THE JOURNAL OF BIOLOGICAL CHEMISTRY VOL. 282, NO. 16, pp. 11914 –11920, April 20, 2007
© 2007 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in the U.S.A.
Tests of the Extension and Deadbolt Models of
Integrin Activation*
Received for publication, January 9, 2007, and in revised form, February 13, 2007 Published, JBC Papers in Press, February 13, 2007, DOI 10.1074/jbc.M700249200
Jieqing Zhu‡, Brian Boylan§, Bing-Hao Luo‡, Peter J. Newman§¶, and Timothy A. Springer‡1
From the ‡The CBR Institute for Biomedical Research and Departments of Pathology, Harvard Medical School, Boston,
Massachusetts 02115, §Blood Research Institute, BloodCenter of Wisconsin, Milwaukee, Wisconsin 53201, and the
¶
Departments of Pharmacology and Cellular Biology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226
Integrins are cell adhesion molecules that transmit bidirectional signals across the plasma membrane and regulate many
biological functions, including cell differentiation, cell migration, and wound healing (1, 2). A bent conformation observed
by x-ray crystallography (3) and electron microscopy (4, 5) represents the physiological low affinity state. Integrin activation is
regulated by binding of intracellular proteins such as talin to the
integrin -subunit cytoplasmic tail (6), leading to separation
between the ␣- and -subunits at their transmembrane and
cytoplasmic domains (7–9) and propagation of conformational
changes from the transmembrane domains to the ligand binding headpiece, which increases integrin affinity for ligand (10,
11). These long range conformational changes involve integrin
extension. Conversely, ligand binding also induces integrin
extension, which leads to the separation of the ␣- and -subunit
legs and the transmembrane and cytoplasmic regions (11).
Recent co-crystal structures of integrin ␣IIb3 headpiece
bound to ligands revealed the molecular basis for the large
* This work was supported by National Institutes of Health Grants HL48675
(to T. A. S.) and HL44612 (to P. J. N.). The costs of publication of this article
were defrayed in part by the payment of page charges. This article must
therefore be hereby marked “advertisement” in accordance with 18 U.S.C.
Section 1734 solely to indicate this fact.
1
To whom correspondence should be addressed: The CBR Institute for Biomedical Research and Departments of Pathology, Harvard Medical School,
200 Longwood Ave., Boston, MA 02115. E-mail: springeroffice@cbr.
med.harvard.edu.
11914 JOURNAL OF BIOLOGICAL CHEMISTRY
conformational changes that accompany ligand binding (12).
The conformational changes in the ligand binding site that
increase affinity for ligand are allosterically linked by a
crankshaft-like ␣7-helix displacement in the 3 I domain to
a 60° change in the angle between the 3 I and hybrid
domains and to a 70 Å increase in separation between the ␣and -subunits at their knees. Since the hybrid domain and
connecting -leg domains are central in the interface
between the integrin headpiece and legs in the bent conformation, integrin extension enables hybrid domain swingout. The crystal structures are in excellent agreement with
studies using electron microscopy (4, 5, 13, 14), NMR (15),
small angle x-ray scattering (16), and mutational studies and
mapping of the epitopes of allosteric activating and inhibiting antibodies (17–20), which all support the switchblade
model for integrin activation, in which integrin extension
and leg separation are coupled with swing-out of the hybrid
domain and the displacement of the -I domain ␣7 helix.
Two effects of extension are important for integrin activation. First, extension places the ligand binding site 150 –200 Å
further above the cell surface and orients it pointing away from,
rather than toward, the surface of the cell on which it expressed.
This favors accessibility to ligands on other cells and in the
extracellular matrix. Second, extension enables hybrid domain
swing-out and conversion of the headpiece from the closed, low
affinity to the open high affinity state (11).
There is controversy about the requirement of extension for
integrin activation. An electron microscopy (EM)2 study
reported that soluble ␣V3 bound to a fibronectin fragment
was still in the bent conformation (21). As a supplement or
alternative to integrin extension, regulation of integrin affinity
by a “deadbolt” has been proposed. This proposal is based on
the observation of an interaction at a small 60 Å2 interface
between the -tail domain CD loop (the deadbolt) and the -I
domain 6 strand-␣7 helix region in the unliganded ␣V3 crystal structure (22). It was hypothesized that 1) this interaction
restricts the displacement of the -I domain 6-␣7 loop and
thus keeps the integrin in the low affinity state and 2) that loss of
this interaction would induce inside-out activation without
extension. Here, we directly tested the deadbolt model by
mutating or deleting the -tail domain CD loop and show that
2
The abbreviations used are: EM, electron microscopy; mAb, monoclonal
antibody; I-EGF, integrin-epidermal growth factor-like; LIBS, ligand-induced binding site; Fn, fibronectin; Fg, fibrinogen; FITC, fluorescein isothiocyanate; MFI, mean fluorescence intensity; CHO, Chinese hamaster
ovary.
VOLUME 282 • NUMBER 16 • APRIL 20, 2007
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Despite extensive evidence that integrin conformational
changes between bent and extended conformations regulate
affinity for ligands, an alternative hypothesis has been proposed
in which a “deadbolt” can regulate affinity for ligand in the
absence of extension. Here, we tested both the deadbolt and the
extension models. According to the deadbolt model, a hairpin
loop in the 3 tail domain could act as a deadbolt to restrain the
displacement of the 3 I domain 6-␣7 loop and maintain integrin in the low affinity state. We found that mutating or deleting
the 3 tail domain loop has no effect on ligand binding by either
␣IIb3 or ␣V3 integrins. In contrast, we found that mutations
that lock integrins in the bent conformation with disulfide
bonds resist inside-out activation induced by cytoplasmic
domain mutation. Furthermore, we demonstrated that extension is required for accessibility to fibronectin but not smaller
fragments. The data demonstrate that integrin extension is
required for ligand binding during integrin inside-out signaling
and that the deadbolt does not regulate integrin activation.
Testing Extension and Deadbolt Models of Integrin Activation
these mutations have no effect on ligand binding by ␣IIb3 or
␣V3 integrins. We further demonstrate that integrins locked
in the bent conformation are resistant to inside-out activation
and that extension is important to promote accessibility of cell
surface integrins for soluble ligands.
APRIL 20, 2007 • VOLUME 282 • NUMBER 16
RESULTS
Deleting or Mutating Residues of the -Tail Domain CD Loop
Has No Effect on Ligand Binding—According to the deadbolt
model, a hairpin loop (between -strands C and D) of the 3 tail
domain (the CD loop) contacts the 3 I domain and restrains
integrin activation (Fig. 1). Two groups of investigators independently tested the deadbolt model and have combined their
data in this paper. One group produced and assayed ligand
binding to CHO cell lines stably expressing either wild-type or
mutant ␣IIb3 with five residues (Asp-672–Lys-676) deleted
from the -tail CD loop. The activation state of these integrins
was evaluated by measuring the ability of CHO cell transfectants to bind macromolecular ligands. Wild-type and mutant
␣IIb3 were expressed similarly in CHO cells (Fig. 2A). Basal
binding of fibrinogen and the ligand-mimetic, activation-dependent antibody PAC-1 (28, 29) in the presence of Ca2⫹ was
low for both wild-type ␣IIb3 and the ␣IIb3_⌬672– 676
mutant (Fig. 2B), i.e. it was similar to that in mock transfectants.
In contrast, Mn2⫹ induced similar binding of fibrinogen and
PAC-1 mAb to the wild-type and ␣IIb3_⌬672– 676 CHO cell
transfectants (Fig. 2B). These data show that deletion of the
deadbolt does not constitutively activate ␣IIb3 integrin.
The other group deleted three residues (Aps-672–Ser-674)
of the CD loop or mutated them to Ala individually. These
mutations had no effect on the expression of ␣IIb3 or ␣V3
on the cell surface (data not shown). The activation state of
these mutant integrins was evaluated by measuring soluble
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EXPERIMENTAL PROCEDURES
Plasmid Construction, Transient Transfection, and Immunoprecipitation—Plasmids coding for full-length human
␣IIb or ␣V and 3 were subcloned into pEF/V5-HisA and
pcDNA3.1/Myc-His (⫹), respectively (4). Cysteine substitutions (G307C in ␣V and R563C in 3) and the N305T mutant of
3 were described previously (4, 23). Other mutants were made
using site-directed mutagenesis with the QuikChange kit
(Stratagene, La Jolla, CA). Constructs were transfected into
293T cells using FuGENE 6 transfection reagent (Roche Diagnostics) according to the manufacturer’s instruction. Transfected cells were metabolically labeled with [35S]cysteine/methionine (PerkinElmer Life Science) and immunoprecipitated
with the conformation-independent 3 mAb AP3, which recognizes residues in the plexin-semaphorin-integrin (PSI) and
hybrid domains (24 –26) as described (7).
Soluble Ligand Binding—The fragments of human fibronectin (Fn) type III domains 7–10 (Fn7–10) and 9 –10 (Fn9 –10)
were prepared as described previously (27). The purified fragments were labeled by Alexa Fluor 488 using Alexa Fluor 488
protein labeling kit A10235 (Molecular Probes, Eugene, OR).
Binding of fluorescein isothiocyanate (FITC)-labeled human
fibrinogen (Fg) (Enzyme Research Laboratories, South Bend,
IN), FITC-labeled human Fn (Sigma), and Alexa Fluor 488labeled Fn7–10 and Fn9 –10 and PAC-1 IgM, which recognizes
activated ␣IIb3 at its ligand binding site (28, 29) (BD Biosciences), was determined as described (23). Briefly, transfected
cells suspended in 20 mM HEPES-buffered saline (pH 7.4) supplemented with 5.5 mM glucose and 1% bovine serum albumin
were incubated with 50 g/ml fluorescently labeled ligands or
10 g/ml PAC-1 in the presence of 5 mM EDTA, 1 mM Ca2⫹/1
mM Mg2⫹, 1 mM Ca2⫹/1 mM Mn2⫹, 1 mM Ca2⫹/1 mM Mn2⫹
plus either 10 g/ml activating mAb PT25-2 to the ␣IIb -propeller domain (29, 30) or 10 g/ml activating mAb LIBS1 to the
3 leg domain (31) at room temperature for 30 min and then
stained with Cy3-labeled anti-3 mAb AP3 on ice for 30 min.
For PAC-1 binding, cells were washed and stained with FITCconjugated anti-mouse IgM and Cy3-labeled AP3 on ice for
another 30 min before being subjected to flow cytometry. Binding activity is presented as the mean fluorescence intensity
(MFI) of FITC-conjugated anti-mouse IgM or ligands, after
subtraction of background MFI in EDTA, expressed as a percentage of the MFI of the Cy3-AP3.
LIBS Epitope Expression—Anti-LIBS mAb LIBS1 and D3 to
the 3 leg domain (31) were kindly provided by Drs. M. H. Ginsberg (University of California, San Diego, La Jolla, CA) (32)and
L. K. Jennings (University of Tennessee Health Science Center,
Memphis, TN) (33). LIBS epitope expression was determined
as described previously (23). In brief, transfected cells suspended in HEPES-buffered saline supplemented with 5.5 mM
glucose and 1% bovine serum albumin were incubated with or
without 25 M cyclo-(Arg-Gly-Asp-D-Phe-Val) (cyclo-RGDfV)
(Bachem Bioscience, Inc., King of Prussia, PA) peptide or 1
mg/ml Fn7–10 in the presence of 1 mM Ca2⫹/1 mM Mg2⫹ or 0.2
mM Ca2⫹/0.2 mM Mn2⫹ at room temperature for 30 min and
then with anti-LIBS mAb on ice for 30 min followed by FITCconjugated anti-mouse IgG and flow cytometry. LIBS antibody
binding is presented as the MFI of anti-LIBS antibody staining
as a percentage of the MFI of conformation-independent mAb
AP3 staining.
Establishment of CHO Stable Cells—The QuikChange
mutagenesis kit (Stratagene; La Jolla, CA) was used to introduce
a 15-nucleotide deletion into human integrin 3 cDNA to generate cDNA encoding ⌬Asp-672–Lys-676. CHO cells were
transfected with human integrin ␣IIb (in pcDNA3.1 Zeo;
Invitrogen) and one of two forms of human integrin 3 (wild
type or ⌬Asp-672–Lys-676 in pcDNA3.1 Neo; Invitrogen)
using a calcium phosphate transfection kit (Invitrogen). 48 h
after transfection, the growth medium (F12K; Mediatech, Inc,
Herndon, VA) was supplemented with 0.6 mg/ml G418 and
0.35 mg/ml Zeocin (Invitrogen).
Evaluation of ␣IIb3 Activation State on CHO Cells—CHO
cells that had been stably transfected with cDNAs encoding
human ␣IIb and either human 3 or human 3_⌬672– 676
were incubated with 5 g/ml anti-integrin 3 monoclonal antibody, AP3, conjugated to Alexa Fluor 647 (Invitrogen) and
either 120 g/ml FITC-fibrinogen or 2.5 g/ml PAC-1 (BD
Biosciences) in the presence of 1 mM Ca2⫹ or 1 mM Mn2⫹.
Purified fibrinogen was kindly provided by Dr. Michael Mosesson (Blood Research Institute, BloodCenter of Wisconsin, Milwaukee, WI) and was labeled with FITC according to previously
described methods (34).
Testing Extension and Deadbolt Models of Integrin Activation
11916 JOURNAL OF BIOLOGICAL CHEMISTRY
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gen, fibronectin, or the ligand
mimetic antibody PAC-1 under
physiological conditions (Fig. 3,
A–C). Furthermore, CD loop
mutants all were activated by Mn2⫹
and activating mAbs to the same
level as wild-type integrins. In addition, CD loop mutations had no
effect on the binding of fibronectin
fragments Fn7–10 or Fn9 –10 (Fig.
3D). As a positive control, the cytoplasmic GFFKR to GAAKR mutation (␣V_GAAKR), which mimics
inside out activation (7), constitutively activated binding of the
fibronectin fragments (Fig. 3D).
These data demonstrate that the
FIGURE 1. Locations of the mutations in the ␣V3 crystal structure (3). The ␣V subunit is in cyan. The 3 -tail domain CD loop does not
hybrid domain is in green, and other 3 domains are in magenta. The 3 tail domain CD loop (residues Asp672–Lys-676) is in blue. Residues mutated to cysteine are shown with spheres at the positions of ␣V G307 C␣ restrain activation of either integrin
(cyan) and 3 R563 C␣ (magenta). The position of the N-glycan wedge introduced by the 3 NIN305T mutation ␣IIb3 or ␣V3.
is shown with a yellow sphere at Asn-303 C␣. A, this view emphasizes the -I domain 6-strand and ␣7-helix, the
Integrins Locked in the Bent
only elements shown as a ribbon, and their proximity to the -tail domain CD loop. B, this view is rotated
relative to A and emphasizes the glycan wedge introduced into a crevice between the hybrid and -I domain Conformation Are Resistant to
that widens in the open headpiece conformation.
Inside-out Activation—Fluorescence resonance energy transfer
and mutagenesis studies revealed that integrin inside-out
activation requires unclasping or separation of the cytoplasmic and transmembrane domains (7, 8, 35, 36). However, it
has been questioned whether integrin extension is required for
inside-out activation (37). Previously, we showed that introducing a disulfide bond between residues 307 of the ␣V -propeller
domain and 563 of the 3 I-EGF domain 4, with the ␣V G307C
and 3 R563C mutations, locked the ␣V3 integrin in the low
affinity, bent conformation (4). Residues ␣V-307 and 3–563
are in close proximity in the bent ␣V3 conformation (3) (Fig.
1). To test the requirement for integrin extension during insideout activation, we combined the disulfide bond-forming ␣V
G307C and 3 R563C mutations with the ␣V_GFFKR/GAAKR
cytoplasmic domain mutation, which induces inside-out activation. The disulfide bond was formed in high efficiency in this
␣V_GAAKR_G307C/3_R563 mutant (Fig. 4A). Consistent
with previous results (7), the GAAKR mutant (␣V_GAAKR/
3) bound anti-LIBS antibodies D3 and LIBS1 to the 3 leg (31)
constitutively, indicating that the integrin was extended (Fig.
4B). However, binding of LIBS antibodies by ␣V_GAAKR/3
was reversed when the intersubunit disulfide bond was introduced in the ␣V_GAAKR_G307C/3_R563C mutant (Fig. 4B).
The high affinity (IC50 ⫽ 2.5 nM) (38) cyclo-RGDfV peptide
induced LIBS binding by wild-type ␣V3 and further enhanced
FIGURE 2. Effect of -tail domain CD loop mutation on ligand binding by LIBS binding by ␣V_GAAKR/3 (Fig. 4B). By contrast, the
␣IIb3 expressed on CHO cells. A, expression of wild-type and mutant mutants ␣V_GAAKR_G307C/3_R563C and ␣V_G307C/
␣IIb/3 integrins in CHO cells is shown as MFI of ␣IIb/3-positive cells stained
with AP3-conjugated with Alexa Fluor 647. B, mock transfectants or ␣IIb/3- 3_R563C did not bind LIBS antibodies even in the presence of
positive cells gated for equivalent 3 expression using AP3 mAb were quan- cyclo-RGDfV (Fig. 4B), showing that these mutants were locked
titated for binding of FITC-Fg or the fibrinogen mimetic antibody, PAC-1, in in the bent conformation. Ligand binding assays showed that
the presence of 1 mM Ca2⫹ or 1 mM Mn2⫹.
mutant integrin ␣V_GAAKR/3 bound fibrinogen and
fibronectin constitutively in Ca2⫹/Mg2⫹ (Fig. 4C). By conligand binding to 293T cells transiently transfected with the trast, the mutants ␣V_GAAKR_G307C/3_R563C and
mutant or wild type receptors (Fig. 3). None of the CD loop ␣V_G307C/3_R563C did not bind fibrinogen and fibronecmutants showed constitutively high affinity for human fibrino- tin even in the presence of Mn2⫹ and activating mAb (Fig.
Testing Extension and Deadbolt Models of Integrin Activation
4C). These data demonstrate that integrins locked in the
bent conformation are resistant to inside-out activation, i.e.
integrin extension is required for inside-out activation as
measured by both LIBS epitope exposure and soluble ligand
binding.
Ligand Accessibility Is Important for Integrin Ligand Binding—A negative stain EM study suggested that the soluble integrin ␣V3 in the bent conformation could bind a fragment of
its ligand, fibronectin (21). These data lead to the suggestion
that integrin extension may not be required for ligand binding. We designed experiments to investigate whether bent
integrins are accessible to ligands on the cell surface. Introduction of a glycan wedge into the hybrid domain at its interface with the -I domain by introducing an NIT305 sequon
with the 3_N305T mutation has been shown to stabilize
integrins in the high affinity state by favoring swing-out of
the hybrid domain (Fig. 1) (23). We combined the
␣V_G307C/3_R563C mutations that form the intersubunit
disulfide bond with the glycan wedge mutation. The disulfide bond was formed with high efficiency in this
␣V_G307C/3_N305T_R563C mutant (Fig. 4A). For wildtype ␣V3 integrin, LIBS epitope exposure was partially
APRIL 20, 2007 • VOLUME 282 • NUMBER 16
induced by Mn2⫹ treatment and fully induced by cyclo-RGDfV peptide or Fn7–10 in the presence of Mn2⫹ (Fig. 5A).
The wedge mutant (␣V/3_N305T) bound LIBS antibodies
constitutively, irrespective of the conditions used. By contrast, the ␣V_G307C/3_N305T_R563C and ␣V_G307C/
3_R563C mutants did not bind LIBS antibodies even in the
presence of cyclo-RGDfV peptides or Fn7–10 (Fig. 5A),
showing that these mutant integrins were locked in the bent
conformation. In contrast to wild-type ␣V3, the disulfidebonded wedge mutant ␣V_G307C/3_N305T_R563C did
not bind full-length human fibronectin, even when activated
(Fig. 5B).
Distinctive results were obtained with small fibronectin fragments. The disulfide-bonded wedge mutant ␣V_G307C/
3_N305T_R563C bound the Fn7–10 and Fn9 –10 fragments
constitutively (Fig. 5, C and D). However, the amount of binding was lesser than that for the other mutants and wild-type
␣V3 (Fig. 5, C and D). Consistent with previous results, the
disulfide-bonded mutant ␣V_G307C/3_R563C did not
bind fibronectin, Fn7–10, and Fn9 –10, even in activating
conditions. As controls, the wedge mutant (␣V/3_N305T)
or the wedge mutation combined with single cysteine mutaJOURNAL OF BIOLOGICAL CHEMISTRY
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FIGURE 3. Effect of -tail domain CD loop mutation on ligand binding by ␣IIb3 and ␣V3 expressed in 293T cells. A, human Fg binding to ␣IIb3
and ␣V3. B, ligand-mimetic PAC-1 antibody binding to ␣IIb3. C, human Fn binding to ␣V3. D, binding of fibronectin fragments Fn9 –10 and Fn7–10 to
␣V3. 293T cell transfectants were treated with the indicated conditions and incubated with 50 g/ml fluorescently labeled ligands or 10 g/ml PAC-1 IgM as
described under “Experimental Procedures.” Binding is expressed as MFI of ligand staining as a percentage of MFI of Cy3-AP3 antibody staining.
Testing Extension and Deadbolt Models of Integrin Activation
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FIGURE 4. Intersubunit disulfide bond formation and effect on LIBS exposure and ligand binding. A, disulfide bond formation by cysteine mutants. Lysates from
35
S-labeled 293T cells that had been transiently transfected with wild-type or mutant integrins as indicated were immunoprecipitated with mAb AP3 (anti-3) and
subjected to non-reducing SDS-7.5% PAGE. Bands of ␣V (␣), 3 (), and ␣V3 heterodimer (␣-) are indicated. Positions of protein molecular size markers are shown
on the left. B, LIBS exposure. 293T cell transfectants were stained with anti-LIBS mAb D3 or LIBS1 in the presence of 1 mM Ca2⫹/1 mM Mg2⫹ or 1 mM Ca2⫹/1 mM Mn2⫹,
with or without 25 M cyclo-RGDfV (RGD). LIBS epitope expression is expressed as MFI of the D3 or LIBS1 staining as a percentage of MFI of AP3 mAb staining. C, soluble
Fn or Fg binding to 293T cell transfectants in the presence of 1 mM Ca2⫹/1 mM Mg2⫹ or 1 mM Ca2⫹/1 mM Mn2⫹ or 1 mM Ca2⫹/1 mM Mn2⫹ plus 10 g/ml activating mAb,
LIBS1. Binding is expressed as MFI of ligand staining as a percentage of MFI of staining with Cy3-AP3 antibody.
tions (␣V_G307C/3_N305T and ␣V/3_N305T_R563C)
bound fibronectin and the Fn7–10 or Fn9 –10 fragments
constitutively (Fig. 5, B–D). These data demonstrate that
even when activated, integrins in the bent conformation are
not accessible to large soluble ligands, although they are
accessible to small soluble ligand fragments.
Fn Fragments Induce Extension of Cell Surface ␣V3—As
mentioned above, a negative stain EM study reported that soluble ␣V3 remained in the bent conformation when exposed to
1 mg/ml Fn fragment in 0.2 mM Ca2⫹/0.2 mM Mn2⫹ (21). Notably, under the same conditions, we found that Fn 7–10 efficiently induced extension of cell surface ␣V3, as reported both
by the LIBS1 and D3 antibodies (Fig. 5A).
11918 JOURNAL OF BIOLOGICAL CHEMISTRY
DISCUSSION
In the ␣V3 structure (3), residue Ser-674 at the tip of the CD
loop of the -tail domain contacts the -I domain. The buried
surface area is quite small at 60 Å2. According to the deadbolt
model, the residues at the tip of the CD loop, by contacting the
6-strand near the 6-␣7 loop of the -I domain, restrain the
integrin in the resting, low affinity state (22, 37). Activating
mutations in the 6-␣7 loop of the 2 I domain (39) have been
interpreted as supporting the deadbolt model (37). However,
rearrangement of this region is directly coupled to rearrangement of the ligand binding site and thus to the affinity change
(12). Introduction of disulfide bonds (40) or other mutations
VOLUME 282 • NUMBER 16 • APRIL 20, 2007
Testing Extension and Deadbolt Models of Integrin Activation
(41, 42) into the 6-␣7 loop and ␣7-helix induced high affinity
by causing the rearrangement of the ligand binding site of the
-I domain, and there is no evidence that these mutations disrupt interaction with the -tail domain CD loop. Therefore,
mutation of the -tail domain CD loop is required to test the
deadbolt model. This model was independently tested using
different mutations and cell lines in two different laboratories,
with similar results. We found that mutations to Ala of three
residues in this loop, or deletions of residues 672– 674 or 672–
676, did not activate ligand binding by integrins ␣V3 or
␣IIb3 and had no effect on the ability of these integrins to be
activated by Mn2⫹ or antibodies. If local rearrangement of the
3 tail domain were sufficient for integrin affinity regulation, as
proposed in the deadbolt model, we should at least detect the
binding of small fibronectin fragments to CD loop mutants.
However, we found that the CD loop mutations had no effect on
the binding of the Fn7–10 and Fn9 –10 fragments. Therefore,
we conclude that the -tail domain CD loop does not regulate
ligand binding and does not act as a deadbolt. Recently, it was
reported that exchanging D658GMD in the -tail domain CD
loop of the 2 subunit with D672SSG of the 3 subunit or with
N658GTD, which introduces a glycan wedge sequence, activated the ␣M2 integrin (43). A crystal structure of ␣M2 is
not available. Whether the -tail domain CD loop of ␣M2
contacts the -I domain is unknown, and the overall orientaAPRIL 20, 2007 • VOLUME 282 • NUMBER 16
tion of the -tail domain relative to the -I domain in 2 integrins is unknown, making the structural consequences of mutations in the CD loop difficult to predict. However, it is known
that multiple substitutions in the 2 subunit, including those in
I-EGF domain 3, are activating, consistent with disruption of
interactions in the bent conformation (15, 44). Integrin activation is observed after many types of mutations. We believe that
our finding of a lack of effect of deletion or mutation of the
-tail domain CD loop, in the context of the structurally characterized ␣V3 integrin, which served as the basis for the deadbolt proposal, is the more telling test of this model.
Our data further demonstrate that transition from the bent
to extended conformation (i.e. integrin extension) is not only
required for integrin affinity regulation but also required for
accessibility to biological ligands. Integrin ␣V3 with the
␣V_GFFKR/GAAKR cytoplasmic mutation, which induces
inside-out activation, bound ligands with high affinity in nonactivating conditions and was in the extended conformation, as
shown by constitutive LIBS epitope exposure. However, this
constitutive LIBS exposure and ligand binding activity was
reversed when a disulfide bond was introduced between the ␣V
-propeller domain and the 3 I-EGF 4 domain, which locked
the integrin in the bent conformation. It has been demonstrated that activating mutations in the transmembrane and
cytoplasmic regions of ␣IIb3 induce constitutive LIBS epitope
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FIGURE 5. LIBS exposure and ligand binding of disulfide-bonded and glycan wedge mutants. A, LIBS exposure. 293T cell transfectants were stained with
anti-LIBS antibodies, D3 or LIBS1, in the presence of 1 mM Ca2⫹/1 mM Mg2⫹ or 0.2 mM Ca2⫹/0.2 mM Mn2⫹, with or without 25 M cyclo-RGDfV or 1 mg/ml Fn7–10.
LIBS epitope expression is expressed as MFI of the D3 or LIBS1 staining as a percentage of MFI of AP3 mAb staining. B, Fn binding. C, fibronectin fragment
Fn7–10 binding. D, fibronectin fragment Fn9 –10 binding. In B–D, 293T cell transfectants were stained with 50 g/ml fluorescently labeled ligands in the
presence of 1 mM Ca2⫹/1 mM Mg2⫹ or 1 mM Ca2⫹/1 mM Mn2⫹ plus 10 g/ml activating mAb, LIBS1, and subjected to flow cytometry. Binding is expressed as
MFI of ligand staining as a percentage of MFI of Cy3-AP3 antibody staining.
Testing Extension and Deadbolt Models of Integrin Activation
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VOLUME 282 • NUMBER 16 • APRIL 20, 2007
Downloaded from http://www.jbc.org/ by guest on June 11, 2020
exposure (7, 35), showing that integrin extension can occur in
the absence of ligand binding. These results are consistent with
a wide body of evidence that suggests that integrin affinity regulation is concomitant with extension during inside-out activation (11). Integrin ␣V3 with a disulfide bond between the ␣V
-propeller domain and the 3 I-EGF 4 domain and activated
with a hybrid domain glycan wedge appeared to remain in the
bent conformation as intended because the LIBS1 and D3
epitopes were not exposed. Although the glycan wedge mutant
activated binding to soluble fibronectin, this binding was completely reversed by the introduced disulfide bond. However, the
double mutant with the glycan wedge and the disulfide to
restrain the bent conformation bound Fn7–10 and Fn9 –10
fragments, although binding was lower than in the absence of
the disulfide. These results demonstrate that the introduced
disulfide did not prevent activation and that integrin extension
is required for access to the large macromolecular ligands to
which integrins bind physiologically.
In addition, we demonstrate that Mn2⫹ and binding of
Fn7–10 induce LIBS epitope exposure in integrin ␣V3, suggesting that these agents induced ␣V3 integrin extension on
the cell surface. Notably, we used the same Mn2⫹ concentration
(0.2 mM) and fibronectin fragment concentration (1 mg/ml) as
used in a recent EM study that suggested that after binding of
these agents, ␣V3 remained in a bent conformation (21).
However, aggregation was present in the EM preparations,
which may have resulted in the absence of sampling of extended
integrins (11, 21). Our results on cell surface ␣V3 are consistent with other EM studies on soluble ␣V3 bound to cycloRGDfV peptide (4), soluble ␣IIb3 bound to RGD peptide (13),
and ␣51 headpiece fragments bound to Fn7–10 (14), which
showed that integrins were extended and/or had the open headpiece conformation with the hybrid domain swung out after ligand
binding. In conclusion, our results fail to show any role for a deadbolt in the integrin in which this model was proposed, ␣V3, or in
integrin ␣IIb3, suggest that integrins extend on the cell surface
under conditions in which extension was not found in one EM
study, and show that integrin extension is important for both affinity regulation and ligand accessibility.
Tests of the Extension and Deadbolt Models of Integrin Activation
Jieqing Zhu, Brian Boylan, Bing-Hao Luo, Peter J. Newman and Timothy A. Springer
J. Biol. Chem. 2007, 282:11914-11920.
doi: 10.1074/jbc.M700249200 originally published online February 13, 2007
Access the most updated version of this article at doi: 10.1074/jbc.M700249200
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