The binding affinity of the uPA–PAI-1 complex to the receptors was greatly reduced by substitution of basic and hydrophobic resi-dues in a-helix D and a-helix E of PAI-1.. These receptors
Trang 1plasminogen activator inhibitor-1 complex for endocytosis receptors of the low-density lipoprotein receptor family, determined by site-directed mutagenesis
Sune Skeldal1, Jakob V Larsen1, Katrine E Pedersen1, Helle H Petersen1, Rikke Egelund1,
Anni Christensen1, Jan K Jensen1,2, Jørgen Gliemann3and Peter A Andreasen1,2
1 Department of Molecular Biology, University of Aarhus, Denmark
2 Interdisciplinary Nanoscience Center (iNANO), University of Aarhus, Denmark
3 Department of Medical Biochemistry, University of Aarhus, Denmark
The low-density lipoprotein receptor (LDLR) family of
endocytosis receptors has been implicated in binding
and endocytosis of a large number of structurally
un-related proteins, including apolipoproteins, protease–
inhibitor complexes, extracellular matrix proteins, and
hormone carriers In mammals, this receptor family includes LDLR itself, low-density lipoprotein receptor-related protein-1A (LRP-1A), LRP-1B, megalin
or LRP-2, very-low-density lipoprotein receptor (VLDLR), and apolipoprotein E receptor-2 These
Keywords
low-density lipoprotein receptor-related
protein; plasminogen activator inhibitor 1;
sorting protein-related receptor; urokinase
plasminogen activator; very-low-density
lipoprotein receptor
Correspondence
P A Andreasen, Department of Molecular
Biology, University of Aarhus, Gustav
Wied’s Vej 10C, 8000 Aarhus C, Denmark
Fax: +45 86 12 31 78
Tel: +45 89 42 50 80
E-mail: pa@mb.au.dk
(Received 17 July 2006, revised 20
Septem-ber 2006, accepted 22 SeptemSeptem-ber 2006)
doi:10.1111/j.1742-4658.2006.05511.x
Some endocytosis receptors related to the low-density lipoprotein receptor, including low-density lipoprotein receptor-related protein-1A, very-low-density lipoprotein receptor, and sorting protein-related receptor, bind pro-tease-inhibitor complexes, including urokinase-type plasminogen activator (uPA), plasminogen activator inhibitor-1 (PAI-1), and the uPA–PAI-1 com-plex The unique capacity of these receptors for high-affinity binding of many structurally unrelated ligands renders mapping of receptor-binding surfaces of serpin and serine protease ligands a special challenge We have mapped the receptor-binding area of the uPA–PAI-1 complex by site-direc-ted mutagenesis Substitution of a cluster of basic residues near the 37-loop and 60-loop of uPA reduced the receptor-binding affinity of the uPA–PAI-1 complex approximately twofold Deletion of the N-terminal growth factor domain of uPA reduced the affinity 2–4-fold, depending on the receptor, and deletion of both the growth factor domain and the kringle reduced the affinity sevenfold The binding affinity of the uPA–PAI-1 complex to the receptors was greatly reduced by substitution of basic and hydrophobic resi-dues in a-helix D and a-helix E of PAI-1 The localization of the implicated residues in the 3D structures of uPA and PAI-1 shows that they form a continuous receptor-binding area spanning the serpin as well as the A-chain and the serine protease domain of uPA Our results suggest that the 10–100-fold higher affinity of the uPA–PAI-1 complex compared with the free components depends on the bonus effect of bringing the binding areas on uPA and PAI-1 together on the same binding entity
Abbreviations
a1-PI, a1-antiproteinase inhibitor; CTR, complement type repeat; HEK293T, human embryonic kidney cell line 293T; LDLR, low-density lipoprotein receptor; LRP, low-density lipoprotein receptor-related protein; PAI-1, plasminogen activator inhibitor 1; RAP, receptor-associated protein; RCL, reactive centre loop; sorLA, sorting protein-related receptor; SPD, serine protease domain; tPA, tissue-type plasminogen activator; uPA, urokinase-type plasminogen activator; uPAR, uPA receptor; VLDLR, very-low-density lipoprotein receptor.
Trang 2receptors are constructed with the same types of
domains, but with variable numbers of each type of
domain The domains include complement-type repeats
(CTRs), YWTD-repeat-containing b-propellers, and
epidermal growth factor precursor domains The
recep-tors also all have a transmembrane a-helix and a
cyto-plasmic C-terminal domain mediating endocytosis via
clathrin-coated pits Generally, the CTRs are believed
to mediate ligand binding The related receptor sorting
protein-related receptor (sorLA), which in addition to
other types of domains also contains CTRs, have a
lig-and repertoire overlapping that of the LDLR family
One ligand common to all these receptors is the
40-kDa receptor-associated protein (RAP) [1]
The crystal structures of the third domain of RAP
in complex with a CTR pair from LDLR and of
human rhinovirus serotype 2 in complex with CTRs
from VLDLR have recently been solved [2,3]
Com-mon to all of these structures is the fact that binding
to the CTRs is mediated through basic and
hydropho-bic residues in the ligand Also, some inferences can be
made from an X-ray structure analysis of LDLR
crys-tals obtained at pH 5, in which the b-propeller bends
back and makes contact with the CTR cluster in a
way believed to mimic ligand binding [4]
The fact that these receptors exhibit high-affinity
binding of so many structurally unrelated ligands
makes their ligand-binding potential a unique case of
molecular recognition Particularly interesting ligands
are the serine protease urokinase-type plasminogen
activator (uPA), its primary serpin inhibitor,
plasmino-gen activator inhibitor-1 (PAI-1), and the
correspond-ing protease-serpin complex, which have been shown
to bind to LRP-1A [5], megalin [6,7], VLDLR [8,9],
LRP-1B [10], and sorLA [11] These receptors mediate
endocytosis of uPA–PAI-1 complex accumulated on
the cell surface by binding to the urokinase-type
plasminogen activator receptor (uPAR), whereas
endo-cytosis directly from the fluid phase is negligible [5]
Although both free uPA and free PAI-1 exhibit a
dis-tinct affinity for these receptors, the uPA–PAI-1
com-plex binds to LRP-1A with an affinity much higher
than that of the free components Thus, the Kd value
for binding of the uPA–PAI-1 complex to these
recep-tors is reported to be 1 nm [6,8,11,12], whereas that
for binding of free PAI-1 is reported to be 30 nm
[11–13], and Kdvalues of 7–200 nm have been reported
for binding of free u-PA [11,12,14]
The molecular recognition between the receptors
and the uPA–PAI-1 complex should therefore be seen
in relation to the mechanism of protease–serpin
com-plex formation X-ray crystal structure analyses have
shown that serpins are globular proteins consisting of
three b-sheets and nine a-helices [15] (Fig 1) Three-dimensional structures of a covalently coupled a1 -anti-proteinase inhibitor (a1-PI)–trypsin complex [16], a covalently coupled a1-PI–elastase complex [17], and several reversible complexes between serpins and pro-teases with the active-site Ser replaced by Ala [18–20] have been reported The structures support biochemi-cal and biophysibiochemi-cal evidence that complex formation is initiated by formation of a reversible docking complex,
in which the P1–P1¢ bond in the surface-exposed react-ive centre loop (RCL) interacts with the actreact-ive site of the protease Next, the P1–P1¢ bond is cleaved, the P1 residue coupled to the active-site Ser of the protease
by an ester bond, the N-terminal part of the RCL inserted as strand 4 in b-sheet A, and the protease translocated to the opposite pole of the serpin [15] Serpins are thus attacked by the proteases as sub-strates, but the normal catalytic cycle stops at the acyl-enzyme intermediate stage From the available 3D structures of stable protease–serpin complexes [16,17],
it was inferred that the catalytic mechanism is halted because of distortion of the active site of the protease The energy needed for the distortion stems from stabil-ization of the serpin in the ‘relaxed’ conformation by insertion of the RCL into b-sheet A, as opposed to the
‘stressed’, relatively unstable active conformation RCL insertion can also occur after abortive complex formation as the result of complete cleavage of the
P1–P1¢ bond or by insertion of the uncleaved RCL in latent PAI-1 [15]
Unfortunately, there is no X-ray crystal structure analysis of any receptor–protease–inhibitor complexes However, on the basis of the established inhibitory mechanism for serpins, there are several possible expla-nations of the increased receptor-binding affinity of the uPA–PAI-1 complex compared with the affinity of the individual components, including increased affinity associated with a conformational change in PAI-1, increased affinity associated with a conformational change in uPA, and⁄ or an avidity effect of two or more binding sites being brought together on the same ligand We have now addressed this problem using site-directed mutagenesis, introducing mutations into both uPA and PAI-1, and studying binding to VLDLR, LRP-1A, and sorLA We initially chose basic residues for mutation on the basis of evidence for receptor binding involving interactions between basic residues in the ligands and acidic residues in the recep-tor [21] Binding of the uPA–PAI-1 complex to LRP-1A and VLDLR was previously shown to require basic residues in a-helix D and a-helix E in the flexible joint-region of the inhibitor [13,22,23] We therefore mutated residues adjacent to this region in
Trang 3the uPA–PAI-1 complex, and we provide evidence that
the complex has an extended receptor-binding area
spanning uPA as well as PAI-1
Results
Screening of the effects of Ala substitutions in
PAI-1 on uPA–PAI-1 complex–receptor binding in
microtiter wells
For the initial screening, our strategy was to substitute
basic and hydrophobic residues in PAI-1 with Ala,
prepare complexes between the variants and 125I-uPA,
measure the binding of 20 pm radioactive complexes to
VLDLR, LRP-1A, or sorLA immobilized in microtiter
wells, and express the binding of the Ala-substituted
complexes relative to the binding of the wild-type
com-plex This relatively simple strategy was based on
several facilitating arrangements
We used PAI-1 expressed in a human cell line,
because of solubility problems with nonglycosylated,
bacterially expressed PAI-1 The source of PAI-1 for the preparation of the complexes was not purified PAI-1, but serum-free conditioned medium from human embryonic kidney cell line 293T (HEK293T) transfected with the corresponding cDNA A well-established pro-cedure [24] was used to re-activate the latent PAI-1 in the medium by the use of denaturation with SDS and refolding by removing the SDS by addition of an excess
of BSA 125I-uPA–PAI-1 complexes were prepared by adding125I-uPA to the medium and purifying the com-plexes by immuno-affinity chromatography [24]
As a control for the integrity of the125I-uPA–PAI-1 complexes during the binding assay, we measured the binding of all Ala-substituted complexes to parallel wells coated with 20 lgÆmL)1 monoclonal antibody to PAI-1 (mAb2) This antibody coat bound 64 ± 10%
of all complexes, and the binding of variant complexes always varied less than 20% from the binding of the wild-type complex
Binding of the uPA–PAI-1 complex to the receptors was assumed to follow the equation:
Fig 1 The 3D structures of PAI-1 and uPA The figure shows ribbon diagrams of the structure of the active conformation of PAI-1 [53] (pdb file 1B3K) and the structure of the SPD of uPA [27] (pdb file 1LMW) The localization of several a-helices and b-strands are indicated The diagrams were constructed with the use of SWISSPDBVIEWER
Trang 4ơRL Ử ơRTơL=đKdợ ơLỡ
in which [RL] is receptor-bound ligand, [R]T is total
receptor concentration, [L] is the concentration of free
ligand, and Kdis the equilibrium dissociation constant
With the 20 pm concentrations of 125I-labeled ligands
used here and Kd values 1 nm (see above),
[L] << Kd, and the above equation is reduced to:
đơRL=ơLỡ Ử ơRT=Kd Hence, the fractional uPAỜPAI-1 complexỜreceptor
binding is expected to be inversely proportional to the
Kd value Control experiments (not shown) confirmed
the linear relationship between [RL] and [L] We
there-fore used the amount of receptor-bound complex
between125I-labelled uPA and mutated PAI-1, as
com-pared with the amount of receptor-bound complex
between 125I-labeled uPA and wild-type PAI-1, as a
measure of the effect of the substitution on the Kd
value With the accuracy and background binding of
this assay, we expected that binding of up to 10-fold
less than the control value would be different from
background
For analyses of the binding of complexes of uPA
with PAI-1 mutants, we first mutated groups of
three residues to Ala If a triple mutation resulted in more than a twofold reduction in binding of the uPAỜPAI-1 complex to the receptor compared with the wild-type complex, we investigated the effect of substituting each residue separately For single mutants, a receptor-binding reduction of more than 1.5-fold was considered to define the residues involved in receptor binding Residues for Ala substi-tutions were selected on the basis of their proximity
to the previously implicated residues in a-helix D and a-helix E, namely Lys71, Arg78, Lys82, Lys90, Arg120 and Lys124 [13,22,23]
With this strategy, out of a total of 41 PAI-1 resi-dues tested, we identified 11 of importance for binding
of the uPAỜPAI-1 complex to one, two, or all three receptors (Fig 2) However, the involvement of Lys124 in receptor binding could not be confirmed
Screening of effects of Ala substitutions in uPA
on uPAỜPAI-1 complexỜreceptor binding in microtiter wells
Besides the C-terminal, 30-kDa serine protease domain (SPD), uPA contains an N-terminal, 25-kDa A-chain, consisting of a growth factor domain
Fig 2 Effect of Ala substitutions of PAI-1 residues on the binding of the uPAỜPAI-1 complex to VLDLR, LRP-1A and sorLA in a solid-phase assay The binding of the variant complexes was expressed relative to the binding of wild-type complex in the same experiment Mean ổ SD values are shown for at least three independent experiments As compared with the binding of wild-type complexes, the binding to the receptors of the complexes between uPA and the following PAI-1 mutants were reduced less than twofold when tested as triple mutants
or less than 1.5-fold when tested individually, and these mutated residues were therefore considered to be unimportant in the binding: H4A, H5A; P6A; P7A; Y9A; Q58A; K67A; D69A; D70A; P75A; L77A; M85A; P87A; W88A; E92A; T96A; R103A; D104A; K106; L107A; Q109A; P113A; H114; F119A; S121A; K124A; Q125A; W141A; H145A; K178A , Significantly different from binding observed with the correspond-ing wild-type (P < 0.01).
Trang 5and a kringle domain [25] In agreement with previous
reports on LRP-1A [12] and sorLA [11], we now
dem-onstrate that a truncated version of uPA without the
A-chain (LMW-uPA) has a sevenfold reduced affinity
for VLDLR (Fig 3) Moreover, we demonstrated that
a uPA variant with deletion of the growth factor
domain (DGF-uPA) had 2–4-fold reduced binding to
VLDLR, LRP-1A, and sorLA (see Fig 3) Suspecting
the involvement of basic residues in the contact with
the receptors, we measured the effect on receptor
bind-ing of substitutbind-ing a cluster of three arginines (R108,
R109, and R110) in the uPA kringle, but found no
change in receptor binding after these mutations
(Fig 3) At the moment, possible endocytosis
receptor-binding residues in the kringle is not known We did
not perform a mutational analysis of the growth factor
domain, as this, under physiological conditions at the
cell surface, is shielded from contact with the
endo-cytosis receptors by binding to uPAR [5,12]
We previously reported that a number of
mono-clonal antibodies against the SPD of uPA have
epi-topes localized in the 37-loop and 60-loop [26]
(Fig 1) To preliminarily investigate whether a
recep-tor-binding site exists in the SPD of uPA, we
studied the effect on uPA–PAI-1 complex–receptor binding of one such monoclonal antibody, mAb3689, with an epitope encompassing Arg179(36), His180(37), and Arg181(37a) in the 37-loop of uPA (amino-acid residues in the SPD of uPA will be referred to by a double numbering system, based on numbering from the N-terminus of the native protein with the chymotrypsin template numbering system in parentheses; residues in the N-terminal A-chain of uPA will be referred to by numbering from the N-terminus of the native protein [27]) The presence
of the antibody reduced the binding significantly (Fig 3) We therefore studied the effect of Ala sub-stitution of clusters of residues in the 37-loop and 60-loop on receptor binding and demonstrated that both sets of mutations reduced receptor binding 2–4-fold (Fig 3) As a control, we found that mAb3689 did not reduce the binding of the variant with the mutations in its epitope in the 37-loop (Fig 3) Sub-stitution of a number of other residues was without effect on receptor binding (Fig 3) We therefore con-cluded that two or more residues in the 37-loop and 60-loop of the SPD of uPA are part of the ligand– receptor interface
Fig 3 Effect of Ala substitutions of uPA
residues on the binding of the uPA–PAI-1
complex to VLDLR, LRP-1A, and sorLA in a
solid-phase assay The binding of the variant
complexes was expressed relative to the
binding of the wild-type complex in the
same experiment Mean ± SD values are
shown for at least three independent
experi-ments Compared with binding of wild-type
complexes, binding to the receptors of the
complexes between PAI-1 and the following
uPA mutants were reduced less than
1.5-fold, and the mutated residues were
therefore considered unimportant for
bind-ing: K212(62)A; E213(62a)A; D194(63)A;
I216(65)A; Y218(67)A; N227(76)A;
Q229(78)A; E235(84)A; K264(110a)A;
E265(110b)A; R267(110d)A; H402(241)A;
K404(243)A , Significantly different from
binding observed with the corresponding
wild-type complex (P < 0.01) ND, not
deter-mined.
Trang 6Screening of effects of Ala substitutions in both
uPA and PAI-1 on uPA–PAI-1 complex–receptor
binding in microtiter wells
To extend the results obtained by the microtiter
well-binding assays by Biacore well-binding analysis and assays
of receptor-mediated endocytosis, we used a complex
between a PAI-1 variant with low receptor affinity and
a uPA variant with low receptor affinity We used a
PAI-1 variant with a quadruple mutation in a-helix D,
i.e K71A-R78A-Y81A-K82A This variant did not
dif-fer significantly from wild-type PAI-1 with respect to
specific inhibitory activity (wild-type 69 ± 4% and
mutant 59 ± 7% of the theoretical maximum),
second-order rate constant for the reaction with
uPA (wild-type 3.3 ± 0.7· 106m)1Æs)1 and mutant
3.7 ± 0.5· 106m)1Æs)1), and relative vitronectin
bind-ing (mutant 1.03 ± 0.11 times that of wild-type) We
used the uPA variant with the triple mutation in the
37-loop, i.e R178(35)A-R179(36)A-R181(37a)A This
variant did not differ significantly from wild-type uPA
with respect to Kmfor hydrolysis of S-2444
(pyro-Glu-Gly-Arg-p-nitroanilide), the Km values for wild-type
and variant being 87 ± 11 lm and 85 ± 0.01 lm,
respectively The resulting complex showed the
expected more than 10-fold reduction in affinity for
VLDLR and LRP-1A in microtiter well-binding assays
(Fig 4)
Surface plasmon resonance analysis of receptor
binding of uPA–PAI-1 complexes
We analysed receptor binding of the complexes
between the quadruple a-helix D PAI-1 mutant
(K71A-R78A-Y81A-K82A PAI-1) and the 37-loop
uPA mutant [R178(35)A-R179(36)A-R181(37a)A uPA]
by surface plasmon resonance In this case,
nonradio-active wild-type and mutant uPA–PAI-1 complexes
were prepared from PAI-1 purified by immuno-affinity
chromatography and re-activated by denaturation with
guanidinium chloride and refolding by dialysis, and
uPA purified by immuno-affinity chromatography The
complexes were purified from unreacted uPA and
PAI-1 by immuno-affinity chromatography VLDLR,
LRP-1A, or sorLA was immobilized on Biacore chips,
and wild-type and mutant uPA–PAI-1 complexes
injec-ted on to the chips at concentrations of 1.5–100 nm
The time course of the binding obtained with VLDLR
is shown in Fig 5 Similar results were found with the
two other receptors It is evident from the figure that
the binding was reduced with the mutant
uPA–wild-type PAI-1 complex, the wild-uPA–wild-type uPA–mutant PAI-1
complex, and, in particular, with the mutant
uPA–mutant PAI-1 complex Fitting the binding data
to a Langmuir 1 : 1 binding model by the use of the biaevaluation 3.0 software (global fitting) resulted in
a Kdvalue for binding of the wild-type uPA–wild-type PAI-1 complex to either of the three receptors of
1 nm, while Kd for the mutant uPA–mutant PAI-1 complex was increased more than 10-fold However, the fit to the simple 1 : 1 binding model was poor As appears from Fig 5, the rapid first phase of the associ-ation was followed by a second, slower phase, and a corresponding rapid dissociation phase, amounting to 20% of total binding, particularly with relatively high ligand concentrations We therefore carried out a more
Fig 4 Effect of Ala substitutions of both uPA and PAI-1 residues
on binding of the uPA–PAI-1 complex to VLDLR, LRP-1A, and
sor-LA in a solid-phase assay Binding of wild-type and variant com-plexes to the receptors was estimated Binding of the variant complexes is expressed relative to that of the wild-type complex in the same experiment Mean ± SD values are shown for at least three independent experiments , Significantly different from bind-ing observed with the correspondbind-ing wild-type (P < 0.01) ND, not determined.
Trang 7reliable estimation of the Kdvalues by determining the
association rate constants from the initial rate of
association and the dissociation rate constant from the
slow phase of dissociation (see Experimental
proce-dures) and calculated the Kd values as the ratio
between the association and dissociation rate constants
(Table 1) The Kd values obtained for the wild-type
uPA–wild-type PAI-1 complex, 1 nm, were in good
agreement with those reported previously [6,8,11,12]
The Kd values of the complexes between mutant uPA
and wild-type PAI-1 and wild-type uPA and mutant
PAI-1 were increased 1.5–5-fold, mostly as the result
of increased dissociation rate constants A particularly
large change, corresponding to 10–30-fold increased Kd
values, was observed for the complex with mutations
in both uPA and PAI-1 This increase was the result
of a decreased association rate constant and an
increased dissociation rate constant The fold reduction
in the Kdvalues determined by surface plasmon
reson-ance did not differ significantly from those expected
from the microtiter well-binding assays, although the
average fold reductions of the Kdvalue were smaller in
the Biacore experiments than expected from the
microtiter well-binding assays
Effects of Ala substitutions of uPA and PAI-1 on
receptor-mediated endocytosis of the uPA–PAI-1
complex
We measured the receptor-dependent degradation of
the complexes between the quadruple a-helix D PAI-1
mutant (K71A-R78A-Y81A-K82A PAI-1) and the
37-loop uPA mutant
[R178(35)A-R179(36)A-R181(37a)A uPA] in cell lines expressing VLDLR and
LRP-1A For VLDLR-mediated endocytosis, we used
U937 cells These cells were previously shown to
con-tain VLDLR-II mRNA, i.e a VLDLR variant without
exon 16 encoding the O-linked sugar domain [28]
Lig-and blot analysis of membrane fragments from U937
cells revealed a RAP-binding membrane protein
co-migrating with VLDLR-II (Fig 6) For
LRP-1A-mediated endocytosis, we used COS-1 cells, in which
the only RAP-binding receptor detectable by RAP
lig-and blotting analysis is LRP-1A [29] In both cell lines,
Fig 5 Surface plasmon resonance analysis of binding of wild-type
and variant uPA–PAI-1 complexes to VLDLR Binding was
meas-ured using chips with 50 fmol ⁄ mm 2 immobilized VLDLR The
chips were superfused with the indicated complexes at
concentra-tions of 12, 6, 3 and 1.5 n M , followed by buffer alone at 480 s.
Mutant uPA ¼ R178(35)A-R179(36)A-R181(37a)A uPA Mutant
PAI-1 ¼ K71A-R78A-Y81A-K82A PAI-1.
Trang 8the receptor-mediated degradation of wild-type and variant complexes was reduced largely in parallel with receptor binding (Fig 6)
Using another PAI-1 mutant, i.e a triple mutant con-taining three different substitutions each resulting in reduced receptor binding, we also showed that reduced complex–receptor binding was associated with increased accumulation of the complexes on the cell surface, compared with the wild-type complex Another triple mutant without reduced receptor binding did not result
in accumulation on the cell surface (Table 2)
It is not possible to study the endocytosis of variant complexes with truncations of the A-chain of uPA These variants do not bind to uPAR and their endo-cytosis is therefore negligible
Discussion
In this work, we used site-directed mutagenesis to map the VLDLR, LRP-1A, and sorLA binding surfaces of the uPA–PAI-1 complex The mapped interaction sur-face spans both PAI-1 and uPA In PAI-1, residues His79, Tyr81, Met112, Phe116, Arg117, and Arg270 are implicated in the interaction surface Also, the pre-viously reported reduced binding of the double mutants K82A-R120A and R78A-K124A [22] could be tracked back to reveal the importance of Arg120, as well as Lys82 and Arg78, in receptor binding, and con-firm the previous reports of the importance of Lys71, Lys82, and Lys90 [13] and Arg78 [23] in a-helix D Of the 41 PAI-1 residues tested because of their proximity
to a-helix D, 12 were Arg or Lys Of the 11 residues implicated in binding, seven were Arg or Lys Thus, basic residues constituted a much higher percentage of
k 1
Kd
Kd
paren-theses uPA
k1 ·
1 Æs
k)
Kd (nM
k1 ·
1 Æs
k)
Kd (nM
k1 ·
1 Æs
k)
Kd (nM
R178(35)A-R179 (36)A-R181(37a)A
K71A-R78A- Y81A-K82A
R178(35)A-R179 (36)A-R181(37a)A
K71A-R78A- Y81A-K82A
a Significantly
c Significantly
Table 2 Effect of substitutions of PAI-1 residues on the accumula-tion of uPA–PAI-1 on U937 cell surface U937 cells were allowed
to bind the respective uPA–PAI-1 mutant complexes for 1 h on ice and subsequently washed before incubation for 8 min at either 0 or
37 C Cell surface-associated complex was then released with a low-pH buffer Cell surface-associated and internalized complexes are expressed as percentage of total amount of cell-associated complex Mean ± SD values are given for experiments performed
in triplicate.
PAI-1 variant
0 C, 8 min 37 C, 8 min
Inside
Cell surface associated Inside
Cell surface associated Wild-type 5.7 ± 0.1 94.3 ± 0.1 39.2 ± 1.1 60.8 ± 1.1 H5A-P6A-Q109A 5.0 ± 0.2 95.0 ± 0.2 35.8 ± 1.8 64.2 ± 1.8 H79A-F116A-R117A 4.5 ± 0.2 a 95.5 ± 0.2 a 20.6 ± 0.2 a 79.4 ± 0.2 a
a Significantly different from the corresponding number for wild-type PAI-1 (P < 0.01).
Trang 9the PAI-1 residues implicated in binding than of all
the residues tested This finding agrees well with the
hypothesis that ligand recognition by this receptor
class relies on electrostatic interactions between basic
residues in the ligands and acidic residues in the
recep-tors [21] However, X-ray crystal structure analysis of
LDLR at pH 5 [4] and receptor–ligand complexes
[2,3], site-directed mutagenesis of CTRs [30] (see
below) and our results presented here (Fig 2) suggest
that hydrophobic interactions may also be important
Almost all the residues implicated in binding were
localized in and around a-helix D and a-helix E, but,
interestingly, substitution of Arg270, localized in
b-strand 2C, 2.7 nm from a-helix D, also affected
binding, suggesting an even more extended binding
surface (Fig 7)
In uPA, we have shown that substitutions in and near
the 37-loop and 60-loop caused a substantial reduction
in binding Thus, our findings for the first time implicate
the SPD of uPA in the binding of the complex to the
receptors Obviously, we cannot exclude the possibility
that additional residues in the SPD contribute to the
binding In addition, we could confirm the previously implicated importance of the A-chain of uPA in the binding of the uPA–PAI-1 complex to the receptors Moreover, we demonstrated that deletion of the uPA growth factor domain resulted in reduced binding As the reduction was smaller than that caused by deletion
of the entire A-chain, it seems likely that both the growth factor domain and the kringle contribute to the binding, but putative endocytosis receptor-binding resi-dues in the kringle remain unknown The involvement
of the growth factor domain readily explains the previously reported reduction of the affinity of the uPA–serpin complexes for LRP-1A and SorLA in the presence of the cellular receptor for uPA, uPAR [11,12,14], as uPAR binds to the growth factor domain
of uPA [31] Binding to uPAR would thus shield endo-cytosis receptor-binding residues
In the model of the complex between PAI-1 and the uPA SPD, constructed from the 3D structure of the
a1-PI–trypsin complex, as determined by X-ray crystal structure analysis [16], the residues of the 37-loop and 60-loop of uPA studied here are relatively close to
Fig 6 Receptor-mediated degradation of uPA–PAI-1 wild-type and variant complexes in U937 and COS-1 cells U937 cells were incubated with 10 p M125I-uPA–PAI-1 complex for 45 min at 37 C, by which time the amount of degraded complex was determined as the fraction
of complex soluble in 7% trichloroacetic acid The degradation of wild-type 125 I-uPA–PAI-1 complex was set equal to 1, and the degradation
of mutant complexes expressed relative to that The figure shows mean ± SD for triple determinations in a typical experiment out of a total of three with U937 cells and two with COS-1 cells Insert, 125 I-RAP ligand blotting analysis of a membrane preparation from U937 Lane
1, purified VLDLR type II [44]; lane 2, membranes from U937 cells The migration of molecular mass markers is indicated on the right.
Trang 10Fig 7 The receptor binding surface in the uPA–PAI-1 complex The uPA–PAI-1 complex shown is a SWISSPDBVIEWER surface display of a model constructed from the structure of the a1-PI–trypsin complex [16] (pdb file 1EZX), by overlayering the a1-PI part of that structure with the structure of cleaved PAI-1 [54] (pdb file 9PAI) and the trypsin part of that structure with the structure of the SPD of uPA [27] (pdb file 1LMW) Although the relative orientation of uPA and PAI-1 in the model is realistic, it does not allow any predictions of exact distances between amino-acid residues The effects of Ala substitutions on the binding of 20 p M125I-uPA–PAI-1 complex to VLDLR, LRP-1A, and
sor-LA individually are depicted in the model Ala substitution of residues colored red resulted in a more than fivefold reduction in binding Ala substitution of residues colored pink resulted in a 1.5–5-fold reduction in binding PAI-1 residue Tyr81 and uPA residue Arg178(35), which would have been colored pink, are not visible with the orientation of the structure used Ala substitution of residues colored blue resulted in
a less than 1.5-fold reduction in binding when tested individually or less than twofold reduction when tested as part of triple mutants One structure is shown for binding to each receptor In addition, the complex model is also shown as a ribbon diagram in which b-strand 2C of PAI-1 is colored yellow and a-helix D and E of PAI-1 are colored green The 37-loop and 60-loop of uPA are shown in red and pink, respect-ively Also indicated is a ribbon diagram of the pH 5 structure of the CTRs from LDLR [4] (pdb file 1 N7D).