From the Department of Biochemistry and Cell Biology, Institute for Cell and Developmental Biology, State University of New York at Stony Brook, Stony Brook, New York 11794-5215
Received for publication, December 2, 2002, and in revised form, December 16, 2002
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ABSTRACT |
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Fringe plays a key role in the specification of
boundaries during development by modulating the ability of Notch
ligands to activate Notch receptors. Fringe is a fucose-specific
The Notch signaling pathway plays an essential role in multiple
stages of development in metazoans (1). In humans, deregulation of the
Notch pathway can result in a number of serious diseases, including T
cell leukemia (2), cerebral arteriopathies (CADASIL) (3), and Alagille
syndrome (4). Recently, defects in Notch signaling have been implicated
in the pathogenesis of multiple sclerosis (5). Whereas there is only
one Notch in Drosophila, four Notch homologues exist in
mammalian systems. Notch becomes activated upon binding to its ligands
(members of the Delta, Serrate/Jagged families) on the surfaces of
adjacent cells, initiating a series of proteolytic events resulting in
release of the Notch intracellular domain from the membrane (reviewed
in Ref. 6). The Notch intracellular domain translocates to the nucleus,
where it binds to members of the CSL (CBF1/suppressor of
hairless/Lag-1 family of transcriptional regulators, activating
transcription of downstream gene products.
The Notch receptor is a large, cell surface membrane glycoprotein
containing multiple domains (1, 6). The extracellular domains of Notch1
and Notch2 consist largely of 36 tandem epidermal growth factor-like
(EGF)1 repeats. Notch3 and
Notch4 contain 34 and 29 EGF repeats, respectively. EGF repeats are
defined by the presence of six conserved cysteine residues that form
three disulfide bonds (7). Many of the EGF repeats on Notch1 contain
evolutionarily conserved consensus sites for two unusual forms of
glycosylation: O-fucose and O-glucose (8). The
consensus sites for these modifications were determined by comparison
of sites of glycosylation from the EGF repeats of several
serum glycoproteins (9). Based on these analyses, O-glucose modifications were shown to occur between the first and second conserved cysteine (C1 and C2, respectively) of
the EGF repeat at the sequence
C1XSXPC2, and
O-fucose modifications occurred between the second and third conserved cysteines (C2 and C3, respectively)
at the sequence
C2XXGG(S/T)C3 (9). We
demonstrated that the Notch1 protein from Chinese hamster ovary cells
is modified with both O-fucose and O-glucose, suggesting that these consensus sites can be used to accurately predict
whether a protein will bear the modifications (8). In similar studies,
we have shown that Notch ligands (Drosophila Delta and
Serrate, mammalian Jagged1 and Delta1), which also contain O-fucose consensus sites, are modified with
O-fucose (10). Interestingly, mutation of all of the
predicted O-fucose sites on Drosophila Serrate
failed to eliminate O-fucosylation, suggesting that
O-fucose was modifying sites not predicted using the
original consensus sequence (10). Based on these results, we proposed a
broadened consensus site for O-fucosylation:
C2X3-5(S/T)C3.
Mutation of these sites on Drosophila Serrate eliminated
O-fucosylation completely. Thus, the number of EGF repeats
in Notch and other proteins predicted to bear O-fucose has
increased. Nonetheless, individual O-fucosylation sites have
not been mapped on either Notch or its ligands.
Recent studies have demonstrated that O-fucose modifications
play an essential role in Notch function. Reduction of
O-fucosyltransferase expression in Drosophila
using RNAi (11) or in mice by gene ablation2 causes Notch-like
phenotypes, suggesting that O-fucose modifications are
essential for Notch function. In addition, we and others have shown
that Notch activation is modulated by extension of O-fucose on Notch with the fucose-specific
To better understand the mechanism of how the O-fucose
structures modulate Notch function, we have begun to map sites of both O-fucose and Fringe modification on mouse Notch1. Here we
used fragments of the mNotch1 extracellular domain to identify sites of
O-fucose modification and subsequent elongation by Mfng. We show that the broadened O-fucose consensus sequence
(C2X3-5(S/T)C3)
(10) can be used to accurately predict sites of
O-fucosylation, and we observed that Fringe modifies
O-fucose on EGF repeats that have important biological
roles, including ligand binding and cell-autonomous inhibition by
ligand. These findings suggest specific mechanisms for how the change
in O-fucose glycan structure can modulate Notch function.
Materials--
N-Acetylneuraminidase I
( Production of Mouse Notch1 EGF Fragments and
Mutants--
Constructs encoding fragments (EGF 1-5, 6-10, 11-15,
16-18, 19-23, 24-28, 29-36, 24, and 26) of the mouse Notch1
extracellular domain were generated using PCR using pcDNA-Notch1myc
as template (see Table I for a list of
the primers used). The fragments were designed to contain less than two
predicted O-fucose sites based on the original consensus
sequence: C2XXGG(S/T)C3
(9). Restriction sites for HindIII and XhoI were
designed into the primers for subsequent cloning. PCR was carried out
for 30 cycles with the following conditions: denaturing at 95 °C for 0.5 min; annealing at 65 °C for 1 min; elongating at 72 °C for 2 min/kb. The PCR product was then digested with HindIII and
XhoI and subcloned into the corresponding sites in the
mammalian expression vector pSecTag (Invitrogen). To make site mutants
for some of the EGF fragments, the QuikChange site-directed mutagenesis
protocol (Stratagene) was used. All constructs were sequenced to
confirm nucleotide sequence.
To express and metabolically radiolabel the fragments, each construct
was transiently transfected into Lec1 cells using Geneporter (Gene
Therapy Systems) essentially as described previously (16). Following
transfection (24 h), the medium was replaced with fresh medium
containing 20 µCi/ml [6-3H]fucose. After 48 h, the
medium was collected, and the fragments were purified by rotating the
medium with Ni2+-nitrilotriacetic acid-agarose (30 µl of
beads/100-mm plate; Qiagen) for 1 h at 4 °C. After extensive
washing (five times with 50 mM Tris-HCl, pH 8.0, 150 mM NaCl, 1% Nonidet P-40, 0.5% deoxycholate, and 0.1%
SDS), the fragments were eluted with 100 mM EDTA, pH 8.0.
Analysis of O-Fucose Saccharide Structures--
Release of
O-fucose saccharides from the fragments by alkali-induced
Fringe Assays--
Fringe assays were performed as described
(16) using His6-tagged Drosophila Fringe as the
enzyme, UDP-[3H]GlcNAc (PerkinElmer Life Sciences)
as the donor substrate, and factor VII EGF repeat (modified with
O-fucose) as acceptor substrate. Factor VII
EGF-O-fucose was reduced and alkylated in 8 M
urea with dithiothreitol and iodoacetamide as described previously (23). Both the control EGF repeat and the reduced and alkylated EGF
repeat were repurified on reverse-phase high pressure liquid chromatography as described (23).
Other Methods--
SDS-PAGE and fluorography with
En3Hance (DuPont) and Western blots with anti-Myc epitope
antibodies were performed as described (8, 10).
Expression and O-Fucosylation of Notch1 Extracellular Domain
Fragments--
The mouse Notch1 extracellular domain contains 36 tandem EGF repeats, 10 of which contain the O-fucose
consensus sequence C2XXGG(S/T)C3 (EGF
repeats 3, 5, 16, 18, 20, 24, 26, 27, 31, and 35) and five of which are
evolutionarily conserved (EGF repeats 3, 20, 24, 26, and 31) (Fig.
1b). To simplify the analysis
of O-fucose and Fringe modification sites, we divided the
mouse Notch1 extracellular domain into fragments. Constructs encoding
EGF 1-5, 6-10, 11-15, 16-18, 19-23, 24-28, and 29-36 were
generated from mouse Notch1 by PCR. The desired fragments were obtained
by PCR and cloned into a mammalian expression vector encoding a signal
sequence for secretion and C-terminal Myc epitope and His6
tags (see "Experimental Procedures"). Each of the fragments was
expressed transiently in Lec1 cells and metabolically radiolabeled with
[3H]fucose. Although several constructs encoding EGF
6-10 were generated, no protein was detected when transfected into
cells, so no further experiments were performed with these constructs.
In Lec1 cells, no complex-type N-glycans are synthesized
(24), and the majority of [3H]fucose is incorporated into
O-fucose structures (25). Thus, the presence of
[3H]fucose on a fragment suggests
O-fucosylation (as shown previously (8)). Notch
extracellular fragments were purified from medium using
Ni2+-nitrilotriacetic acid-agarose and analyzed by SDS-PAGE
and fluorography (Fig. 2). Each of the
fragments expressed in Lec1 cells was labeled with
[3H]fucose, indicating that they were all modified with
O-fucose. Interestingly, the extent of fucosylation varied
from fragment to fragment. For instance, much more of the EGF 16-18
fragment needed to be loaded onto a gel than any of the other fragments to be readily detected by Western blot (Fig. 2), indicating that EGF
16-18 was more heavily radiolabeled than the other fragments. These
results suggest that the stoichiometry of O-fucosylation on
different EGF repeats may differ.
To investigate which EGF repeats actually bear O-fucose
residues, site mutants were generated at sites predicted using both the
original consensus
(C2XXGG(S/T)C3) and the
broadened consensus
(C2X3-5(S/T)C3)
(Fig. 3a). EGF 19-23 has one
C2XXGG(S/T)C3 consensus
site at EGF 20 (C2VNGGTC3).
Mutation of the threonine to alanine at EGF 20 (T20A) caused significant reduction, but not the elimination, in the fucosylation of
this fragment (Fig. 3, b and c), suggesting that
the broader sites are utilized. Mutation of the site at EGF 23 (S23A)
also resulted in a reduction in fucosylation (Fig. 3, b and
c). A double mutant where the sites at both 20 and 23 were
eliminated (20A/23A) lost the majority of the fucosylation, although
some residual fucosylation remained. The residual radioactivity on the
double mutant is probably on EGF 21 (C2LNQGTC3). Attempts at expressing
and analyzing the triple mutant (EGF 20A/21A/23A) were unsuccessful.
These data demonstrate that both the
C2XXGG(S/T)C3 consensus
sites and the broader sites
(C2X3-5(S/T)C3)
are modified with O-fucose.
To further refine the broader consensus sites, mutants were generated
on EGF 11-15. Although no
C2XXGG(S/T)C3 consensus
sites exist in this fragment, both EGF 12 and 15 contain C2X3-5(S/T)C3
sites (Fig. 3a). Mutation of the site at EGF 12 completely
eliminated the fucosylation of the fragment (Fig. 3, b and
c), suggesting that EGF12 is modified with
O-fucose. Interestingly, the site in EGF 12, believed to be
a portion of the ligand binding site (26), is conserved in all Notch
homologues in data bases (Fig. 1b and data not shown). These
data also suggest that the site at EGF 15 is not modified with
O-fucose. Since the site at EGF 15 has only three amino
acids between the second cysteine and the modified serine or threonine
(Fig. 3a), these results indicate that a minimum distance in
this space is necessary for efficient fucosylation. Thus, the
O-fucose consensus may be refined to
C2X4-5(S/T)C3
(Fig. 1).
Localization of O-Fucose Residues on Mouse Notch1 Modified by
Fringe--
To analyze which EGF repeats contain O-fucose
that can be modified by Fringe, the mouse Notch1 extracellular domain
fragments (Fig. 2) were expressed in Lec1 cell lines stably expressing
Mfng or the corresponding empty vector (16). These cell lines were used
previously in cell-based Notch signaling assays to demonstrate that
Mfng inhibits Notch activation by Jagged1 (16, 19). Interestingly, CHO
cells (including Lec1) have been shown to possess transcripts encoding
Lfng and Rfng but not Mfng (19, 27). The presence of endogenous Lfng
and/or Rfng explains the presence of elongated O-fucose
structures seen previously on Notch isolated from Lec1 cells (8).
Plasmids encoding the different fragments from mouse Notch1 were
transiently transfected into both Lec1 cell lines, the cells were
metabolically radiolabeled with [3H]fucose, and the
fragments were purified from the medium using Ni2+-nitrilotriacetic acid-agarose chromatography. To
determine whether any of the Fringes had elongated O-fucose
on the fragments, the O-linked sugars were released by
alkali-induced
Interestingly, O-fucose was elongated by Mfng to some extent
on most EGF repeats (EGF 11-15, 16-18, 19-23, 24-28, and 29-36) but not on EGF 1-5 (Fig. 4). The extent of elongation varied
significantly, from 15% on EGF 19-23 to 80% on EGF 29-36. These
results suggest that Fringe shows a preference for O-fucose
on some EGF repeats relative to others. In addition, several fragments
showed significant elongation in the absence of Mfng (EGF 16-18,
24-28, and 29-36), presumably due to the action of the endogenous
Fringes. Significantly, O-fucose on EGF 11-15 was elongated
to a small extent by the endogenous Fringes but significantly by
Mfng. Since EGF 12 is the only site bearing O-fucose in EGF
11-15 (Fig. 3), these results suggest that EGF 12 is a substrate for
Mfng modification.
Signals for Fringe Recognition Are Embedded within the Sequence of
EGF Repeats--
The data in Fig. 4 suggest that Fringes show a
preference for O-fucose on some EGF repeats over others. To
begin addressing the basis of this preference, we first analyzed
whether Fringe recognizes O-fucose in the context of a
simple primary amino acid sequence or in the context of a properly
folded EGF repeat. Bacterially expressed EGF repeat-1 from factor VII
modified with O-fucose was unfolded by reduction and
alkylation (see "Experimental Procedures"), and both the native and
unfolded EGF-O-fucose were analyzed as substrates for Fringe
in an in vitro assay (Fig.
5a). The denatured EGF-O-fucose was a very poor substrate for Fringe compared
with the folded EGF-O-fucose, showing that a correctly
folded EGF is the optimal substrate for Fringe, as it is for protein
O-fucosyltransferase (28) and protein
O-glucosyltransferase (23).
To examine whether the determinants for this preference are encoded
within individual EGF repeats, Fringe-mediated elongation of
O-fucose on individual EGF repeats expressed in Lec1 cells was analyzed. EGF repeats 24 and 26 from mouse Notch1 were chosen because both contain evolutionarily conserved
C2XXGG(S/T)C3-type
O-fucose sites (see Fig. 1) and are within a fragment (EGF 24-28) showing partial elongation of O-fucose even in the
absence of Mfng (Fig. 4). The partial modification suggested that the endogenous Fringes (Lfng and Rfng) may be modifying O-fucose
on one of the EGF repeats but not the other. This indeed turned out to
be the case. As before, plasmids expressing EGF 24 or 26 were transiently transfected into Lec1 cells, the cells were metabolically radiolabeled with [3H]fucose, and the EGF repeats were
purified from the medium using Ni2+-nitrilotriacetic
acid-agarose chromatography. In contrast to what was seen with EGF
24-28, the only sugar structure detected on EGF 24 was a
monosaccharide (Fig. 5b), whereas both the tetra- and
monosaccharide forms of O-fucose were seen on EGF 26 (Fig. 5c). Thus, the endogenous Fringes (Lfng and Rfng) can
discriminate between EGF 26 and EGF 24 based solely on the sequence
information in the individual EGF repeats. The O-fucose on
EGF 24 is elongated when expressed in Lec1 cells with Mfng (data not
shown), indicating that there may be some differences in the
specificity of the individual Fringes. Nonetheless, since the Mfng is
expressed at a higher level than the endogenous Fringes, this effect
may be due to overexpression rather than specificity differences. Thus,
a properly folded EGF repeat modified with O-fucose appears
to be the basic unit of Fringe recognition. Although we have not yet
determined the specific signals for Fringe recognition, these data
suggest that such signals are embedded within the EGF sequence itself.
O-Fucosylation and Fringe Modification on Larger EGF
Fragments--
To determine whether analysis of small fragments of
Notch is reflective of what happens in the intact molecule, we analyzed for O-fucose and Fringe modification on larger fragments.
Although EGF 1-36 was modified with O-fucose and Mfng,
individual point mutants caused undetectable changes in elongation
(data not shown). Thus, fragments expressing EGF 1-18 and EGF 19-36
were prepared. To evaluate whether some of the same sites are modified
in these larger fragments as in the smaller fragments, a mutation was
introduced into EGF 1-18 at the O-fucose site on EGF 12, and a mutation was introduced into EGF 19-36 at the
O-fucose site on EGF 26. These fragments were analyzed for
Fringe-mediated elongation of O-fucose in the same manner as
the smaller fragments in Fig. 4. (Table II). Mutation of serine on EGF 12 had no
effect on the relative amounts of mono- and multisaccharide on EGF
1-18 in control cells, but it resulted in a significant decrease in
the multisaccharide from the Mfng cell line, suggesting that EGF 12 is
one of the major sites of Mfng action in this larger fragment.
Similarly, mutation of serine on EGF 26 caused a significant decrease
in the amount of multisaccharide on EGF 19-36 from control cells, indicating that EGF 26 is a major site for endogenous Fringe activity. These data indicate that the Fringe effects we observed on smaller fragments such as EGF 11-15 and EGF 24-28 are similar to those seen
with these larger fragments. They also indicate that EGF 12 is a major
target for Mfng and that EGF 26 is a major target for endogenous
Fringes. The fact that we can observe similar Fringe effects from both
the smaller and larger EGF fragments suggests that analysis of the
small fragments of mouse Notch1 accurately predicts how Fringe will act
on the whole Notch protein.
Abruptex Mutants Can Interfere with Fringe Action--
Abruptex
mutants are a class of Drosophila Notch missense mutations
localized in EGF repeats 24-29 (29) that result in a hyperactivatable
form of Notch (30). The Abruptex phenotype is believed to be caused by
abolition of cell-autonomous inhibition of Notch by ligands (31). In
addition, some of the Abruptex mutants are refractory to Fringe (31),
suggesting some relationship between the mechanism of Fringe action and
the Abruptex mutations. Within the Abruptex mutation region (EGF
repeats 24-29), there are several O-fucose modification
sites (Fig. 1). The O-fucose modifications in this region
are also significantly elongated by Fringe (Fig. 4). We were interested
to determine whether any of the Abruptex mutations would alter the
ability of Fringe to induce elongation of O-fucose and
particularly if the mutants would be refractory to Fringe action. We
chose two Abruptex mutants, Ax9B2 (Asp948 To begin understanding how Fringe-mediated alterations on
O-fucose saccharide structure may modulate Notch function,
we have analyzed sites of O-fucose and Fringe modification
on mouse Notch1. During these studies, we identified
O-fucose modifications on sites predicted using the
C2X3-5(S/T)C3
consensus site proposed in earlier studies (10), demonstrating that
this broader consensus site can be used to accurately predict O-fucose modification. The fact that O-fucose
modifies a broader set of sites than previously predicted (summarized
in Fig. 7) increases both the number of
sites on the Notch receptors and the number of proteins predicted to
bear this modification. Many of these sites are evolutionarily
conserved on Notch (Fig. 1b). In particular, the site at EGF
12, believed to be essential for ligand binding (26), is conserved in
the analogous EGF repeat in all Notch known homologues, including
Glp1 and Lin12 from Caenorhabditis elegans. We have also examined the recognition of
O-fucose by Fringe and shown that some EGF repeats in Notch
extracellular domain are modified by Fringe, whereas others are not.
Interestingly, some of the sites modified by Fringe overlap with
functional regions of the Notch extracellular domain (summarized in
Fig. 7). Each of these results points to a potential mechanism for how
elongated O-fucose affects Notch activation.
1,3-N-acetylglucosaminyltransferase that modifies
O-fucose moieties on the epidermal growth factor-like (EGF)
repeats of Notch. To investigate how the change in sugar structure
caused by Fringe modulates Notch activity, we have analyzed the sites
of O-fucose and Fringe modification on mouse Notch1. The
extracellular domain of Notch1 has 36 tandem EGF repeats, many of which
are predicted to be modified with O-fucose. We recently proposed a broadened consensus sequence for
O-fucose,
C2X3-5(S/T)C3
(where C2 and C3 represent the second and third
conserved cysteines), significantly expanding the potential number of
modification sites on Notch. Here we demonstrate that sites predicted
using this broader consensus sequence are modified with
O-fucose on mouse Notch1, and we present evidence
suggesting that the consensus can be further refined to
C2X4-5(S/T)C3.
In particular, we demonstrate that EGF 12, a portion of the ligand-binding site, is modified with O-fucose and that
this site is evolutionarily conserved. We also show that endogenous
Fringe proteins in Chinese hamster ovary cells (Lunatic fringe and
Radical fringe) as well as exogenous Manic fringe modify
O-fucose on many but not all EGF repeats of mouse Notch1.
These findings suggest that the Fringes show a preference for
O-fucose on some EGF repeats relative to others. This
specificity appears to be encoded within the amino acid sequence of the
individual EGF repeats. Interestingly, our results reveal that Manic
fringe modifies O-fucose both at the ligand-binding site
(EGF 12) and in the Abruptex region. These findings provide insight
into potential mechanisms by which Fringe action on Notch receptors may
influence both the affinity of Notch-ligand binding and cell-autonomous
inhibition of Notch signaling by ligand.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
1,3-N-acetylglucosaminyltransferase Fringe (for recent
reviews, see Refs. 12 and 13). Fringe was first identified in
Drosophila and shown to inhibit Notch's ability to respond
to Serrate but to potentiate its ability to respond to Delta (14).
Three Drosophila Fringe homologues have been identified in
mammals: Lunatic fringe (Lfng), Manic fringe (Mfng), and Radical fringe
(Rfng) (15). The
1,3-N-acetylglucosaminyltransferase activity of Fringe proteins is essential for their biological activity
in Drosophila (16-18) and in cell-based Notch signaling assays (19), and the O-fucose residues are required for
Fringe to modulate Notch activity (16, 19). Thus, Fringe mediates its
effects on Notch signaling by the addition of GlcNAc to
O-fucose moieties. Nonetheless, neither the specific role of
O-fucose in Notch function nor the mechanism of how a change
in sugar structure alters Notch function is known.
EXPERIMENTAL PROCEDURES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
2,3-specific sialidase) was obtained from Glyko, Inc., and
-galactosidase (Diplococcal pneumoniae) was from Roche
Molecular Biochemicals. [6-3H]Fucose was purchased from
American Radiolabeled Chemicals, Inc. (St. Louis, MO). A plasmid
expressing full-length mouse Notch1 with a C-terminal Myc tag
(pcDNA-Notch1myc) was generously provided by Dr. Jefferey Nye
(Northwestern University). Drosophila Fringe bearing an
N-terminal His6 tag was generously provided by Dr. Kenneth
Irvine (Rutgers University). The Chinese hamster ovary (CHO) Lec1 cell
line (20) and Lec1 cells stably transfected with either mouse Manic
fringe or control vectors (16) were developed in and generously
provided by the laboratory of Dr. Pamela Stanley (Albert Einstein
College of Medicine). All CHO cells were grown as described previously
(8). Alditol sugar standards were prepared by reduction of the
corresponding sugar with sodium borohydride as described previously
(21). All other reagents were of the highest quality available.
Primer list for production of EGF fragments from mNotch1 and site
mutants
-elimination and subsequent analysis by gel filtration chromatography on a Superdex peptide column was done essentially as
described (8, 16, 22). Tetrasaccharide and trisaccharide forms of
O-fucose were confirmed using exoglycosidase digestions essentially as described (8), although the
N-acetylneuraminidase I (
2,3-specific sialidase) (10 milliunits) and
-galactosidase (Diplococcal pneumoniae)
digestions were done concurrently. The disaccharide GlcNAc
1,3Fucitol
was confirmed using high pH anion exchange chromatography as
described previously (8).
RESULTS
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
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Fig. 1.
O-Fucose sites on the Notch
extracellular domain. a, mouse Notch1 extracellular
domain. b, evolutionarily conserved sites in the
extracellular domain of Notch proteins. Data were obtained by comparing
Drosophila Notch, mouse Notch1 and -2, and human Notch1 and
-2. Ligand binding sites (26) and Abruptex mutation sites (29) are
labeled with L and A, respectively.
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Fig. 2.
Expression and
O-fucosylation of EGF fragments from mouse
Notch1. Plasmids encoding fragments (EGF 1-5, 11-15, 16-18,
19-23, 24-28, and 29-36) of mouse Notch1 extracellular domain were
transfected into Lec1 cells, the cells were metabolically radiolabeled
with [3H]fucose, and the expressed fragments were
purified from medium as described under "Experimental Procedures."
Equivalent amounts of each purified fragment (based on radioactivity)
were analyzed by SDS-PAGE and fluorography (left) or Western
blot analysis with anti-Myc antibodies (right). To more
clearly detect the protein species for EGF 16-18, a second Western
blot was performed with a larger amount of protein (lane
marked EGF 16-18* has approximately 3 times more protein). The migration position of molecular weight
standards (× 10 3) is shown.
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Fig. 3.
Identifying O-fucosylation
sites using mutants of EGF 11-15 and EGF 19-23. a,
sequences between the first and second conserved cysteines for
potential O-fucose sites from EGF 12, 15, 20, 21, and 23 are
shown, with the predicted modification sites underlined.
b, wild type and mutant fragments were expressed in Lec1
cells and analyzed for fucosylation as described under "Experimental
Procedures." Protein levels were determined by Western blot using
anti-Myc antibody. Equivalent amounts of protein were loaded on the
gels for both the Western blot and fluorograph. c, the level
of fucosylation was normalized to the amount of protein by dividing the
cpm for each sample by the amount of protein (determined by
densitometric scanning of the Western blot shown in b). The
data are presented as percentage of wild type (100%).
-elimination and analyzed by gel filtration
chromatography on a Superdex peptide column (8, 16). The structures of
the resulting tetra-, tri-, and disaccharide species were confirmed to
be Sia
2,3Gal
1,4GlcNAc
1,3Fucitol, Gal
1,4GlcNAc
1,3Fucitol, and GlcNAc
1,3Fucitol,
respectively, by a combination of exoglycosidase digestions and high pH
anion exchange chromatography analysis (see "Experimental
Procedures") (8). Since the synthesis of di-, tri-, and
tetrasaccharide is dependent on the action of Fringe (16), the effect
of Fringe can be quantified by calculating the ratio of the elongated,
multisaccharide species (multisaccharide includes di-, tri-, and
tetrasaccharide) to the total amount of O-fucose saccharides
(sum of multisaccharide and monosaccharide species) from each fragment
(Fig. 4).
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Fig. 4.
Mfng modifies O-fucose on
most but not all EGF fragments of mouse Notch1. Plasmids
encoding mouse Notch1 extracellular domain fragments were
transfected into Lec1 cell lines expressing Mfng or empty vector as
shown in Fig. 2. The O-linked sugars derived from each
fragment were size-fractionated on a Superdex peptide column. The
open circles/dashed lines
represent data from control cell lines (C), and
closed circles/solid lines
represent data from Mfng cells (M). The O-fucose
monosaccharide elutes at ~35 min, whereas the tetrasaccharide elutes
at ~17-19 min. Di- and trisaccharide species migrate at ~34 and 33 min, respectively. The structures of tetra-, tri-, and disaccharide
species were confirmed using exoglycosidase digestion and high
pH anion exchange chromatography analysis as described under
"Experimental Procedures." The extent of Fringe-mediated elongation
was quantified by determining the percentage of total radioactivity of
the monosaccharide versus that of multisaccharide species
(sum of di-, tri-, and tetrasaccharide). Each analysis has been
performed at least twice. The data shown here are representative of all
experiments.
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Fig. 5.
Signals for recognition by Fringe are encoded
within individual EGF repeats. a, Fringe assay was
carried out as described previously (16), using either native or
reduced and alkylated factor VII EGF-O-fucose (see
"Experimental Procedures") as substrate. b and
c, plasmids encoding individual EGF 24 (b) or 26 (c) from mouse Notch1 were transfected into control Lec1
cells. The O-fucose saccharides derived from the expressed
proteins were evaluated as described under "Experimental
Procedures." The migration position of tetrasaccharide
(TS) and monosaccharide (MS) forms of
O-fucose are shown.
Fringe modification of O-fucose on EGF 1-18 and EGF 19-36
Val) and Ax59b (Cys972
Gly), for study
because the sites are well conserved across species (Fig.
6). Both of these Abruptex mutations
occur within Drosophila Notch EGF 24. We co-transfected
mouse Notch EGF 24-28, bearing one or the other mutation, with mouse
Mfng or Dfng. Sugar analysis showed that elongation of
O-fucose on Ax9B2 (Asp948
Val)
was not increased by Mfng, indicating that this mutation interferes
with the ability of Mfng to recognize this fragment (Table
III). Dfng could elongate
O-fucose on Ax9B2 (65% multisaccharide), but
less extensively than the wild type EGF 24-28 (78.5% multisaccharide)
(Table III). In contrast, increases in elongation of
O-fucose on Ax59b (Cys972
Gly)
were caused by both Fringes, similar to the wild-type EGF 24-28 (Table
III). These results suggest that although some Abruptex mutations may
be refractory to Fringe action (e.g. Ax9B2), the
Abruptex phenotypes are not necessarily linked to a block in Fringe
action (e.g. Ax39b).
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Fig. 6.
Conservation of sequences from EGF 24 sequence. Notch EGF 24 sequences from different species are
listed. The positions of the Ax9B2 and Ax59b
Abruptex mutation sites (underlined) are shown.
Fringe modification of O-fucose on EGF 24-28 bearing Abruptex
mutations
DISCUSSION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
View larger version (21K):
[in a new window]
Fig. 7.
Summary of O-fucose and
Fringe modification sites on mouse Notch1 extracellular domain.
Ligand binding sites (26) and Abruptex mutation sites (29) are labeled
with L and A, respectively. O-Fucose
sites are identified as described in Fig. 1. , no elongation by any
Fringe; +, individual sites known to be elongated by Fringe; *, sites
in a fragment (e.g. EGF 11-15, 16-18, 19-23, 24-28, and
29-28) elongated by Fringe.
Although our prior work on O-fucosylation of Notch ligands resulted in the proposal of a broader consensus site, C2X3-5(S/T)C3 (10), the results reported here allow us to further refine the consensus. We have demonstrated the presence of O-fucose on three EGF repeats from mouse Notch1 predicted to be modified using the broader consensus: EGFs 12, 21, and 23. Each of these new sites contains four amino acids between C2 and the modified serine or threonine. The fact that the serine prior to C3 in EGF 15 (C2HYGSC3) was not modified suggests that more than three amino acids may be necessary between C2 and the modified Ser/Thr for O-fucosylation, limiting the sites to C2X4-5(S/T)C3. Using this broader consensus site identifies 21 potential O-fucosylation sites on mouse Notch1 (13 evolutionarily conserved), instead of nine sites (five evolutionarily conserved) (Fig. 1). Work to further define the consensus site for O-fucose modification is currently being carried out in our laboratory.
In addition to refining the O-fucose consensus site, we have localized the O-fucose residues on mouse Notch1 modified by Fringe. Analysis of Fringe-mediated elongation of O-fucose on fragments from mouse Notch1 extracellular domain revealed that none of the Fringes (endogenous Lfng and Rfng, exogenous Mfng) modified O-fucose residues on EGF 1-5. Within the fragments that were elongated by Mfng, dramatically different efficiencies of elongation were observed (e.g. O-fucose on EGF 24-28 was elongated to a much greater extent than O-fucose on EGF 19-23). Significantly, Fringe-mediated elongation of O-fucose was observed on regions of the mouse Notch1 extracellular domain corresponding to the ligand binding sites and the Abruptex region (Fig. 7). We showed that a properly folded EGF repeat was required for Fringe recognition, demonstrating that the enzyme recognizes specific features of the three-dimensional structure of an EGF repeat. Analysis of individual EGF repeats (24 and 26) in cells showed that the endogenous Fringes elongated O-fucose on EGF 26, but not on EGF 24. These results suggest that the basic unit of recognition is the EGF repeat and that signals required for Fringe modification are encoded within the individual EGF repeat.
A previous report showed that Dfng can modify O-fucose on a Drosophila Notch EGF 1-3 fragment in vitro (17). Although we found that mammalian Fringes do not modify O-fucose on EGF 1-5 in Lec1 cells, this may reflect the differences in specificity between mammalian and Drosophila Fringe or differences in in vitro versus in vivo assays. Additionally, Shimizu and co-workers (27) have previously compared Lfng and Mfng action on mouse Notch2 by analyzing the shift in migration of the protein on an SDS-PAGE caused by the Fringe-mediated change in sugar structure. Their data suggested that Lfng modifies O-fucose on mouse Notch2 EGF 1-15, whereas Mfng modifies O-fucose on mouse Notch2 EGF 23-29. Whereas analyzing changes in carbohydrate structure using a shift in migration on SDS-PAGE is less definitive than the analysis of Fringe-mediated alterations in O-fucose structures described here, it will be interesting to determine whether the Fringe effect we observed on EGF 12 is specific for Notch1 and Mfng.
Although it is clear that Fringe modulates Notch function by altering O-fucose structure (16, 17, 19), the mechanism by which the change in sugar structure affects Notch activity is unknown. We have previously proposed several potential models for how the change in sugar structure alters Notch function (for details, see Refs. 13 and 16). The present results offer support for two of these models. The first is based on the observation that Mfng modifies O-fucose on EGF 12. EGF repeats 11 and 12 of Drosophila Notch are both necessary and sufficient for interaction with Delta and Serrate (32). In this model, the O-fucose moiety is predicted to be directly involved in binding to ligand or to an accessory protein that influences ligand binding. Thus, the inhibition of Serrate/Jagged signaling by Fringe could be caused by a decrease in binding to Notch due to steric hindrance of the binding site by the elongated sugar structure. A direct role for the O-fucose in Notch-ligand binding interactions is supported by the finding that Notch signaling is reduced in Lec13 cells (which have reduced levels of fucosylation) (16, 19). The recent demonstration that elimination of protein O-fucosyltransferase using RNAi in Drosophila (11) or gene ablation in the mouse2 causes severe Notch-like phenotypes also implicates a direct role for O-fucose in Notch-ligand interactions. The fact that essentially all Notch functions were affected by loss of O-fucose, not just those involving Fringe, strongly suggests that O-fucose modifications are essential for Notch function. This is consistent with a model where O-fucose is part of an essential binding event necessary for Notch activation. Direct effects of Fringe on Notch ligand interactions have been demonstrated in cell binding assays, although some of the results are conflicting. For instance, Shimizu et al. (27) showed that Mfng and Lfng cause a reduction in Jagged1 binding to Notch2 in cell-based binding assays, but other reports using different cell types showed no change in Serrate/Jagged binding caused by Fringe (17, 27, 33). Data showing a Fringe-mediated increase in Delta binding also exist (17), raising the intriguing possibility that Delta could contain a lectin-like activity, specific for the altered carbohydrate structures on EGF 12. More research, such as in vitro binding studies, must be done to better understand how Fringe modification alters Notch interaction with its ligands.
The fact that Fringes modify O-fucose on sites within the
Abruptex region (Fig. 7) supports a second possible model of how the
change in sugar structure could alter Notch function. Cis-interactions between Notch and Notch ligands present in the same cell have been
demonstrated to reduce the ability of Notch to receive signals from
adjacent cells (30). The Abruptex mutants are believed to abrogate this
cell-autonomous inhibition by ligands, resulting in hyperactivatable
forms of Notch (31, 34). Abruptex mutations are located within EGF
repeats 24-29 (29), overlapping with several O-fucose sites
that can be elongated by Fringe (Fig. 7). Interestingly, some Abruptex
mutants are refractory to Fringe, suggesting that the Abruptex
mutations may prevent Fringe from functioning properly (31). The fact
that EGF 24-28 is a major site of modification by Fringe (see Fig. 4)
suggests that Fringe modification on this region may function to
regulate cis-interactions. Here, we identified at least one Abruptex
mutant (Ax9B2: Asp948 Val) that affects
recognition by Fringe. Consistent with our results, Ju et
al. (35) have shown that Ax9B2 abolishes Fringe-Notch
interactions. However, since some Abruptex mutants, such as
Ax59b (Cys972
Gly), can still be elongated
by Fringe, we conclude that there must be additional mechanisms that
cause the Abruptex phenotype.
Our recent demonstration that Fringe modifies O-fucose on
the ligands as well as Notch raises the intriguing possibility that ligand modification could also play a role in modulation of Notch signaling (10). Since the majority of Fringe effects appear to be
cell-autonomous with respect to Notch, the modification of ligands
would most likely affect cis-interactions between Notch and ligands in
the same cell. Determination of which of these models or others are
involved in Notch regulation is an area of continuing research.
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ACKNOWLEDGEMENTS |
---|
We thank Yi Luo, Kelvin Luther, Raajit Rampal, Nadia Rana, and Malgosia Skowron for critical reading of the manuscript and helpful discussions. We also thank Dr. Pamela Stanley (Albert Einstein College of Medicine), Dr. Thomas Vogt (Merck), and Dr. Kenneth Irvine (Rutgers) for providing stably transfected CHO cell lines, mammalian Fringe constructs, and Drosophila Fringe constructs, respectively. We also thank Dr. Irvine and Dr. Stanley for helpful discussions and communication of results prior to publication.
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FOOTNOTES |
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* This work was supported by National Institutes of Health Grant GM 61126.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
To whom correspondence should be addressed. Tel.: 631-632-7336;
E-mail: Robert.Haltiwanger@stonybrook.edu.
Published, JBC Papers in Press, December 16, 2002, DOI 10.1074/jbc.M212221200
2 S. Shi and P. Stanley, submitted for publication.
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ABBREVIATIONS |
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The abbreviations used are: EGF, epidermal growth factor-like; Lfng, Lunatic fringe; Mfng, Manic fringe; Rfng, Radical fringe; Dfng, Drosophila fringe; CHO, Chinese hamster ovary; C1, C2, and C3, conserved cysteine 1, 2, and 3, respectively.
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