From the
Alternate splicing of a single exon encoding an
NH
Two tandem Ig-like disulfide loops (Loops II and III) connected
by an interloop sequence are sufficient for the binding of
glycosaminoglycan and ligand to the fibroblast growth factor receptor
(FGFR)
Here we report that FGFR1
In the accompanying paper
(1) , we demonstrated that
the base ligand-binding site of the FGFR kinase ectodomain that is
shared by FGF-1, FGF-2, and FGF-7 consists of one Ig-like disulfide
loop (Loop II), which contains a heparan sulfate binding domain, a
short interloop sequence that connects Loop II and Loop III, and a part
of the NH
Molecular models suggest that
the interloop exon between Loops I and II may serve as a hinge that
facilitates the interaction of Loop I with Loops II and III in the
FGFR1
The model presented here in which the major
glycosaminoglycan binding domain of FGFR enjoys greater exposure to
nearby co-factors explains the lack of requirement of FGFR
We thank Maki Kan, Kerstin McKeehan, and Suzanne Tran
for excellent technical assistance.
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
FOOTNOTES
ACKNOWLEDGEMENTS
REFERENCES
-terminal immunoglobulin (Ig) disulfide loop in the
ectodomain of the fibroblast growth factor receptor (FGFR) types 1 and
2 results in
and
isoforms that exhibit 3- and 2-Ig loops,
respectively. Previously we demonstrated that alternately spliced Loop
I has no independent ligand binding activity but is sufficiently
interactive with the ligand- and heparin-binding site formed by Loops
II and III to lower affinity for the same fibroblast growth factor
(FGF) ligand. Here we show that a lower affinity of FGFR1
for
heparin parallels the lower affinity for FGF-1. A mutant of FGFR1
in which the sequence between Loops I and II was deleted exhibits high
affinity for both FGF-1 and heparin and other properties of the
FGFR1
isoform, which include resistance to degradation by trypsin
and display of specific antibody epitopes. This suggests that the
interloop sequence facilitates the interaction of Loop I with Loops II
and III. Lack of expression of both exons coding for Loop I and the
sequence between Loops I and II in the FGFR2 gene characterizes rat
prostate tumor cells, which exhibit a loss of the low affinity class of
FGF receptors. Although the exon coding for the sequence between Loops
I and II is alternately spliced in the FGFR2
isoform, coordinate
expression with the exon coding for Loop I results in the functional
differences between the FGFR
and FGFR
variants.
(
)
kinase ectodomain
(1) . Loop II,
the interloop sequence, and the NH
terminus of Loop III
appear to comprise the minimal structure required for the binding of
members of the FGF ligand family in general, while both the invariant
core and COOH-terminal domains encoded in alternately spliced exons of
Loop III determine specificity for particular FGF ligands
(1) .
The NH
terminus of Loop II contributes a conserved heparan
sulfate binding domain that is obligatory for ligand binding
(2) . Alternate splicing of a single exon in the FGFR1 and FGFR2
genes results in an isoform, FGFR
, which exhibits an additional
Ig-like loop (Loop I) NH
-terminal to Loops II and III of
the FGFR
isoform
(3) . Only the 3-loop products of the
FGFR3 and FGFR4 genes have been reported
(3) . The
NH
-terminal Loop I is the least conserved domain among the
FGFR
isoforms (21-38% relative to the 63-82%
conservation between Loops II and III)
(3) . In the FGFR
isoform, Loops I and II are separated by an interloop sequence encoded
by a single exon that exhibits a characteristic cluster of acidic
residues called the ``acidic box''
(3) . The sequence
composes the NH
terminus of the FGFR
isoform, which
can be alternately spliced
(3) . Both the natural variants of
FGFR2
and mutants of FGFR1
with a deletion of all or a
portion of the exon coding for the acidic box sequence had no
qualitative effects on ligand binding
(3, 4, 5) . However, the presence of Loop I
causes recombinant FGFR1
to exhibit an affinity for FGF-1 that is
12.5% of that of the FGFR1
isoform in the presence of heparin and
complete loss of binding to FGF-1 in the absence of the
glycosaminoglycan
(6) . The fact that distal antibody epitopes
within Loop I, the inter-Loop II/III sequence, and the COOH terminus of
Loop III are masked in specifically native FGFR1
relative to
denatured FGFR1
or FGFR1
suggested that Loop I and Loops II
and III are sufficiently interactive to affect both the interaction of
glycosaminoglycan and ligand with the 2-loop structure
(7) .
and FGFR1
exhibit differences in
affinity for heparin that parallel the differences in affinity for
FGF-1. Deletion of the interloop sequence between Loops I and II caused
reversion of multiple properties (affinity for heparin and FGF,
lability to trypsin, display of antibody epitopes) of the FGFR1
isoform to those of the FGFR1
isoform despite the presence of the
Loop I structure. We show that lack of expression of both exons coding
for Loop I and the sequence between Loops I and II in the FGFR2 gene in
prostate tumor cells correlates with the loss of low affinity binding
sites. Although the interloop sequence is alternately spliced in the
FGFR2
isoform, the two exons are expressed coordinately in the
FGFR
variant, which results in unique properties of the isoform.
Construction of Mutant FGFR1
The
following oligonucleotide primer sequences (5` to 3`) were synthesized
on an ABI PCRMATE DNA synthesizer (Applied Biosystems, Inc., Foster
City, CA): P1a, atatgaattcCGAGCTCACTGTGGAGTATCCATG; ADP1,
TTTGGTGTTATCTGTTTCTTTCGAGGAGGGGAGAGCATC; ADP2, AAAGAAACAGATAACACCAAA;
P1b, GACCTTGTAGCCTCCAATTCTGTGG; ADP3, TCCGTCAATGTTTCACCCGTAGCTCCATAT;
and ADP4, ATATGGAGCTACGGGTGAAACATTGACGGA. Residues not in human FGFR1
cDNA are indicated in lowercase, and restriction sites are indicated in
boldface. Mutant cDNA fragments for construction of FGFR1 cDNAs
(
A)
(see Fig. 1) were generated by previously described methods
(1, 2, 8) between primers P1a and ADP1 and
between primers ADP2 and P1b in the polymerase chain reaction (PCR)
using FGFR1
cDNA as a template. PCR reactions were performed as
described
(1) . Mutant fragments for construction of
FGFR1
(
AE) were generated between P1a and ADP3 and between
ADP4 and P1b. After digestion with BglI and BstXI,
the mutant cDNAs were ligated with flanking FGFR1 cDNA sequences at
BglI and BstXI sites and cloned into pBlueScript SK
vector (Stratagene, La Jolla, CA) at EcoRI and SalI
sites. The resulting 1222- and 1273-base pair (bp) cDNAs were recovered
by partial digestion with EcoRI and cloned into mammalian
expression vectors p91023B and the baculovirus transfer vector pVL1393
at EcoRI sites. Recombinant baculoviruses bearing mutant
FGFR1
cDNAs were prepared as described
(1, 7) . The
nucleotide sequences of all cDNA fragments generated by the PCR and the
sequence across ligation sites in all cDNA constructions were
determined.
Figure 1:
Schematic diagram of the FGFR
extracellular domain. The three disulfide Ig loops (I, II, and III),
primer positions, and restriction enzyme sites described under
``Experimental Procedures'' are indicated. Dotted lines indicate sequence deletions, and zigzag lines indicate vector sequence. The translational
initiation site and secretory signal sequence ( S), the acidic
box ( A), the putative heparin binding domain ( HB),
and the transmembrane domain ( TM) are indicated by solid rectangles. The Loop I and interloop exons are indicated
by arrows. Inset, covalent cross-linking of labeled
FGF-1 to wild-type (
,
) and mutant
constructions of the FGFR1
ectodomain expressed in COS-7 cells.
Each analysis is from an assay that contained 5
10
cells.
Binding of Heparin and FGF-1 to Recombinant
FGFR
I-Labeled FGF-1 binding and covalent affinity
cross-linking were performed in transfected COS-7 cells and
baculovirus-infected Sf9 cells as described
(1) . Scatchard
analysis was carried out by addition of various concentrations of
I-labeled FGF-1 and 2 µg/ml heparin. Specific binding
was calculated by subtraction of the amount of labeled FGF-1 bound in
the presence of a 50-fold excess of unlabeled FGF-1. For heparin
binding, Sf9 insect cells were infected with recombinant baculovirus
for 2 days and then harvested and distributed into 1.7-ml Eppendorf
tubes at 5
10
cells/tube by modification of
procedures described for FGF-1 binding
(6) . Concentrations
(0.7-8 µg/ml) of [
H]heparin
( M
= 6000-20,000; 0.7 Ci/mg, DuPont)
were added to each sample in 0.5 ml of binding buffer. After incubation
for 4 h at 4 °C, the cells were collected in 96-well filtration
plates (Millipore, Bedford, MA) and washed three times with
phosphate-buffered saline, pH 7 (PBS). The membrane from each well was
excised, and radioactivity was determined by liquid scintillation.
Specific binding was determined by the difference between total binding
and nonspecific binding remaining in the presence of a 100-fold excess
of unlabeled heparin or the amount of [
H]heparin
bound to an equivalent amount of cells infected with an unrelated
recombinant virus at the same titer.
Tryptic Fragmentation of Recombinant
FGFR1
Complexes of recombinant FGFR1 constructions and heparin
were fragmented with trypsin by modification of a previously described
procedure
(1, 2) . Sf9 cells infected with baculoviruses
bearing recombinant FGFR1 cDNAs were lysed with 0.2% Triton X-100 in
PBS, and receptor products were then immobilized by incubation of the
cell lysates with heparin-agarose beads (1 ml of packed beads/10cells) for 4 h at 4 °C. After washing with PBS, half of the
complex was retained as an untreated control, and the other half was
treated with 100 µg/ml trypsin in PBS at 4 °C for 1 h. After a
wash with PBS containing 2 mM p-methylsulfonyl
fluoride, material retained on the beads was eluted with 1 M
NaCl in PBS, concentrated, and subjected to analysis by sodium dodecyl
sulfate-polyacrylamide gel electrophoresis and immunoblot
(1, 7) .
Analysis of FGFR2 Isoforms in Rat Prostate Tissues by
PCR
Variants of the ectodomain of FGFR2 were analyzed by paired
5` and 3` primers PB1 (5`-gccgaatTCAAGTGTGCAGATGGGATTAACG) and CB
(5`-AGGATGGGCCGGTGCGGTGATCGCTC). Primer PB1 began 67 bp upstream of the
translational initiation site, and CB was complementary to an invariant
sequence within Loop II (see Fig. 1). A 5` primer PB9
(5`-AGCCACCAACCAAATACCAA) complementary to a coding sequence within
Loop I of FGFR2 was paired with the CB primer to detect
specifically the FGFR
isoform. Template cDNAs were prepared, and
the PCR was performed as described previously
(9, 10) .
Impact of Loop I and the Inter-Loop I/II Domains on
FGF-1 Binding
Previous studies indicated that deletion of the
39-bp coding sequence for the 13 core residues (EDDDDDDDSSSEE) that
compose the acidic box (denoted as the A domain in Fig. 1) in the
NHterminus of FGFR1
(5) or splice variants of
the FGFR2 gene that do not exhibit the entire exon (acidic exon
designated AE in Fig. 1) containing the acidic residues
(3, 4) had no effect on the binding of FGF-1 to
recombinant products expressed on the surface of transfected monkey
kidney COS cells. To test whether the two deletions affected ligand
binding to the 3-loop FGFR1
isoform, we constructed mutants
FGFR1
(
A) and FGFR1
(
AE) with deletions of the 13
residues (Glu
-Glu
) of the acidic box
and the whole exon coding for the interloop sequence
(Asp
-Met
), respectively. Neither
deletion had a notable qualitative effect on the amount of FGF-1
covalently cross-linked to recombinant product in assays containing
heparin and
I-FGF-1 at 120 pM (Fig. 1,
inset). We previously reported that although both isoforms
bind FGF-1 when expressed on the surface of baculovirus-infected Sf9
cells, the affinity of FGFR1
for FGF-1 was 12.5% that of
FGFR1
in the presence of heparin
(6) .
Fig. 2
demonstrates that mammalian COS cells transfected with the
FGFR1
ectodomain also exhibit a lower affinity for FGF-1 than
those transfected with FGFR1
. The FGFR1
(
A) construction,
which exhibits a deletion of only the 13-residue acidic box between
Loops I and II, retained the lower affinity exhibited by wild-type
FGFR1
; however, deletion of the entire interloop sequence resulted
in a higher affinity similar to two-loop FGFR1
despite the
presence of Loop I directly at the NH
terminus of Loop II
(Fig. 2).
Figure 2:
Scatchard analysis of
I-FGF-1 binding to variant FGFR1 ectodomains. Specific
binding of different concentrations of
I-FGF-1 to COS-7
cells transfected with the indicated cDNA constructions was performed
as described under ``Experimental Procedures.'' The values
are means of duplicate samples. B, bound ligand; F,
free ligand.
Impact of Loop I and the Interloop Domains on Interaction
with Heparin
When expressed on the surface of
baculovirus-infected Sf9 cells, FGFR1 failed to bind FGF-1 at all
in the absence of heparin relative to the FGFR1
isoform, which
continues to bind FGF-1 with a reduced affinity
(6) . Both
deletions in the FGFR1
(
A) and FGFR1
(
AE)
constructions relieved the absolute requirement for exogenous heparin
for the binding of FGF-1 to wild-type FGFR1
on the surface of Sf9
cells (Fig. 3). Since heparan sulfate glycosaminoglycans bind
directly to the FGFR ectodomain in the absence of FGF
(2) , we
designed a radioreceptor assay to quantitate the binding of
[
H]heparin to recombinant FGFR isoforms expressed
on the surface of infected Sf9 cells. Binding was saturable and in the
presence of unlabeled heparin reversible to a plateau level exhibited
by cells infected with virus bearing an unrelated cDNA. Parallel to the
differences in affinities for FGF-1, FGFR1
exhibited a lower
affinity for the glycosaminoglycan than FGFR1
. Absence of the
13-residue acidic box in FGFR1
(
A) did not lower affinity for
heparin, while deletion of the entire interloop sequence increased the
affinity to that of FGFR1
(Fig. 4). Solubilized ectodomains
immobilized either on antibody-agarose beads or constructions bearing a
fusion partner (glutathione S-transferase) at the COOH
terminus immobilized on GSH-agarose exhibited similar differences (not
shown).
Figure 3:
Requirement for heparin for the binding of
FGF-1 to variant FGFR1 ectodomains. Covalent affinity cross-linking of
I-FGF-1 to Sf9 cells (10
) infected with
recombinant baculovirus bearing cDNA coding for the indicated variant
ectodomains of FGFR1 from binding assays performed in the absence
(
) or presence (+) of 2 µg/ml heparin was analyzed by
SDS-polyacrylamide gel electrophoresis as described under
``Experimental Procedures.''
Figure 4:
Scatchard analysis of
[H]heparin binding to variant ectodomains of
FGFR1. Specific binding of various concentrations of
[
H]heparin in the presence or absence of a
100-fold excess of unlabeled heparin to Sf9 cells expressing the
indicated cDNA constructions was determined. For example, in a typical
binding assay, 10
cells infected with FGFR1
bound
271,540 dpm of labeled heparin, and the same number of cells infected
with virus bearing an unrelated cDNA bound 64,859 dpm. Addition of
excess unlabeled heparin to FGFR1
-infected cells reduced the
binding to near that exhibited by uninfected or mock-infected cells.
The values are the mean of duplicate samples. B, bound ligand;
F, free ligand.
The interaction of the Sf9 cell-derived recombinant
FGFR ectodomain with heparin protects an active 30-kDa core
fragment against proteolysis by trypsin
(2) (Fig. 5). In
contrast, heparin failed to protect the FGFR1
isoform from
proteolysis. The FGFR1
(
A) product retained the properties of
intact FGFR1
, while the FGFR1
(
AE) product exhibited
properties of FGFR1
despite the presence of Loop I.
Figure 5:
Differential protection of variant
ectodomains of FGFR1 against tryptic degradation in the presence of
heparin. The indicated variants of the FGFR1 ectodomain were absorbed
on heparin-agarose beads from lysates of baculovirus-infected 10Sf9 cells. Equal portions of the beads were then incubated in the
absence ( top) and presence ( bottom) of trypsin as
described under ``Experimental Procedures.'' Eluates of the
beads were then analyzed by SDS-polyacrylamide gel electrophoresis and
immunoblot.
Impact of Loop I and the Interloop Domains on Reaction of
FGFR1
Specific antibody
epitopes within defined sequence domains in Loop I and the two loops of
FGFR1 with Domain-specific Antibodies
are masked by the native conformation of the FGFR1
isoform
(7) . To determine whether the inter-Loop I/II sequence
affected the display of epitopes within native FGFR1
, we tested
the reaction of the mutant construction products with monoclonal M5G10,
which reacts with an epitope within the sequence spanning the COOH
terminus of Loop II, the inter-Loop II/III sequence, and the NH
terminus of Loop III
(7) . As reported previously
(7) , M5G10 fails to react with native FGFR1
but reacts
with denatured FGFR1
(Fig. 2) and FGFR1
(Fig. 6).
Deletion of the 13 residues of the acidic box and the entire interloop
exon sequence in the mutant FGFR1
(
A) and the
FGFR1
(
AE) constructions, respectively, restores reactivity
with antibody M5G10 (Fig. 6).
Figure 6:
Differential reactivity of variant native
ectodomains with monoclonal antibody M5G10. The indicated native
recombinant ectodomains from lysates of baculovirus-infected Sf9 cells
were reacted with anti-FGFR1 mouse monoclonal antibody M5G10 and rabbit
polyclonal antiserum (Ab) A50 immobilized to protein A beads as
described (7). Extracts of immunocomplexes of both antibodies were
subjected to SDS-polyacrylamide gel electrophoresis (denaturing
conditions) and analyzed by immunoblot using antibody M5G10 followed by
reaction with rabbit anti-mouse IgG conjugated to alkaline
phosphatase.
Lack of Expression of FGFR2
A distinctive difference between isolated epithelial
cells from normal rat prostate and transplantable rat prostate tumors
is the loss of the low affinity class of FGF receptor sites in the
tumor cells
(11) . To determine whether the difference reflected
loss of expression of the Isoforms in Rat Prostate
Tumors
isoform of the FGFR2 gene, which is
selectively expressed in the epithelial cells relative to the FGFR1
gene
(9) , mRNAs coding for isoforms of the FGFR2 ectodomain
were analyzed by PCR in normal prostate; a slow growing, well
differentiated, androgen-responsive tumor (Dunning R3327PAP tumor); and
a malignant variant that arises from the latter after castration of
male hosts (Dunning R3327AT3 tumor)
(9) . Normal prostate tissue
exhibits three mRNA species characteristic of FGFR2
at 849 bp,
FGFR2
at 585 bp, and FGFR2
(
AE) at 504 bp, while both
types of tumors exhibited predominantly the FGFR2
(
AE) isoform
(Fig. 7 A). The FGFR2
isoform exhibiting the
NH
-terminal AE sequence is detectable in Dunning R3327PAP
tumors but undetectable in the Dunning R3327AT3 tumors. Analysis of the
cDNA template from normal prostate tissue with a primer complementary
to specific coding sequences for Loop I of FGFR2
revealed a single
664-bp band characteristic of the FGFR2
isoform containing the
interloop exon coding sequence AE (Fig. 7 B). No band at
583 bp indicative of the FGFR2
(
AE) variant could be detected.
Figure 7:
Lack of expression of FGFR2 in rat
prostate tumors. A, paired primers for invariant coding
sequences of the FGFR2 ectodomain that span the exons coding for Loop I
and the inter-Loop I/II sequence (Fig. 1) were employed in the PCR with
cDNA templates from the indicated rat prostate tissues. AT3,
fast growing, undifferentiated malignant Dunning R3327AT3 tumor;
DT, slow growing, nonmalignant, differentiated Dunning
R3327PAP tumor; NP, normal rat prostate tissue. B, a
Loop I-specific 5` primer was employed together with the same 3` primer
(CB) used in panel A. NP, normal prostate;
, FGFR2
cDNA; -, no cDNA template. Lanes at the right of panels A and B are
standards.
terminus of Loop III extending through lysine
189. A constitutive structural domain just downstream of lysine 189
within Loop III and mutually exclusive alternately spliced exons coding
for the COOH-terminal half of Loop III interact to determine the
affinity for FGF-2 and whether FGF-7 binds at all. In addition to
tandem Loops II and III, which compose the FGFR
isoform, alternate
splicing of a single exon in the FGFR1 and FGFR2 genes results in an
isoform (FGFR
) containing an NH
-terminal Ig-like loop
(Loop I) that is connected to Loop II by an interloop sequence that is
also encoded by a single exon. In previous reports, we have proposed
that although Loop I does not directly participate in formation of a
ligand-binding site, it is sufficiently interactive with the
base-binding site formed by structural domains within Loops II and III
to alter affinity for the same FGF ligand and to influence the nature
of the interaction of the base-binding site with heparin
(6, 7) . In this report, we show by Scatchard analysis
that FGFR1
exhibits a lower affinity for heparin that parallels
its lower affinity for FGF. This and the fact that heparin fails to
protect the active binding site from degradation by trypsin in the
FGFR1
isoform confirmed that the nature of the interaction of the
glycosaminoglycan with FGFR1
is quite different from that of
FGFR1
and may underlie the differences in affinity between the two
splice variants for the same ligand.
isoform
(7) . Here we show that a mutant of
FGFR1
(FGFR1
(
AE)) in which the inter-Loop I/II sequence
was deleted exhibits properties of FGFR1
despite the presence of
Loop I immediately adjacent to the NH
terminus of Loop II.
This included high affinity binding to both FGF and heparin, loss of
the absolute requirement for heparin for FGF-1 binding, resistance to
tryptic degradation in the presence of heparin, and display of antibody
epitopes. Deletion of only the 13 residues that compose what is
commonly referred to as the acidic box within the interloop sequence
was insufficient to abrogate the low affinity of the FGFR1
isoform
for FGF or heparin or the resistance of the active core of the
ligand-binding site to trypsin in the presence of heparin. This
suggests that the remaining 17 residues of the 30-residue interloop
sequence are sufficient to facilitate the functional impact of Loop I
with the active ligand- and heparin-binding site. It is noteworthy that
deletion of the 13-residue sequence in FGFR1
was sufficient to
expose a monoclonal antibody epitope within Loops II and III that is
normally hidden and relieved the qualitative requirement for heparin
for FGF-1 binding to the FGFR1
variant despite the fact that the
deletion had no impact on the K
for
heparin and FGF-1.
relative to FGFR
for exogenous heparin when expressed on the
surface of baculovirus-infected Sf9 insect cells, which express
unidentified mimetic co-factors for the FGFR ectodomain
(6) but
do not express the type and amounts of long chain heparan sulfate
glycosaminoglycans comparable with mammalian cells.
(
)
The impact of the differences between the FGFR
and
FGFR
ectodomains in requirement for heparan sulfates in a
physiological context remains to be established. Although differences
in the binding of FGF-1 and FGF-2 to FGFR1
and FGFR1
isoforms
are quantitative when heparin is used as the glycosaminoglycan in the
FGFR ternary complex, the presence of Loop I may restrict the range of
natural heparan sulfate co-factors that will support the ternary FGFR
complex with a particular FGF ligand. Regulation of the ratio of
expression of FGFR
and FGFR
at the level of alternate
splicing has been reported. In well differentiated human hepatoma cells
that are growth-inhibited by high concentrations of FGF, expression of
the
isoform of the FGFR1 gene exceeds that of the
variant,
and only the
ectodomain is combined with a kinase-defective
intracellular domain, which also arises by an alternate splice event
(6) . A selective reduction in expression of the FGFR1
variant correlates with malignancy of human pancreatic
(12) and
brain
(13) tumors. Here we showed that lack of expression of
the
splice variant of the FGFR2 variant characterizes
transplantable rat prostate tumors relative to normal prostate tissue.
This correlates with the loss of low affinity receptors on cells
derived from the tumors relative to normal epithelial cells
(11) . The FGFR2 gene is specifically expressed in the
epithelial cells from normal prostate and well differentiated prostate
tumors that exhibit both epithelial and stromal compartments
(9) . Alternate splicing of exons coding for the COOH terminus
of Loop III of FGFR2 determine whether the epithelial cells respond to
stromal cell-derived FGF-7 or to other FGF ligands that are abnormally
activated in malignant tumors
(9) . Last, we showed that, in
normal prostate tissue that expresses the FGFR2
isoform, the
single exons coding for Loop I and the inter-Loop I/II sequence are
coordinately expressed, although the interloop exon is alternately
spliced in the FGFR2
variant
(3, 4) . This is
consistent with the failure to demonstrate the
AE variant of
FGFR1 and FGFR2 in a variety of tissues
(3, 13)
(
)
and the results of this
study that indicate that the splice variant would be functionally
redundant in respect to the FGFR
isoform.
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.