardized Quantum Simply Cellular
bead calibration plot (35). The bead standards consist of four populations of microbeads coated with goat anti-mouse antibody, which bind
different numbers of mouse IgG monoclonal antibody molecules (5686,
18,329, 50,908, and 150,477 molecule binding capacities) in addition to
a blank population. The beads were stained in the same way as the
CHO cells.
Protein Iodination—RAP and scuPA (50 mg) were iodinated by using
the IODO-GEN method as described previously (36).
Kinetic Analysis of Endocytosis—Kinetic analysis of endocytosis was
performed according to previously published methods (37, 38). Stably
transfected CHO cells were plated in 12-well plates at a density of 2 3
105 cells/well and used after overnight culture. Cells were rinsed twice
in ice-cold ligand binding buffer (minimal Eagle’s medium containing
0.6% BSA), and 125I-RAP was added at 5 nM final concentration in cold
ligand binding buffer (0.5 ml/well). The binding of 125I-RAP was carried
out at 4 °C for 30 min with gentle rocking. Binding of 125I-RAP was
specific, i.e. the addition of 100-fold excess unlabeled RAP inhibited
binding by 90 –95%. Unbound ligand was removed by washing cell
monolayers three times with cold binding buffer. Ice-cold stop/strip
solution (0.2 M acetic acid, pH 2.6, 0.1 M NaCl) was added to one set of
plates without warming up and kept on ice. The remaining plates were
then placed in a 37 °C water bath, and 0.5 ml of ligand binding buffer
prewarmed to 37 °C was quickly added to the well monolayers to initiate internalization. After each time point, the plates were quickly
placed on ice and the ligand binding buffer was replaced with cold
stop/strip solution. Ligand that remained on the cell surface was
stripped by incubation of cell monolayers with cold stop/strip solution
for a total of 20 min (0.75 ml for 10 min, twice) and counted. Cell
monolayers were then solubilized with low SDS lysis buffer (62.5 mM
Tris-HCl, pH 6.8, 0.2% SDS, 10% v/v glycerol) and counted. The sum of
ligand that was internalized, plus that which remained on the cell
surface after each assay was used as the maximum potential internalization. The fraction of internalized ligand after each time point was
calculated and plotted.
Analyses of LRP Ligand Binding Activity and Ligand Degradation
Efficiency—Cells (2 3 105) were seeded into 12-well dishes 1 day prior
to assays. Assay buffer (minimal Eagle’s medium containing 0.6% BSA
with 5 nM radioligand, 0.6 ml/well) was added to cell monolayers, in the
absence or the presence of unlabeled 500 nM RAP, followed with incubation for 1 h at 4 °C. Thereafter, overlying buffer containing unbound
ligand was removed, and cell monolayers were washed and lysed in low
SDS lysis buffer and counted.
Ligand degradation efficiency was measured using the methods as
described (24, 39). Briefly, 2 3 105 cells were seeded into 12-well dishes
1 day prior to assays. Pre-warmed assay buffer was added to cell
monolayers, in the absence or the presence of unlabeled 500 nM RAP,
followed with incubation for 4 h at 37 °C. Thereafter, the medium
overlying the cell monolayers was removed and proteins were precipitated by addition of BSA to 10 mg/ml and trichloroacetic acid to 20%.
Degradation of radioligand was defined as the appearance of radioactive fragments in the overlying medium that were soluble in 20%
trichloroacetic acid. The protein concentrations of each cell lysates were
measured in parallel dishes that did not contain LRP ligands. The
ligand degradation efficiency is the value of degraded ligand (cpm/mg
cell protein) divided by the number of cell surface LRP minireceptors (as
determined by flow cytometry, and calculated relative to wild type
mLRP4T100).
Immunofluorescence Microscopy—Stably transfected CHO cells expressing various proteins were grown on glass coverslips, fixed in 4%
paraformaldehyde in phosphate-buffered saline for 20 min, and permeabilized with 0.2% Triton X-100 for 10 min at room temperature. The
cells were then incubated for 30 min with anti-HA antibody followed by
a 30-min labeling with Cy3 goat anti-mouse IgG. Confocal microscopy
was performed on a Bio-Rad MRC 1024 model using a Zeiss 633
(nominal aperture, 1.4) oil immersion lens.
RESULTS
Endocytosis Rate of LRP Minireceptors with Truncated
Tails—The very large size of LRP (;600 kDa) limits molecular
LRP Endocytosis Signals
17189
FIG. 1. Endocytosis rate of LRP minireceptors with truncated cytoplasmic tails. A, schematic representation of mLRP4T100.
mLRP4T100 is depicted in comparison to the full-length LRP molecule. The four putative ligand-binding domains are labeled with Roman
numerals I, II, III, and IV. B, the sequence of LRP cytoplasmic tail. The first amino acid following the transmembrane domain is numbered 1. C,
schematic representation of potential endocytosis signals within the tails of mLRP4T100, and its deletion variants. D, effects of tail truncation on
LRP minireceptor-mediated endocytosis. LRP-null CHO cells stably transfected with mLRP4T100, and its deletion variants were incubated with
5 nM 125I-RAP at 4 °C for 30 min, and then shifted to 37 °C for the indicated times. The amounts of ligand internalized as the fraction of the total
cell-associated ligand (the sum of the internalized ligand plus the ligand remaining on the cell surface at the end of the assay; see “ Experimental
Procedures” for further explanation) are plotted against time. Values are the average of triple determinations with the S.E. indicated by error bars.
This experiment is a representative of three such experiments performed with similar data.
manipulations at the cDNA level and the expression of this
protein via transfection. Thus, we generated an LRP minireceptor that mimics the function and trafficking of LRP (Fig.
1A). This LRP minireceptor encodes residues 2462– 4525 of the
full-length LRP (7), which includes the fourth cluster of ligandbinding repeats through the carboxyl terminus of the receptor
(designated mLRP4T100, with “m” representing membranecontaining, “4” representing the fourth cluster of ligand-binding repeats, “T” representing cytoplasmic tail, and “100” representing the 100 amino acid residues within the LRP tail). To
facilitate immunoprecipitation and Western blot analysis, an
HA epitope was included near the amino terminus of
mLRP4T100. Our studies have demonstrated that this LRP
minireceptor mimics the function and trafficking of full-length
endogenous LRP (30).
Fig. 1B shows the sequence of the cytoplasmic tail of LRP
with the first amino acid following the transmembrane domain
numbered as 1. Using LRP-null CHO cells (31), we generated
stably transfected cell lines expressing LRP minireceptors with
different potential endocytosis signals after truncation (Fig.
1C), and measured the endocytosis rate of these truncated LRP
minireceptors. Our previous studies have shown that the
fourth ligand-binding domain of LRP binds RAP with high
affinity (30, 32, 33). Fig. 1D shows the effects of tail truncation
on LRP minireceptor-mediated endocytosis. As shown in the
figure, the stable cell line bearing mLRP4T100 rapidly internalized 125I-RAP, with a half-time of less than 30 s. In contrast,
125
I-RAP was poorly internalized by cells expressing either
mLRP4Ttailess or mLRP4T25, while the endocytosis rates of
mLRP4T59 and mLRP4T72 were variably greater than those of
mLRP4Ttailess and mLRP4T25. These results suggest each of
the potential endocytosis motifs (NPXY, YXXØ, and di-leucine)
within the LRP tail may contribute to LRP endocytosis.
The YXXL Motif, but Not the Two NPXY Motifs, Serves as the
Dominant Endocytosis Signal for LRP—Based on the above
results, we generated eight LRP minireceptors with specific
mutations in the cytoplasmic tail based on mLRP4T100 (Fig.
2A). These mutations were targeted to each of the putative
endocytosis signals, which allowed us to evaluate the contribution of these signals in LRP endocytosis. Fig. 2B shows the
endocytosis rates of wild type mLRP4T100 and its proximal
NPXY motif mutants. Alteration of the asparagine or the tyrosine to alanine resulted in only a slight decrease in RAP internalization, with the initial endocytosis rates being indistinguishable from that of mLRP4T100. These results indicate that
the proximal NPXY motif is not important for LRP endocytosis.
Fig. 2C shows the endocytosis rates of wild type mLRP4T100
and its distal NPXY motif mutants. As the phenylalanine is the
important residue within the FXNPXY motif for LDLR endocytosis (24), we also mutated this residue to alanine. Replacement of the phenylalanine or the asparagine by alanine resulted in only a slight decrease in RAP internalization, with the
initial endocytosis rates being indistinguishable from that of
mLRP4T100. In contrast, alteration of the tyrosine to alanine
resulted in a dramatic decrease in RAP internalization. At 15,
30, and 60 s, this mutant internalized 3%, 5%, and 12%, respectively, of the total cell-associated 125I-RAP, corresponding
to a 80 –90% impairment in endocytosis relative to wild type
mLRP4T100. These results indicate that the tyrosine residue
at position 63 is critical for LRP endocytosis. Because the
tyrosine within the distal NPXY motif is also within the YXXL
motif, the decreased endocytosis rate of mLRP4T100(Y63A)
may have resulted from the mutation of the YXXL motif. Fig.
2D shows the endocytosis rates of wild type mLRP4T100 and
its YXXL motif mutants. As shown in the figure, the endocytosis rate of mLRP4T100(L66A) was significantly decreased, and
was indistinguishable from that of mLRP4T100(Y63A). Taken
together, these results clearly demonstrate that the YXXL motif, but not the distal NPXY motif, serves as the major endocytosis signal for LRP.
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LRP Endocytosis Signals
FIG. 2. Endocytosis rate of LRP minireceptors with site-mutated cytoplasmic tails. A, schematic representation of potential endocytosis signals
within the tails of LRP4T100, and its mutants. The introduced amino acid substitutions are shown in bold. B–E, effect of
mutations in LRP tail on LRP minireceptor-mediated endocytosis. 125I-RAP internalization in LRP-null CHO cells stably
transfected with mLRP4T100, and its
mutants was carried out for indicated
time points as described in the Fig. 1D
legend. Values are the average of triple
determinations with the S.E. indicated by
error bars. This experiment is a representative of three such experiments performed with similar data. B, endocytosis
of 125I-RAP by LRP minireceptors with
mutations in the proximal NPXY motif. C,
endocytosis of 125I-RAP by LRP minireceptors with mutations in the distal
NPXY motif. D, endocytosis of 125I-RAP
by LRP minireceptors with mutations in
the YXXL motif. E, endocytosis of 125IRAP by LRP minireceptors with mutations of the two di-leucine motifs.
Distal Di-leucine Motif, but Not the Proximal Di-leucine Motif,
Contributes to LRP Endocytosis—The di-leucine motif is another
well characterized endocytosis signal that is present in many transmembrane cell surface proteins. Thus, we next compared the endocytosis rate of mLRP4T100 with that of its di-leucine motif mutants (Fig. 2E). Mutation of both leucines to alanines in the
proximal di-leucine motif resulted in little or no change in RAP
internalization. However, the mLRP4T100(L86A,L87A) mutant
clearly demonstrates a defect in internalization, although not as
substantial as mLRP4T100(Y63A) or mLRP4T100(L66A) (Fig. 2, D
and E). At 15, 30, and 60 s, mLRP4T100(L86A,L87A) internalized
18%, 30%, and 40%, respectively, of the total cell-associated 125IRAP, corresponding to a 30–40% impairment in endocytosis relative to wild type mLRP4T100. These results clearly demonstrate
that the distal di-leucine motif, but not the proximal di-leucine
motif, contributes to LRP endocytosis.
The YXXL Motif and the Distal Di-leucine Motif Function
Independently in LRP Endocytosis—To address whether the
YXXL motif and the distal di-leucine motif function independently in LRP endocytosis, we generated a double mutant,
mLRP4T100(Y63A,L86A,L87A). The endocytosis rate of this
mutant receptor was compared with that for wild type
mLRP4T100 as well as mLRP4T100(Y63A), mLRP4T100(L86A,L87A), and mLRP4Ttailess, in stably transfected
LRP-null CHO cells. As shown in Fig. 3, the endocytosis rate of
mLRP4T100(Y63A,L86A,L87A) was significantly lower than
that of mLRP4T100(Y63A). These results thus clearly demonstrate that the YXXL motif and the distal di-leucine motif
function independently in LRP endocytosis.
FIG. 3. The YXXL motif and the distal di-leucine motif function
independently for LRP endocytosis. 125I-RAP internalization in
LRP-null CHO cells stably transfected with mLRP4T100, mLRP4T100(L86A,L87A), mLRP4T100(Y63A), mLRP4T100(Y63A,L86A,L87A),
and mLRP4Ttailess was carried out for indicated time points as described in the Fig. 1D legend. Values are the average of triple determinations with the S.E. indicated by error bars. This experiment is a
representative of three such experiments performed with similar data.
The Proximal NPXY Motif and the Proximal Di-leucine Motif
Function in a Truncated Receptor Tail—In Fig. 2 (B and E), we
demonstrated that the proximal NPXY motif and the proximal
di-leucine motif exhibit little or no role in LRP endocytosis
within the full-length LRP tail. However, comparison of the
LRP Endocytosis Signals
17191
FIG. 4. The proximal NPXY motif and the proximal di-leucine
motif function in truncated receptor tail. 125I-RAP internalization
in LRP-null CHO cells stably transfected with mLRP4T100, mLRP4T59, mLRP4T59(Y29A), mLRP4T59(L43A,L44A), and mLRP4Ttailess
was carried out for indicated time points as described in the Fig. 1D
legend. Values are the average of triple determinations with the S.E.
indicated by error bars. This experiment is a representative of three
such experiments performed with similar data.
endocytosis rate of mLRP4T100 with that of mLRP4T59 indicated that mLRP4T59 is still capable of internalizing RAP (Fig.
1D). To address this question, we generated two mLRP4T100
mutants mLRP4T59(Y29A) and mLRP4T59(L43A,L44A) using
mLRP4T59 as the template. Fig. 4 shows the endocytosis rates
of the stably transfected CHO cells expressing mLRP4T100,
mLRP4T59, mLRP4T59(Y29A), mLRP4T59(L43A,L44A), and
mLRP4Ttailess. As shown in the figure, the endocytosis rates
of mLRP4T59(Y29A) and mLRP4T59(L43A,L44A) were significantly lower than that of mLRP4T59, indicating that the proximal NPXY motif and the proximal di-leucine motif are functional within the truncated receptor tail.
mLRP4T100 Mutants Defective in Endocytosis Accumulate
at the Cell Surface—We hypothesized that mLRP4T100 mutants that are defective in endocytosis signals would accumulate at the cell surface. We initially compared the ligand binding activity of wild type mLRP4T100 with those of endocytosis
mutants expressed in stably transfected CHO cells. As shown
in Fig. 5A, CHO cells expressing wild type mLRP4T100 exhibited a moderate level of cell surface RAP binding, while CHO
cells transfected with the pcDNA3 vector exhibited only ;10%
of RAP binding compared with those transfected with
mLRP4T100. The residual RAP binding to pcDNA3-transfected
cells is likely mediated by cell surface heparan sulfate proteoglycan (40). Interestingly, CHO cells expressing mLRP4T100
mutants defective in endocytosis signals exhibited significant
increase in cell surface RAP binding activity. To analyze
whether the increase of RAP binding to CHO cells expressing
endocytosis mutants is due to an accumulation of mature minireceptor on the cell surface, we examined the steady-state
distribution of various forms of minireceptors via Western blotting analysis using anti-HA antibody (Fig. 5B). For
mLRP4T100 and its mutants, two distinct bands are seen on
6% SDS-PAGE gel under reducing conditions. The 120-kDa
band represents the mature furin-processed minireceptor form
that corresponds to the LRP-ligand binding domain 4 (see Fig.
1, and Ref. 30). Since the HA epitope is within the amino
terminus of the LRP minireceptors, Western blot analyses with
anti-HA antibody do not detect the LRP-85 band. However, the
presence of this band was confirmed by metabolic labeling with
[35S]cysteine and immunoprecipitation (data not shown). The
upper band that migrates ;200 kDa represents the full-length
ER precursor form that lacks complex sugar modification (see
Ref. 30). mLRP4Ttailess exhibits a similar banding pattern on
SDS-PAGE, except that the ER form migrates faster than that
FIG. 5. Ligand binding activity and Western blotting analysis
of LRP minireceptor expression in LRP-null CHO cells. A, binding of 125I-RAP (5 nM) to LRP-null CHO cells stably transfected with
pcDNA3 or various LRP minireceptors was carried out for 1 h at 4 °C in
the absence (f) or presence (M) of 500 nM RAP. Values are the average
of triple determinations with the S.E. indicated by error bars. B, Western blotting analysis of LRP minireceptor expression in LRP-null CHO
cells. Same amounts of cell lysates from LRP-null CHO cells stably
transfected with LRP minireceptors were separated via 6% SDS-PAGE
under reducing conditions and Western blotted with anti-HA antibody.
The positions of the ER forms and the extracellular subunits representing the ligand-binding domain (120 kDa) are labeled. The bottom panel
shows densitometric analysis of data described in top panel. Data are
calculated as the ratio of 120-kDa/ER form of LRP minireceptors. This
experiment is a representative of two such experiments performed with
similar data.
of mLRP4T100 due to the tail truncation. The results show
that, although the ER forms of mLRP4T100 and those mutants
defective in endocytosis as well as mLRP4Ttailess are expressed at a similar level, the furin-processed 120-kDa forms
are detected at distinctly different levels, i.e. the 120-kDa forms
of mLRP4T100(Y63A), mLRP4T100(L66A), mLRP4T100(Y63A,L86A,L87A), and mLRP4Ttailess are significantly increased, while the 120-kDa form of mLRP4T100(L86A,L86A) is
only slightly greater than that of mLRP4T100 (Fig. 5B). The
ratio of 120-kDa/ER of mLRP4T100 is 0.37, and the ratios of
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LRP Endocytosis Signals
FIG. 7. Scanning confocal images of LRP minireceptors. LRPnull CHO cells stably transfected with mLRP4T100 (A), mLRP4T100(L86A,L87A) (B), mLRP4T100(Y63A) (C), mLRP4T100(L66A) (D),
mLRP4T100(Y63A,L86A,L87A) (E), and mLRP4Ttailess (F) were immunostained with anti-HA antibody and detected with Cy3-Goat antimouse IgG. Images represent single 0.16-mm-thin midsections from
scanned cells.
FIG. 6. Flow cytometric analysis of cell surface LRP minireceptor expression in LRP-null CHO cells. A, histograms of LRP
minireceptor cytofluorimetric analysis in LRP-null CHO cells. LRP-null
CHO cells stably transfected with LRP minireceptors were labeled with
anti-HA antibody and detected with goat anti-mouse IgFITC. Background fluorescence intensity was assessed in the absence of primary
monoclonal antibody (thin line). The x axis represents relative fluorescence intensity, and the y axis represents relative cell number. B, the
number of cell surface LRP minireceptors in LRP-null CHO cells. LRPnull CHO cells stably transfected with LRP minireceptors were stained
as described above. The number of cell surface minireceptors per cell
were determined by using Quantum Simply Cellular microbead standard as described under “Experimental Procedures.” Values are the
average of triple determinations with the S.E. indicated by error bars.
This experiment is a representative of two such experiments performed
with similar data.
those mutants defective in endocytosis as well as
mLRP4Ttailess are ranged from 0.74 to 1.9 (Fig. 5B). Thus, at
steady state, mutation of LRP endocytosis signals or deletion of
the cytoplasmic tail of LRP results in a significant increase in
expression of the mature minireceptors.
To address whether the increases in RAP-binding and the
expression of the 120-kDa forms are results of cell surface
accumulation of the LRP minireceptors, we next measured the
cell surface LRP minireceptors by flow cytometric analysis in
intact cells. As shown in Fig. 6A, CHO cells transfected with
only the pcDNA3 vector yields no specific staining with anti-HA antibody. Wild type mLRP4T100 exhibited a moderate
level of cell surface staining. mLRP4T100 mutants defective in
endocytosis, however, exhibited dramatic increase in cell surface staining. Using Quantum Simply Cellular bead standards
(35), the numbers of LRP minireceptors per cell surface was
assessed (Fig. 6B). Higher numbers of LRP minireceptors were
found on all mLRP4T100 mutants defective in endocytosis
(ranging from ;49,300 to ;388,000 sites/cell), when compared
with wild type mLRP4T100 (;36,700 sites/cell), with the high-
est numbers observed with mLRP4Ttailess and mLRP4T100(Y63A,L86A,L87A). Taken together, these data clearly demonstrate that mLRP4T100 mutants defective in endocytosis accumulate at the cell surface.
To confirm the above results, we determined the steady-state
distribution of LRP minireceptors stably transfected in CHO cells
via confocal microscopy. As seen in Fig. 7A, wild type mLRP4T100
exhibits both cell surface and vesicular staining, similar to that
seen with endogenous LRP (41). Compared with mLRP4T100, the
change of cell surface mLRP4T100(L86A,L87A) expression is not
significant (Fig. 7B). However, mutations in LRP dominant endocytosis motif YXXL resulted in high levels of expression at the cell
surface (Fig. 7, C and D). mLRP4T100(Y63A,L86A,L87A) and
mLRP4Ttailess also show markedly increased cell surface localization (Fig. 7, E and F). These observations of increased expression at
the cell surface for the various endocytosis mutants support the role
for the YXXL motif as a dominant signal in LRP endocytosis.
mLRP4T100 Mutants Defective in Endocytosis Exhibit Reduction in the Efficiency of Ligand Degradation—Having established
that LRP endocytosis is mediated by the YXXL motif and the distal
di-leucine motif, we then investigated the efficiency of LRP ligand
degradation for wild type mLRP4T100 and its endocytosis mutants. Fig. 8A shows that the 125I-RAP degradation efficiency of the
LRP endocytosis mutants decreased significantly with
mLRP4Ttailess showing the lowest level of 125I-RAP degradation.
As expected, mLRP4T100(L86A,L87A) shows partially reduced
degradation, while mLRP4T100(Y63A), mLRP4T100(L66A), and
mLRP4T100(Y63A,L86A,L87A) show only 12.9%, 8.8%, and 6%
that of wild type mLRP4T100, respectively.
Our recent studies have shown that mLRP4T100 is able to
degrade scuPA efficiently.2 Unlike RAP, scuPA is a physiological ligand for LRP. Thus, we utilized scuPA to measure the
efficiency of ligand degradation for the LRP minireceptors.
Similar to that seen with 125I-RAP, LRP endocytosis mutants
exhibited impaired degradation of 125I-scuPA. mLRP4Ttailess
showed lowest level of scuPA degradation. mLRP4T100(L86A,
2
L. M. Obermoeller, Y. Li, and G. Bu, unpublished results.
17193
LRP Endocytosis Signals
FIG. 8. mLRP4T100 mutants defective in endocytosis exhibit
lower ligand degradation efficiency. LRP-null CHO cells stably
transfected with mLRP4T100, mLRP4T100(L86A,L87A), mLRP4T100(Y63A), mLRP4T100(L66A), mLRP4T100(Y63A,L86A,L87A), and
mLRP4Ttailess were incubated with 5 nM 125I-RAP (A) or 125I-scuPA (B)
at 37 °C for 4 h in the presence or absence of 500 nM RAP. The ligand
degradation efficiency was determined as described under “Experimental Procedures.” Values are the average of triple determinations with
the S.E. indicated by error bars. This experiment is a representative of
two such experiments performed with similar data.
L87A) showed partially reduced scuPA degradation, while
mLRP4T100(Y63A), mLRP4T100(L66A), and mLRP4T100(Y63A,L86A,L87A) exhibited only 30%, 23.1%, and 8.7%
that of wild type mLRP4T100, respectively. Taken together,
these data clearly demonstrate that cells expressing mLRP4T100 mutants defective in endocytosis exhibit reduced ligand
degradation efficiency.
DISCUSSION
The NPXY motif serves as the signal for rapid endocytosis
through coated pits for the LDLR (1, 24). It is the only sequence
element within the cytoplasmic tail that is conserved among all
members of the LDLR family. The tail of LRP contains two
copies of this motif, suggesting that the NPXY sequences might
also function as endocytosis signals for LRP. In the present
study, we provide direct evidence that the YXXL motif, but not
the two NPXY motifs, serves as the dominant signal for LRP
endocytosis. Substitution of tyrosine or leucine within the
YXXL motif with alanine significantly reduced the endocytosis
rate. We also found that the distal di-leucine motif contributes
to LRP endocytosis, and that its function is independent from
the YXXL motif. Additionally, studies of the cell surface expression and efficiency of ligand degradation for LRP endocytosis
mutants strongly support these findings. These data suggest
that each member of the LDLR family may use different potential endocytosis signal(s) within their cytoplasmic tails.
In the present study, we also demonstrated that the proximal NPXY motif and the proximal di-leucine motif function in
truncated but not in wild type full-length receptor tails. This
suggests that these motifs might be embedded within the wild
type receptor tail but exposed in the truncated tails. A similar
finding has been reported for the growth hormone receptor (42).
Although endocytosis of the wild type growth hormone receptor
depends on the ubiquitin system, a truncated form of growth
hormone receptor, which lacks the ubiquitin-dependent endocytosis motif, utilizes a di-leucine motif for its endocytosis (42).
Alternatively, the function of these potential endocytosis motifs
within wild type receptor tail may be overridden by the presence of dominant YXXL motif.
Recently, studies have revealed new roles of LDLR family
members as transducers of extracellular signals. It has been
demonstrated that VLDLR and apoER2 function as obligate
components in the Reelin/Disabled-mediated neuronal migration pathway (25–27, 43). A signaling pathway involving the
extracellular protein Reelin and the intracellular adaptor protein Disabled-1 is involved in the control of cell position during
mammalian brain development (44 – 47). It has been shown
that Disabled-1 interacts with the NPXY motifs within the tails
of several members of the LDLR family (25, 26). Mice lacking
the genes for both VLDLR and apoER2 demonstrate a neurological and neuroanatomical phenotype that is indistinguishable from animals deficient in either Reelin or Disabled-1 (26).
Potential signaling functions for members of the LDLR family
have also been suggested from other observations. For example, Goretzki and Mueller (48) have shown that the LRP tail
interacts with a GTP-binding protein and induces cyclic-AMPdependent protein kinase activity. Similar signal transduction
event down stream from LRP was also implicated in hippocampal neurons (49). At present, it is still unclear whether the
signaling event(s) initiated by lipoprotein receptors can be regulated by endocytosis.
Studies using different experimental systems have revealed
that tyrosine-based as well as di-leucine-based sorting signals
of membrane proteins can be recognized by adaptor complexes,
which in turn associate with clathrin and other accessory molecules to generate clathrin coats and coated transport vesicles
(18, 50). Adaptor complex AP-2 plays a critical role in two early
steps of the endocytic pathway at the plasma membrane: the
formation of the clathrin lattice and the selection of specific
cargo proteins for internalization (18, 51). AP-2 interacts with
clathrin through its b subunits and promotes coat formation.
Interaction of the AP-2 m subunit with receptors containing
tyrosine-based as well as di-leucine-based internalization motifs contributes to their localization to coated pits. In the present studies, we have shown that the YXXL motif serves as the
dominant motif for LRP endocytosis, and that the distal dileucine motif further contributes to LRP endocytosis. Thus, it
will be interesting to examine whether AP-2 plays a major role
in LRP endocytosis and whether it interacts directly with the
receptor endocytosis signals.
In conclusion, our results show that the YXXL motif, but not
the two NPXY motifs, serves as the dominant signal for LRP
endocytosis. We have also defined that the distal di-leucine
motif contributes to LRP endocytosis, and that its function is
independent of the YXXL motif. Mutants defective in endocytosis accumulate at the cell surface, and exhibit reduced efficiency for ligand degradation. These data together with that of
other’s, suggest that the YXXL motif and the distal di-leucine
motif of the LRP tail serve as endocytosis signals, while the
NPXY motifs serve as binding sites for cytosolic signaling
proteins.
Acknowledgments—We are grateful to Alan Schwartz for critical
reading and suggestions on the manuscript. We also thank David
FitzGerald (National Institutes of Health, Bethesda, MD) for providing
the LRP-null CHO cell line
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