(Received for publication, September 16, 1996)
From The R. W. Johnson Pharmaceutical Research Institute, Drug Discovery Research, Raritan, New Jersey 08869
Mutagenesis of the erythropoietin receptor (EPOR) permits analysis of the contribution that individual amino acid residues make to erythropoietin (EPO) binding. We employed both random and site-specific mutagenesis to determine the function of amino acid residues in the extracellular domain (referred to as EPO binding protein, EBP) of the EPOR. Residues were chosen for site-specific alanine substitution based on the results of the random mutagenesis or on their homology to residues that are conserved or have been reported to be involved in ligand binding in other receptors of the cytokine receptor family. Site-specific mutants were expressed in Escherichia coli as soluble EBP and analyzed for EPO binding in several different assay formats. In addition, selected mutant proteins were expressed as full-length EPOR on the surface of COS cells and analyzed for 125I-EPO binding in receptor binding assays. Using these methods, we have identified residues that appear to be involved in EPO binding as well as other residues, most of which are conserved in receptors of the cytokine receptor family, that appear to be necessary for the proper folding and/or stability of the EPOR. We present correlations between these mutagenesis data and the recently solved crystal structure of the EBP with a peptide ligand.
Erythropoietin (EPO)1 is a
glycoprotein hormone that functions as the primary regulator of
erythropoiesis by binding a specific receptor (EPOR) on the surface of
erythrocyte precursor cells, signaling their proliferation and
differentiation into mature red blood cells (reviewed in Ref. 1). The
human EPOR is a 484-amino acid glycoprotein comprised of extracellular
and cytoplasmic domains of nearly equal size and a single transmembrane
domain (2). The extracellular domain of the EPOR contains a 225-amino
acid region referred to as the cytokine receptor homology (CRH) domain that shares conserved features with an expanding family of cytokine and
growth factor receptors (3, 4) including the receptors for many
interleukins (IL), colony stimulating factors, growth hormone (GH),
thrombopoietin, leptin, interferons (IFN), and tissue factor among
others. The CRH domains consist of two motifs of approximately 100 amino acids each that are structurally related to fibronectin type III
domains. Based on this homology, Bazan (3, 4) proposed that the CRH
domains consist of two motifs of seven -strands each which adopt
-sheet structures with fibronectin type III-like or immunoglobulin
(Ig)-like folds. These structural predictions have been confirmed by
the solution of the crystal structures of the ligand-bound
extracellular domains of the GH receptor and IFN-
receptor
(IFN-
R
), the GH bound extracellular domain of the prolactin
receptor, the extracellular domain of tissue factor, and most recently,
the extracellular domain of the EPOR with a peptide ligand (5-10).
Alignment of the CRH domains of this family of receptors based on their
predicted
-strand secondary structural elements reveals several
conserved characteristics (3, 4, 11). Of these, the most highly
conserved are four spatially conserved cysteine residues that form two
cysteine bridges in the amino-terminal domain and a WSXWS
sequence at the membrane proximal end of the carboxyl-terminal domain.
The overall amino acid identity between CRH domains in receptors of
this family is generally less than 25% (11). Thus, the general
structural topography of these domains is more highly conserved than
the primary amino acid sequences.
Amino acid residues involved in ligand binding have been identified in
several receptors of the cytokine receptor family using alanine
substitution site-specific mutagenesis. The importance of many of these
ligand binding determinants was subsequently confirmed in structural
studies. For example, Woodcock et al. (12) used alanine
substitution mutagenesis on the common -chain (
c) of
the human GM-CSF, IL-3, and IL-5 receptors to identify amino acid
residues critical for the formation of the high affinity receptors for
GM-CSF and IL-5. Using mutation complementation, they also identified a
residue in GM-CSF that is likely to interact with the critical residues
on the
c. Support for these results was provided by a
molecular model of the GM-CSF receptor complex (ligand,
-chain, and
c) in which the binding determinants identified by
mutagenesis were predicted to be within bonding distance of each other
(13). Alanine substitution mutagenesis has also been used to identify
important ligand binding determinants in the extracellular domain of
the growth hormone receptor (14), and many of these residues were
subsequently shown to interact with residues on GH in the crystal
structure of the GH-growth hormone receptor complex (5). Although the
structure of the EBP with a peptide ligand has recently been determined
(10), the precise nature of the EPO-EBP interaction remains to be
determined. Site-specific mutagenesis provides a sensitive method for
investigating which residues of the extracellular domain of the EPOR
are involved in EPO binding. In retrospect, we are able to compare the
EBP mutagenesis data to the EBP-peptide structure which may help
identify amino acid residues that are involved in EPO binding.
Previously, we reported that a single residue, Phe93, is a critical ligand binding determinant of the EPOR (15). Here, in a more extensive mutagenesis analysis of the 225-amino acid extracellular domain of the EPOR, we report the effect on EPO binding of single amino acid substitutions created by site-specific mutagenesis. In some cases, residues were selected for site-specific mutagenesis based on random mutagenesis results. The functions of other amino acid residues were investigated based on their conservation among receptors of the cytokine receptor family or their homology to ligand binding determinants reported for other receptors of this family. Wild-type and mutant proteins were expressed in Escherichia coli as soluble EPO binding proteins (EBP), consisting of amino acids 1-225 of the extracellular domain of the mature EPOR (16), and analyzed for EPO binding in several different assay formats. Some mutant proteins were expressed as full-length receptor on the surface of COS cells and analyzed in receptor binding assays with 125I-EPO. We have identified residues that appear to be involved in EPO binding as well as other residues, most of which are conserved in receptors of the cytokine receptor family, that appear to be necessary for the proper folding and/or stability of the EPOR. From these results, we can begin to build the structure-activity relationships that will help to delineate the EPO binding face of the EPOR.
The cloning,
expression, protein recovery, and refolding of the EBP has been
described in detail elsewhere (16). Briefly, the nucleotide sequence of
the first 225 amino acids of the extracellular domain of the human EPOR
(EBP) was cloned into plasmid pSAM3 that contains a synthetic pelB
signal sequence. Expression of the EBP was under the control of the T7
promoter in E. coli strain BL21(DE3)pLysS and was induced by
the addition of isopropyl--D-thiogalactopyranoside for
3-5 h at 37 °C. The EBP protein was produced as insoluble protein
localized to inclusion bodies. The EBP was recovered, solubilized, and
refolded as described (16). The protein refolding process was monitored
by high performance-size exclusion liquid chromatography (HP-SEC), and
the active protein was detected by peak shift analysis through the
addition of purified recombinant EPO.
Mutant EBPs created by site-specific mutagenesis were expressed in E. coli and purified as described for the wild-type EBP (16). Some mutant EBPs were purified by a modification of these methods in which the hydrophobic interaction chromatography step was replaced by a preparative high performance-size exclusion chromatography (HP-SEC) step. The purity of mutant and wild-type EBP was estimated by SDS-polyacrylamide gel electrophoresis and analytical HP-SEC as described (16). The concentrations of purified mutant and wild-type EBP were estimated using the experimentally determined extinction coefficient for the wild-type EBP of 2.3 absorbance units per mg/ml at 280 nm (16).
EBP-bead Binding AssayEBP mutants were assayed in the EBP-bead binding assay as described previously (16). Briefly, 50 µl of EBP beads (wild-type EBP covalently attached to agarose beads) were added to tubes containing varying amounts of mutant EBP protein and 0.5 nM 125I-EPO (DuPont NEN, approximately 100 µCi/µg). The reaction volume was increased to 0.5 ml with phosphate-buffered saline, 0.2% BSA, and the tubes were rocked gently at room temperature overnight. The reaction mixture was loaded onto a 1.0-ml micro column (Isolab), and the trapped beads were washed 3 × with 1-ml washes of phosphate-buffered saline, 5.0% BSA. The columns, containing the EBP-bound 125I-EPO, were counted in a gamma counter (ICN Micromedic, Huntsville, AL).
MutagenesisRandom mutations were introduced into the EBP
using the method of Cadwell and Joyce (17). The EBP DNA was amplified
by the polymerase chain reaction (PCR) under conditions that would
increase incorrect base incorporation by Taq polymerase.
EBP-specific PCR primers contained a SalI restriction site
on the 5 primer and a SpeI restriction site engineered
within the 3
primer. Plasmid pCOMB3 DNA (gift of C. Barbas, Scripps
Research Institute) was cut with XhoI and SpeI.
Due to an internal XhoI site in the EBP, the EBP-specific
PCR primers contained a SalI restriction site which is
compatible with XhoI. The PCR reaction conditions were as
follows: 10 mM Tris-HCl, pH 8.3, 50 mM KCl, 7 mM MgCl2, 0.5 mM MnCl2,
0.2 mM dGTP, 0.2 mM dATP, 1 mM
dCTP, 1 mM dTTP, plus 5 units of Taq polymerase
per 100-µl reaction. Reaction cycle conditions were 94 °C for 1 min, 55 °C for 1 min, 72 °C for 1 min, for 25 cycles on a thermal
cycler (Perkin-Elmer 9600). The resulting amplification product was cut
with restriction enzymes and ligated into the vector pCOMB3 generating
a fusion with the carboxyl-terminal half of the M13 geneIII protein
(18). The DNA was then transformed into E. coli strain XL-1
blue (Stratagene, La Jolla, CA).
In most cases site-specific mutants were generated using the polymerase chain reaction (PCR) in which single primers adjacent to convenient restriction endonuclease sites were designed to include the specific desired mutation. Other mutants required the use of overlap PCR (19) to facilitate site-specific mutagenesis. All mutants were verified by DNA sequence analysis to confirm the presence of the desired mutation and the absence of any unintentional mutations.
Colony Ligand BlotBacterial colonies containing the random
EBP library were tested for 125I-EPO binding using a colony
ligand blot assay. Transformed bacterial colonies were picked and grown
on LB-amp plates at 37 °C for 4 h. The plates were overlaid
with nitrocellulose filters soaked in 5 mM
isopropyl--D-thiogalactopyranoside and placed at
30 °C overnight. The filters were removed from the plates and placed in a chloroform chamber for 15 min and subsequently incubated in
lysozyme buffer (50 mM Tris-HCl, pH 8.0, 150 mM
NaCl, 5 mM MgCl2, 3% BSA, 0.4 mg/ml lysozyme,
1 unit/ml DNase) for 1 h with shaking. The filters were incubated
in 0.2 M Tris-HCl, pH 8.0, 5 M urea for 45 min,
followed by 45 min in 50 mM Tris-HCl, pH 8.0, 0.1 M urea. The filters were blocked for 1 h in 5%
non-fat milk, 3% BSA in Tris-buffered saline (TBS). Fresh,
milk/BSA/TBS containing 125I-EPO (200-300 pM)
was added, and the filters were incubated for 2 h at room
temperature with shaking. The filters were washed 2 × for 5 min
with milk/BSA/TBS, 2 × for 5 min with TBS, and then dried and
exposed to x-ray film (Kodak). Selected non-binding colonies were
analyzed by PCR to verify the presence of the EBP insert. Clones
carrying the insert were grown in liquid culture, induced for 3 h
with 1 mM
isopropyl-
-D-thiogalactopyranoside, and analyzed by
Western blot using anti-EBP antiserum. Selected non-binding clones
confirmed to contain the EBP insert and produce a fusion protein of the
expected size were grown for DNA purification and sequence
analysis.
COS cells were maintained in DMEM (Life Technologies, Inc.), 10% fetal calf serum (FCS, Hyclone Laboratories, Logan, UT), 1% L-glutamine, 1% penicillin, 0.1% streptomycin. Cells were seeded into 100-mm tissue culture dishes at approximately 1.0 × 106 cells per plate. The cells were transfected with the EPOR-containing plasmid (wt or mutant) using 10 µg of DNA/plate and DEAE-dextran (0.14 mg/ml) in DMEM minus fetal calf serum. The cells were incubated overnight at 37 °C with the DNA mixture. Cells were then washed with DMEM-FCS and incubated in DMEM-FCS containing 100 µM chloroquine for 2.5 h at 37 °C. The cells were washed with DMEM-FCS; fresh media were added, and the cells were incubated for 48 h prior to use in the EPO binding assay.
COS Cell Binding AssayTransfected COS cells were removed from the plates with dissociation buffer (phosphate-buffered saline, 0.5 mM EDTA). Cells from duplicate plates were pooled, centrifuged, washed, and resuspended in binding buffer (RPMI 1640, 5% BSA, 25 mM Hepes, pH 7.5, 0.02% sodium azide) and counted. Binding assays were essentially as described (20). Briefly, tubes containing cells and varying concentrations of 125I-labeled EPO were incubated overnight at 4 °C. Nonspecific binding was determined by the addition of at least 100-fold excess of unlabeled EPO. Following binding, the tubes were centrifuged at 12,000 rpm for 1 min at 4 °C. The supernatant was removed; the cell pellet was resuspended in 100 µl of binding buffer, and the cell suspension was layered onto 0.7 ml of bovine calf serum. The tubes were then centrifuged at 12,000 rpm for 5 min at 4 °C. The supernatant was removed, and the bottom of the tubes were snipped off and counted in a gamma counter. The binding data were transformed using the method of Scatchard (21) in order to determine binding affinities. Detection of EPOR on the surface of COS cells was performed using an anti-EBP monoclonal antibody in a modification of the receptor binding assay described above. This procedure for cell surface detection has been described previously (15).
Inhibition of Proliferation AssayFDC-P1 cells, stably transfected with the human EPO receptor lacking the terminal 40 amino acids (FDC-P1tHER) and demonstrating EPO responsiveness, were used for inhibition of proliferation assays. Briefly, cells were maintained in RPMI 1640 containing 10% fetal calf serum, 2 mM L-glutamine, penicillin/streptomycin, and 1.0 unit/ml recombinant human EPO. Cells were deprived of factor for 18 h and subsequently added to 96-well tissue culture plates at a density of 4 × 104 cells/well. Each well contained cells, 0.3 units/ml EPO, and the wt EBP or EBP mutants at various concentrations. Plates were incubated at 37 °C for approximately 42 h, after which the cells were pulsed with 1.0 µCi/well of [3H]thymidine (20 Ci/mmol, DuPont NEN) for 6 h at 37 °C. Cells were harvested onto glass fiber filters (GF/A) using a cell harvester (Tomtec), and the filters were counted in a scintillation counter (LKB Betaplate 1205).
Previously, the EBP was expressed in E. coli, purified, and extensively characterized (16). The EBP is capable of binding EPO with low nM affinity and can be purified in mg quantities from laboratory scale fermentations, making it suitable for mutagenesis studies directed toward the elucidation of the EPO binding face of the EPOR. EBP residues were chosen for site-specific mutagenesis based on results of a random mutagenesis study or on their homologous locations to conserved and non-conserved residues reported to be involved in ligand binding in other receptors of the cytokine receptor family. Generally, our approach was to screen for mutations resulting in decreased EPO binding activity by testing crude preparations of the mutant EBPs in a high performance-size exclusion chromatography (HP-SEC) assay (16). Although not quantitative, the HP-SEC assay does not require purification of the mutant proteins, is quick and easy to perform, and is capable of identifying mutations resulting in large reductions in EPO binding (15, 16). This assay was successful in identifying an EBP mutant with a 1,000-fold increased IC50 for EPO (15); however, it may not be sensitive enough to detect less dramatic effects on EPO binding, since an EBP mutant (L96A-EBP) with a 200-fold increased IC50 for EPO still showed EPO binding. Following HP-SEC analysis, selected mutant proteins were purified and analyzed in competition binding assays or expressed and characterized as full-length receptor on the surface of COS cells. Using this strategy, we have created over 40 single alanine substitution mutants in the extracellular domain of the EPOR. Most of these mutants were initially analyzed for EPO binding activity in the HP-SEC assay, and approximately half were purified and further characterized in the EBP-bead and/or inhibition of proliferation assays.
Site-Specific Mutagenesis of Amino Acids in the EBPTo
identify amino acid residues of the EBP involved in EPO binding, random
mutagenesis was performed using a PCR mutagenesis method (17). Random
mutants were evaluated for the ability to bind 125I-EPO in
a colony ligand blot assay, and the expression of mutants that did not
bind EPO was confirmed by Western blot analysis of cell lysates using
anti-EBP antiserum. For random mutants exhibiting EBP expression but no
EPO binding, DNA encoding the EBP was sequenced to determine the
location(s) of the mutation(s). Two mutations (V196D and A198V),
resulting in the loss of EPO binding as evaluated by colony ligand blot
analysis, involved residues located in the F
-strand of the EPOR
(data not shown). The F
-strand is a highly conserved region in
receptors of the cytokine receptor family (Fig. 1, Ref.
3). This amino acid conservation, along with the random mutagenesis
results, led to the selection of this region for site-specific
mutagenesis. Single alanine substitution mutants were made for all of
the residues from Leu186 through Met200,
located in the F
-strand and in the loop between the E
and F
-strands (see Fig. 2). The mutant EBPs were expressed
in E. coli, and crude preparations were tested for EPO
binding in the HP-SEC assay. In this assay, EBP bound to EPO is
detected as a peak with a shorter retention time than that of EBP alone
(Fig. 3; Ref. 16). All of the mutants spanning
Leu186 through Met200 exhibited a specific
EPO-EBP-bound peak in this assay, indicating that they were all capable
of binding EPO (data not shown) and suggesting that none of the single
alanine substitution mutants results in a 1,000-fold or greater
reduction in EPO binding.
To assess the effects of mutations in this region, the EPO binding
activity of selected mutants at both conserved and non-conserved residues of the F
-strand was further characterized. The R189A-, R197A-, and M200A-EBP mutants were purified and assayed for EPO binding
in the EBP-bead assay. The R189A and R197A mutations resulted in a
2-3-fold increase in the IC50 value, whereas the M200A
mutation had a greater effect on EPO binding, resulting in a 16-fold
increase in the IC50 (Fig. 4A,
Table I). Full-length EPORs containing the R189A, R197A
F194A, V196A, or R199A mutations were expressed in COS cells and tested
for the ability to bind 125I-EPO. These mutant receptors
bound EPO with affinity values comparable to wild type (Table I). Cell
surface expression levels of the R197A-EPOR and R199A-EPOR in COS cells
were low (
6,300 and 9,800 receptors per cell, respectively) compared
with the wild-type EPOR (consistently
65,000 receptors per cell) or
the R189A-, F194A-, and V196A-EPOR (65,250, 43,150, and 77,500 receptors per cell, respectively, data not shown). The function of the
conserved arginines in the F
-strand (Arg197 and
Arg199, see Fig. 1) may involve receptor folding,
processing, and/or cell surface expression.
|
The sequence conservation in the C -strand was an attractive target
for structure-function studies using site-specific mutagenesis (Fig.
1). Alanine substitutions were introduced at conserved residues Tyr53, Phe55, and Tyr57 and at
non-conserved residue Gln58 in the C
-strand (Fig. 2).
Alanine substitutions were also introduced at non-conserved residues
Glu60 and Glu62 in the loop between the C and D
-strands (Fig. 2). The F55A-, Y57A-, and to some extent Y53A-EBP did
not fold efficiently as judged by analytical HP-SEC experiments (Fig.
3). Although expression levels were normal, the F55A- and Y57A-EBP
exhibited a relatively small peak at the retention time corresponding
to that of the active wt EBP after 1 day of refolding. Protein in this
peak did not accumulate with time (6 days of refolding), as was
observed for the wt EBP and for EBPs with mutations in non-conserved
residues in this region (Fig. 3). The small amount of F55A- and
Y57A-EBP in this peak appeared to be active, as evidenced by the
diminution of the peak upon the addition of excess EPO (Fig. 3);
however, there was not enough protein present to purify and
characterize further.
Despite reduced yields, sufficient amounts of the Y53A-EBP folded for purification and characterization of this mutant. The Y53A-EBP exhibited a 12-fold increase in IC50 value relative to wt EBP in the EBP bead assay (Fig. 4B, Table I). The Q58A-EBP was purified in good yields and exhibited only a 4-fold increase in IC50 value (Fig. 4B, Table I), even though this residue is immediately adjacent to the conserved Tyr57, which when substituted with alanine had a dramatic effect on refolding and protein yield (Fig. 3). The C-D loop mutations in the non-conserved residues E60A and E62A resulted in mutant proteins that folded well (Fig. 3) and were purified in good yields. The E60A- and E62A-EBP exhibited increased IC50 values in the EBP bead assay of 20- and 10-fold, respectively, suggesting that these residues may be involved in EPO binding (Fig. 4B, Table I). Comparable IC50 values for the Q58A- and E60A-EBP were obtained in the inhibition of proliferation assay, confirming the effects of these mutations (Table I).
An examination of the cytokine receptor family alignment in Fig. 1
reveals that Trp40 in the EPOR is analogous to an amino
acid that is invariant in both the class 1 (CX)
and class 2 (LX
) receptors, providing another
conserved amino acid to mutate. The W40A-EBP mutant did refold
following expression in bacteria; however, the mutant protein did not
appear to bind EPO in the HP-SEC assay or bound at a very low level
(Fig. 5). The W40A-EBP was purified and subsequently tested in the EBP-bead and inhibition of proliferation assays. The
IC50 value determined for the W40A-EBP was 0.8 µM for the EBP-bead assay (Fig. 4B, Table I).
An IC50 could not be determined for the W40A-EBP in the
inhibition of proliferation assay, although the trend of the inhibition
would suggest the IC50 to be greater than 1 µM (Table I). These IC50 values are
130-160-fold higher than the wild-type EBP. The W40A mutation was then
expressed as a full-length EPOR in COS cells and failed to bind EPO in
the whole cell binding assay (Table I). Further investigation of this
mutant full-length receptor utilizing a radiolabeled antibody cell
surface detection method (15) revealed no detectable cell surface
expression of the W40A full-length receptor (data not shown). These
data suggest that W40A is a critical structural determinant of the
EPOR.
Additional Trp residues that are conserved in these receptors are contained within the WSXWS motif, itself a conserved feature of the cytokine receptors (Fig. 1). Previously reported data demonstrated that mutations to the WSXWS motif of the EPOR resulted in drastic effects on cell surface expression and ligand binding (22, 23). We chose to evaluate the effect of more conservative substitutions at the Trp residues of this motif. The W209Y and W212F mutants reported here retained an aromatic side chain moiety in these amino acid positions. These mutations had little effect on EPO binding, resulting in IC50 values increased by 8- and 3-fold for the W209Y and W212F mutants, respectively (Table I).
Several receptors in the cytokine receptor family have residues
important for ligand binding located in the loop between the B and C
-strands (5, 12, 14). We have reported previously that
Ser152 in the B
-C
loop of the EPOR may have a role in
EPO binding, based on the 16-fold increase in IC50 value of
the S152A-EBP (15). Alanine substitution of Met150 and
His153 in this loop resulted in slight increases in
IC50 for EPO, indicating that these residues probably do
not make a significant contribution to EPO binding. To expand our
investigation of the function of residues in this region of the EPOR,
we substituted alanine for the charged, aromatic, and polar residues
(Arg155, Tyr156, Glu157,
Asp159, and Ser161) in the predicted C
-strand (residues 154-161, see Fig. 2). These amino acids are not
conserved in the cytokine receptor family, although an aromatic residue
does appear to be maintained for many of the cytokine receptors at
positions analogous to Tyr156. Crude preparations of these
mutant EBPs bound EPO in the HP-SEC assay, indicating that none of
these mutations resulted in a dramatic reduction (
1,000-fold) in EPO
binding (data not shown).
Chemical modification of the primary amines present in the EBP with NHS-biotin eliminates EPO binding (data not shown), suggesting that one or more lysine residues (Lys10, Lys14, Lys65) might contribute to ligand binding. Earlier work with truncated forms of the EBP showed that elimination of the first 10 amino acids of the EBP (including Lys10) had no gross effect on the ability of this mutant EBP to bind EPO. This mutant folds properly and is capable of binding EPO as analyzed by the HP-SEC assay (data not shown). A K14A full-length EPOR mutant expressed in COS cells demonstrated a Kd of 1.6 nM, which is a 3-fold increase relative to the wild-type EPOR (Table I). This relatively minor change in affinity suggested that if a lysine residue is involved with EPO binding it might be Lys65. The K65A-EBP had an IC50 value that was 20-fold higher than wild-type EBP, Fig. 4B, Table I. The non-conserved amino acids in the C-D loop together with Lys65 may represent a charge cluster essential for the binding of EPO to the EPOR.
In the present study, random mutagenesis was used to identify
regions of the EBP that could be involved in EPO binding, based on
non-biased placement of mutations within the protein. The original sequence alignment of the cytokine receptor superfamily (3) identified
conserved motifs and amino acid residues, and this alignment was used
to select amino acids for site-directed mutagenesis (Fig. 1). Both
approaches ultimately result in identifying amino acids that can be
targeted by site-directed mutagenesis which can then help separate
binding determinants from structural determinants. The random
mutagenesis data implicated the F
-strand in ligand binding and led
to an extensive analysis of this region of the EBP using site-directed
mutagenesis. The HP-SEC assay was used to evaluate the binding
characteristics of the F
-strand EBP mutants. Soluble M200A-EBP
(lies at the beginning of the F
-G
loop, Fig. 2) was tested in the
EBP-bead assay and demonstrated a 16-fold increase in IC50
value (Table I). The recently solved crystal structure of the EBP with
a peptide ligand (10) shows that Met200 is buried in the
EBP but appears to be positioned below Phe205, which is in
proximity to the recently identified critical EPO binding determinant
Phe93 (15, 24). The actual contribution of
Met200 to ligand binding is most likely minimal; however,
mutations of Met200 may exert some structural perturbations
on the ligand binding face and alter the ability of this mutant EBP to
bind EPO.
When selected F
-strand mutants were expressed as full-length EPOR
constructs in COS cells, the binding affinities of these receptors were
comparable with wild-type EPOR (Table I). However, receptor expression
levels for both the R197A and R199A mutants as full-length EPOR were
considerably lower than wild-type, suggesting that these mutations are
having a structural effect on the receptor and are not true binding
determinants for EPO. Amino acids that are highly conserved may be
essential for the structural integrity of the protein, receptor
folding, and trafficking to the cell surface, and therefore indirectly
impact on ligand binding. The reduced receptor expression for
Arg197 and Arg199 receptor mutants may be
related to the interaction of these amino acids with the WSAWS motif,
based upon the crystal structure of the EBP with a peptide ligand (10).
The
-cation-
system found in the structure of the EBP relies on
alternating stacking of two aromatic and two positively charged amino
acid residues. This structure demonstrates the interactions of the
Arg197, Arg199, and the WSAWS motif that are
stabilized by main chain H bonds of Ser210 and
Ser213 with the F
-strand. Further stabilization occurs
through the formation of a salt bridge between Glu157 and
Arg199 (10). Receptor cell surface expression for the
full-length R189A, F194A, and V196A receptors was comparable with
wild-type EPOR, most likely because these residues do not appear to be
required for the structural stability of this region of the receptor.
Therefore, mutations at selected amino acids of the F
-strand and
WSAWS motifs of the EPOR may result in a destabilized receptor
structure. The F
-strand, C
-strand, and the WSAWS motif are
conserved sequences across the cytokine receptor superfamily (Fig. 1)
(4). Mutations of the WSAWS box of the EPOR have been shown to have dramatic effects on cell surface expression due to retention of these
receptors in the endoplasmic reticulum (23), although one mutant
receptor resulted in enhanced folding and surface expression (25). An
extensive network of interactions exist in these areas of the EBP, and
combined mutations may be necessary in order to destabilize this
structure and observe a dramatic effect on binding.
The poor folding characteristics of mutants F55A-EBP and Y57A-EBP (Fig.
3) suggest that these conserved residues (Fig. 1) are important for
structure, folding, and possibly protein processing. By day 6 very
little refolded protein was observed, although the folded protein that
was present was able to bind EPO in the HP-SEC assay, suggesting that
these residues are involved in structural maintenance. Deletion of a
short stretch of amino acids (QYFLY) in the C -strand of the human
GM-CSFR
led to little surface expression with the majority of the
mutant receptor accumulated intracellularly (26).
Analysis of the EBP crystal structure suggests that amino acid residues 60-63 (C-D loop, Fig. 2) may participate in the interaction with ligand (10). Glu60, Glu62, and Lys65 may represent a charge cluster that is necessary for interaction with EPO, and the substantial change in IC50 values for these mutants supports this proposal.
Single alanine substitutions in the C-D loop of the GHbp had only
marginal effects on GH binding (14). However, alanine substitutions of
residues within the C-D loop of the human IL-5R (D55, Y57) abolished
IL-5 binding (27). The C-D loop of the EPOR may represent a hormone
contact sequence conserved across the receptor superfamily.
Alternatively, a structural motif may be present in the C-D loop that
may also be a minor contact point with EPO.
The W40A-EBP did not demonstrate EPO binding in the HP-SEC assay (Fig. 5), but further characterization of W40A-EBP revealed an IC50 value of 800 nM in the EBP-bead assay. An IC50 could not be determined for the W40A-EBP in the inhibition of proliferation assay, although the trend of inhibition suggests a value of 1.0 µM or greater (data not shown). Trp40 may be a key structural determinant since it is invariant in the cytokine receptor superfamily (Fig. 1). Furthermore, the full-length W40A-EPOR did not demonstrate cell surface expression when assayed in COS cells. These data suggest that W40A is required for structural integrity and that the lack of cell surface expression indicates a perturbation of structure in this mutant protein. The W40A-EBP may not be stable over the course of the incubation time of the inhibition of proliferation assay. Moreover, the analogous residue in the growth hormone receptor, Trp50, also showed a lack of cell surface expression (14) and, therefore, could not be characterized. The ability to produce a soluble W40A-EBP allows the characterization of a mutant receptor that would otherwise be unavailable for binding studies. Clearly, the bacterial expression, refolding, and purification of soluble EBP mutants have great utility in obtaining data from proteins that may be difficult to express.
The recently reported critical binding determinant of the EPOR, Phe93, exemplifies the impact that a single amino acid residue can have on ligand binding (15). The identification of amino acid residues other than Phe93 that are significant contact points for EPO binding has met with limited success. Rather, among the 40 individual mutations that were examined (Fig. 2), a few (Tyr53, Glu60, Glu62, Lys65, and Met200) had a moderate effect (10-fold or greater increase in IC50 value). The exception is Trp40 which had a 160-fold increase in IC50 value. Trp40 is located distal to the peptide binding face in the EBP crystal structure (10). The fact that a tryptophan is absolutely conserved at the analogous position in the receptor superfamily coupled with the lack of cell surface expression for receptors having mutations at this position suggests that this residue is essential for proper receptor structure. This conservation of amino acids also adds support to the prediction that all members of the cytokine receptor family have similar structures. The remainder of the EBP mutations showed little or no effect on EPO binding. The inability to find another single amino acid mutation that defines a critical binding determinant in this more extensive, albeit not exhaustive, search may not be surprising. Despite the fact that protein-protein interfaces involve extensive surface area interactions, it has been shown for the GH-GHbp complex that the vast majority of the individual intermolecular contact points make only minor contributions to the overall binding affinity. When the 33 GHbp residues shown to be involved in binding to site 1 of GH in the crystal structure of the GH-GHbp complex were examined by mutagenesis for individual contribution to the overall binding affinity, it was found that a subset of 11 contact residues formed the functional epitope (28). Among these 11 residues there were two for which mutation produced drastic reductions in binding affinity (14, 28).
The results of our mutational analysis of the EBP are consistent with the suggestion that a small number of residues comprise the critical binding determinants for such protein-protein interactions. We were unable to identify mutations either through random or site-directed mutagenesis that had as dramatic an effect on binding affinity as the previously identified Phe93 (15). Rather, we found that there were a number of residues with modest effects on binding that may in fact participate in more peripheral interactions between EPO and the EPOR. Moreover, a number of amino acid positions are clearly involved in the structural integrity of the receptor, as evidenced by the disruption of protein folding and/or intracellular trafficking. These data suggest that only a few key contact points exist for the EPOR. Fine mapping of the EPO-EPOR binding patch should permit attempts to identify a smaller functional epitope and aid in the rational design of small molecule ligands.
We thank Greg Price for large scale fermentations of E. coli and Sally Varga for cell culture expertise.