1. doi:10.1016/j.jmb.2011.08.025 J. Mol. Biol. (2011) 414, 194–203
Contents lists available at www.sciencedirect.com
Journal of Molecular Biology
j o u r n a l h o m e p a g e : h t t p : / / e e s . e l s e v i e r. c o m . j m b
Interactions of Interleukin-8 with the Human
Chemokine Receptor CXCR1 in Phospholipid
Bilayers by NMR Spectroscopy
Sang Ho Park, Fabio Casagrande, Leah Cho, Lauren Albrecht
and Stanley J. Opella⁎
Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive,
La Jolla, CA 92093-0307, USA
Received 3 March 2011; CXCR1 is a receptor for the chemokine interleukin-8 (IL-8), a mediator of
received in revised form immune and inflammatory responses. Strategically located in the cell
19 July 2011; membrane, CXCR1 binds to IL-8 with high affinity and subsequently
accepted 11 August 2011 transduces a signal across the membrane bilayer to a G-protein-activated
Available online second messenger system. Here, we describe NMR studies of the
12 October 2011 interactions between IL-8 and human CXCR1 in lipid environments.
Functional full-length and truncated constructs of CXCR1 and full-length
Edited by A. G. Palmer III IL-8 were uniformly 15N-labeled by expression in bacteria followed by
purification and refolding. The residues responsible for interactions
Keywords: between IL-8 and the N-terminal domain of CXCR1 were identified by
membrane protein; specific chemical shift perturbations of assigned resonances on both IL-
solid-state NMR; 8 and CXCR1. Solution NMR signals from IL-8 in q = 0.1 isotropic bicelles
lipid bilayer; disappeared completely when CXCR1 in lipid bilayers was added in a 1:1
CXCR1; molar ratio, indicating that binding to the receptor-containing bilayers
IL-8 immobilizes IL-8 (on the ∼ 10 5 Hz timescale) and broadens the signals
beyond detection. The same solution NMR signals from IL-8 were less
affected by the addition of N-terminal truncated CXCR1 in lipid bilayers,
demonstrating that the N-terminal domain of CXCR1 is mainly responsible
for binding to IL-8. The interaction is tight enough to immobilize IL-8 along
with the receptor in phospholipid bilayers and is specific enough to result in
well-aligned samples in oriented sample solid-state NMR spectra. A
combination of solution NMR and solid-state NMR studies of IL-8 in the
presence of various constructs of CXCR1 enables us to propose a model for
the multistep binding process.
© 2011 Elsevier Ltd. All rights reserved.
*Corresponding author. E-mail address: sopella@ucsd.edu. Introduction
Abbreviations used: 1TM1–72, the first transmembrane
helix domain corresponding to residues 1–72 of CXCR1; The chemokine system regulates many biological
DHPC, 1,2-dihexanoyl-sn-glycero-3-phosphocholine; and pathological processes, including inflammation,
DMPC, 1,2-dimyristoyl-sn-glycero-3-phosphocholine; embryogenesis, metastasis, host defense against
GPCR, G-protein-coupled receptor; IL-8, interleukin-8; infection, and innate immunity. 1 Its broad role in
ND1–38, the N-terminal extracellular domain corresponding regulation is accomplished through the binding of
to residues 1–38 of CXCR1; NT39–350, N-terminal truncated specific chemokines to their respective G-protein-
construct corresponding to residues 39–350 of CXCR1; OS coupled receptors (GPCRs). For example, the release
solid-state NMR, oriented sample solid-state NMR; HSQC, of the chemokine interleukin-8 (IL-8) by several cell
heteronuclear single quantum coherence. types is a response to an inflammatory stimulus and
0022-2836/$ - see front matter © 2011 Elsevier Ltd. All rights reserved.

2. Interactions of Interleukin-8 with CXCR1 195
results in the migration of leukocytes, including various lipid environments enable us to propose a
neutrophils, monocytes, T- and B-lymphocytes, and multistep model for the interactions between IL-
basophils, to these sites. IL-8 has also been shown to 8 and CXCR1 in lipid bilayers.
stimulate self-renewal of breast cancer stem cells in
vitro. 2 In humans, two high-affinity IL-8 receptors,
CXCR1 and CXCR2, have been characterized, 3,4 and
Results
CXCR1 has been identified as a target for blocking
the formation of breast cancer stem cells that drive
tumor growth and metastasis. 5 Interaction of the N-terminal domain of CXCR1
CXCR1 belongs to the family of chemokine with membranes
receptors with seven transmembrane helices that
couple to heterotrimeric G-proteins for signal We expressed, purified, and characterized the N-
transduction. 6 We have demonstrated the expres- terminal extracellular domain of CXCR1 (ND1–38)
sion in Escherichia coli, and purification and refold- that corresponds to the first 38 residues of CXCR1.
ing of functional full-length CXCR1, and numerous The 15N chemical shift oriented sample solid-state
constructs of the receptor, including N-terminal NMR (OS solid-state NMR) spectrum of uniformly
15
truncated CXCR1 (NT39–350), C-terminal truncated N-labeled ND1–38 in magnetically aligned bilayers
CXCR1 (CT1–319), both N- and C-terminal double- demonstrates that the sample is well aligned on the
truncated CXCR1 (DT23–319), the first transmem- surface of the bilayers (Fig. 1a). The signals that
brane helix domain of CXCR1 (1TM1–72), and the result from cross-polarization neither are centered
N-terminal extracellular domain (ND1–38) without at the isotropic frequency nor have the appearance
any residues associated with the first transmem- of a powder pattern, providing strong evidence for
brane helix. 7,8 We have also characterized the local the existence of specific interactions between the
and global dynamics of full-length CXCR1 in phospholipids and amino acid residues in the N-
membrane environments using a combination of terminal domain of CXCR1. As a control, IL-8 was
solution NMR and solid-state NMR techniques. 9 subject to cross-polarization in the presence of
The mechanisms by which chemokines modulate magnetically aligned bilayers not containing a
specific biological activities are central to under- construct of CXCR1, and as expected, no NMR
standing how GPCRs transmit signals through the signals were observed (data not shown) because IL-
membrane bilayer to the interior of the cell. 8 is water soluble and does not interact with
Previously, solution NMR spectroscopy has been phospholipids.
used to characterize the structure of IL-8 alone 10–12 The 1H/ 15N heteronuclear single quantum coher-
and bound to synthetic peptides with sequences ence (HSQC) solution NMR spectrum of ND1–38 in
corresponding to portions of the N-terminal domain aqueous buffer (Fig. 1b, black contours) has a very
of CXCR1. 12,13 Solution NMR is feasible in these limited dispersion of 1H amide chemical shifts
situations because of the small size and high (b 1 ppm), which is typical of relatively small
solubility of IL-8 and the peptides derived from polypeptides with little or no secondary or tertiary
the N-terminal sequence of CXCR1. These studies structure. Moreover, no homonuclear 1H/ 1H nu-
have identified a probable location on IL-8 that clear Overhauser enhancement cross peaks could be
interacts with the N-terminal domain of CXCR1; observed in standard two-dimensional experiments.
however, these model systems lack several essential In contrast, IL-8 yields a well-resolved solution
components of the biological system, namely, the NMR spectrum that is typical of a native globular
additional residues present in full-length GPCR and protein, since it has a wide dispersion (N 6 ppm) of
1
the planar lipid bilayer environment where the H amide chemical shifts and relatively narrow line
receptor resides. For example, the earlier studies widths (Fig. S1b).
using relatively short synthetic peptides could not Compared to aqueous solution, there are signifi-
detect interactions with extracellular loops or other cant chemical shift changes and broadening of a
regions of CXCR1 or the effects of lipid bilayers on subset of backbone amide signals of ND1–38 includ-
the structures, dynamics, and interactions of CXCR1 ing the side-chain signal of Trp10 when lipids are
and IL-8. added to the sample (Fig. 1c and Fig. S1a). In
Here, we describe studies that use uniformly 15N- contrast, IL-8 does not interact with lipid bilayers,
labeled full-length CXCR1, several of its truncated and therefore, no significant spectral changes
constructs, two versions of its N-terminal domain, including to the side-chain signal of Trp57 were
and native IL-8 in both free and bound states. observed in the presence of phospholipid bilayers
Through utilization of both solution NMR and solid- (Fig. S1b). This is consistent with the OS solid-state
state NMR experiments, it was possible to monitor NMR result on IL-8 alone in the presence of lipid
the proteins in a wide range of lipid environments, bilayers.
including phospholipid bilayers. Appropriate con- The samples made from mixtures of long-chain
trol experiments on both of the proteins in the phospholipids [e.g., 1,2-dimyristoyl-sn-glycero-3-

3. 196 Interactions of Interleukin-8 with CXCR1
Fig. 1. Membrane interaction of
ND1–38 and dissociation of the ND1–
38/IL-8 complex from the mem-
brane. (a and b) 15 N chemical
shift OS solid-state NMR spectra of
uniformly 15N-labeled ND1–38 alone
(a) and in complex (b) with unla-
beled IL-8 in q = 3.2 bicelles. (c and d)
1
H/ 15N HSQC solution NMR spec-
tra of uniformly 15N-labeled ND1–38
alone (c) and in complex (d) with
unlabeled IL-8 in aqueous buffer
(black contours and one-dimension-
al spectrum) and in q = 3.2 bicelles
(red contours and one dimensional-
spectrum). The side-chain signal of
Trp10 residue is indicated. One-
dimensional 15N-edited 1H solution
NMR spectra are aligned along the
top of the corresponding two-dimen-
sional spectra to compare the signal
intensities. The molar ratio of the
complex was 1:1.
phosphocholine (DMPC)] and short-chain phospho- was weakly aligned using fd bacteriophage particles
lipids [e.g., 1,2-dihexanoyl-sn-glycero-3-phospho- in aqueous buffer solution.
choline (DHPC)] have their molar ratio (long/
short) characterized by the parameter “q” and are Dissociation of the N-terminal domain of CXCR1
referred to as “bicelles.” These protein-containing bound to IL-8 from membranes
lipid mixtures enable the structures and dynamics of
the proteins to be characterized by solution NMR The spectra of IL-8 bound to ND1–38 in lipid
and solid-state NMR experiments; q values less than bilayers provide insights into the ternary complex of
about 1.5 result in isotropic bicelles that are IL-8, CXCR1, and phospholipid bilayers (Fig. 1b and
generally suitable for solution NMR experiments, d). There were no significant chemical shift changes
and those with values greater than about 2.5 form in the solution NMR spectrum of the ND1–38 bound
magnetically alignable bilayers that immobilize the to IL-8 when lipid bilayers were added to the
protein and require solid-state NMR methods to aqueous buffer. Remarkably, the signals of free
obtain high-resolution spectra. 14–17 ND 1–38 that were broadened out due to the
In isotropic q = 0.1 bicelles, the largest chemical membrane interaction (Fig. 1c, red contours) reap-
shift changes were observed primarily near the N- pear when IL-8 is bound to ND1–38, including the
terminus (residues 2–16) of ND1–38 (Fig. S1a). In Trp10 side-chain signal (Fig. 1d, red contours).
magnetically aligned q = 3.2 bilayer samples, the Overall, the line widths of the signals from ND1–38
most affected signals, including that from the Trp10 bound to IL-8 are only slightly broader than those of
side chain, were broadened beyond detection in free ND1–38. Taken together, these results demon-
solution NMR spectra (Fig. 1c, red contours). This strate that ND1–38 does not interact with lipid
significant broadening of the first 16 residues of bilayers when bound to IL-8, and IL-8 does not
CXCR1 does not result from weak alignment of the interact with bilayers in the absence of the N-
protein in the liquid crystalline phase but rather terminal domain of CXCR1. The inability, despite
from the interactions with the lipid bilayers, since in extensive efforts, to obtain solid-state NMR signals
a control experiment, all the signals that were only from ND1–38 when it is complexed with IL-8 in the
slightly broadened could be observed when ND1–38 presence of aligned phospholipid bilayers further

4. Interactions of Interleukin-8 with CXCR1 197
supports the finding that the binding of IL-8 results in three distinct regions of the IL-8 sequence:
in the dissociation of the N-terminal domain of residues 12, 17, and 20 in the N-loop; residues 44,
CXCR1 from phospholipid bilayers (Fig. 1b). 48, 49, and 50 in the third β-strand; and residues 61
and 62 in the C-terminal helix (Fig. 2d). This
Binding site mapping of the IL-8 and identifies the regions of IL-8 that interact with the
CXCR1 complex N-terminal domain of CXCR1. These findings are
similar to those from previous studies performed
The backbone resonance assignments of free IL- with a synthetic peptide corresponding to the first 40
8 under the experimental conditions used here were residues of the N-terminal domain of CXCR1 18 and
made by comparisons to the previously assigned with a 17-residue peptide, corresponding to residues
spectra 10 and confirmed by comparisons with 9–29 of CXCR1 where residues 15–19 were replaced
1
H/ 15N HSQC spectra of selectively Leu, Ile, Val, with a single six-amino hexanoic acid moiety. 13
and Phe 15N-labeled samples as well as convention- It has been reported that not only the N-terminal
al triple-resonance experiments performed on uni- domain but also the extracellular loops of CXCR1
formly 13C/ 15N-labeled samples. are involved in the interaction with IL-8. 19 The
The amino acid residues that form the binding spectral changes in IL-8 by the addition of
sites of IL-8 and of ND1–38 were identified by N-terminal truncated CXCR1 (NT39–350) in q = 0.1
mapping the chemical shift perturbations resulting isotropic bicelles provide evidence for the specific
from complex formation between one uniformly interactions between IL-8 and extracellular loops of
15
N-labeled polypeptide in the presence of its CXCR1 (Fig. 2b). Although the extent of the
unlabeled counterpart. The expanded region of chemical shift perturbations of IL-8 by NT39–350
1
H/ 15N HSQC solution NMR spectra of uniformly was not as large as those by ND1–38, significant
15
N-labeled IL-8 shows the specific chemical shift line broadening of the signals, except the first six
perturbation of backbone amide resonances follow- N-terminal residues, and relatively large chemical
ing the addition of unlabeled ND1–38 (Fig. 2a). The shift changes in Leu17 and Lys23 of IL-8 were
plot of chemical shift changes as a function of observed (Fig. 2e).
residue number indicates that relatively large The binding site of the N-terminal region of
chemical shift changes (N0.06 ppm) are observed CXCR1 has been characterized by the measurement
Fig. 2. Interaction of IL-8 with truncated CXCR1 constructs. (a–c) Expanded region of 1H/ 15N HSQC solution NMR
spectra: (a) uniformly 15N-labeled IL-8 alone (black contours) and in complex with unlabeled ND1–38 (red contours) in
aqueous buffer; (b) uniformly 15N-labeled IL-8 alone (black contours) and in complex with unlabeled NT39–350 (red
contours) in q = 0.1 isotropic bicelles; (c) uniformly 15N-labeled ND1–38 alone (black contours) and in the presence of
varying amounts of unlabeled IL-8 in aqueous buffer. The molar ratios of the IL-8 monomer to ND1–38 were 0.25 (green
contours), 0.5 (blue contours), and 1 (red contours), respectively. (d–f) Chemical shift perturbation plot of backbone amide
signals as a function of residue number: (d) plot of IL-8 by addition of an equimolar concentration of ND1–38 to the IL-
8 monomer; (e) plot of IL-8 by addition of an equimolar concentration of NT39–350 to the IL-8 monomer; (f) plot of ND1–38
as a function of the residue number by addition of 0.25 (green), 0.5 (blue), and 1 (red) ratios of the IL-8 monomer to ND1–38.

5. 198 Interactions of Interleukin-8 with CXCR1
and 15N chemical shifts. With increasing concentra-
tions of IL-8, the amide resonances of the affected
residues shift incrementally from the frequencies
observed in the free state to those of the fully bound
state (Fig. 2c). The chemical shift frequencies stop
changing when approximately one equivalent of
the unlabeled IL-8 monomer has been added to the
solution containing labeled ND1–38 (Fig. 2f). The
binding affinity of ND1–38 and IL-8 was determined
by treating the binding-induced chemical shift
changes as a titration. 20 The Kd is approximately 70
μM under these conditions. Previously, N-terminal
fragments of CXCR1 have been shown to bind IL-
8 with an affinity 3–5 orders of magnitude weaker
than that of the full-length receptor. 13,18
Binding of IL-8 to full-length CXCR1 in membrane
environments
Interactions of IL-8 with polypeptides whose
sequences are derived from the N-terminal region
Fig. 3. Interaction of IL-8 with full-length CXCR1. 15N- of CXCR1 have been described previously. 13,18,21
edited 1H solution NMR spectra of uniformly 15N-labeled IL-
8 in the presence of unlabeled full-length CXCR1 in q=0.1
However, information about the interaction of IL-
isotropic bicelles. The molar ratios of CXCR1 to IL-8 monomer 8 with full-length CXCR1 is scarce largely because
are listed on the right side of their respective spectra. of the experimental difficulties encountered in the
study of large membrane proteins in phospholipid
bilayers. We have developed protocols for the
expression, purification, and refolding of various
and analysis of intermolecular nuclear Overhauser CXCR1 constructs in phospholipid bilayers includ-
enhancements observed between IL-8 and the 17- ing the full-length protein. 7,8 This enables us to
residue peptide derived from CXCR1 described study the interactions of IL-8 with full-length and
above. 13 Here, we take advantage of having truncated constructs of CXCR1 in membrane
prepared an isotopically labeled polypeptide by environments.
bacterial expression corresponding to the N-terminal Figure 3 shows the effects of adding increasing the
domain of CXCR1 to map the binding site using amounts of CXCR1 in bilayers to a q = 0.1 isotropic
heteronuclear solution NMR experiments. The bicelle solution containing uniformly 15N-labeled
changes in the spectrum of ND1–38 resulting from IL-8. In the absence of the receptor-containing
the addition of unlabeled IL-8 have the characteris- bilayers, the 15N-edited 1H solution NMR spectrum
tics of “fast exchange” on the timescales of the 1H of the amide region has narrow and well-dispersed
Fig. 4. Interaction of IL-8 with three constructs of CXCR1 in phospholipid bilayers. 15N chemical shift OS solid-state
NMR spectra of uniformly 15N-labeled IL-8 bound to the constructs of CXCR1 in q = 3.2 aligned bicelles: (a) full-length
CXCR1; (b) the first transmembrane helix domain of CXCR1 (1TM1–72); (c) N-terminal truncated CXCR1 (NT39–350). The
molar ratio of IL-8 to CXCR1 in each sample was 1:1.

6. Interactions of Interleukin-8 with CXCR1 199
resonances, typical of a small globular protein in 8, 21 and their N-terminal domains have high
aqueous solution. As the addition of the receptor sequence homology (Fig. S2). Tryptophan residues
approaches a 1:1 molar ratio of CXCR1:IL-8, nearly are commonly found near the membrane surface,
all signals from labeled IL-8 broaden systematically since the polar amide group and hydrophobic ring
and disappear into the baseline, with the exception structure of this amino acid facilitate its localization
of a few signals that have been assigned to residues at the polar/apolar interface. 25 Significantly, signals
near the N- and C-termini. The result was more from both the backbone and the side chain of the
dramatic in lipid bilayers, because with CXCR1 in tryptophan residue in ND1–38 are broadened beyond
proteoliposomes at a 1:1 molar ratio with IL-8, all of detection in the presence of lipid bilayers (Fig. 1c),
the IL-8 signals disappear as a result of their immo- suggesting that the tryptophan residue may serve as
bilization upon binding to the CXCR1-containing an anchor on the membrane surface. The tryptophan
bilayers. Refolded CXCR1 prepared by our methods residues located in the N-terminal domain of rabbit
has been shown to bind IL-8 with an affinity (Kd of CXCR1, one of which is located in the same position
1–5 nM) and to couple to G-protein (EC50 ∼ 1 nM), 7,8 as a tryptophan in the human CXCR1 sequence,
which are similar to the values previously reported have been shown to be involved directly in
in the literature. 3 membrane interactions. 24
The chemical shift perturbation plot for labeled IL-
Critical role of the N-terminal domain of CXCR1 8 in Fig. 2d obtained by the addition of unlabeled
for IL-8 binding ND1–38 shows substantial changes in three regions
of the primary sequence. The residues that contrib-
Comparisons of 15N chemical shift OS solid-state ute to the binding cleft identified in the three-
NMR spectra of uniformly 15N-labeled IL-8 bound dimensional structure of IL-8 were the ones most
to unlabeled full-length CXCR1 and constructs strongly affected by the interaction with ND1–38. The
consisting of the first transmembrane helix domain central region of the ND1–38 primary sequence
(1TM1–72) and the N-terminal truncated (NT39–350) (residues 18–27) was most strongly affected by
receptors in lipid bilayers are shown in Fig. 4. These binding to IL-8. This suggests that ND1–38 may
results demonstrate that the N-terminal domain of adopt an extended conformation when complexed
CXCR1 is mainly responsible for the binding of IL-8. to IL-8. Although the proline residues of ND1–38
The OS solid-state NMR signals of IL-8 were intense were not monitored in our experiments, alanine-
and well resolved when IL-8 was added to full- scanning studies have shown that the two prolines,
length and 1TM1–72 receptors aligned in lipid 21 and 29, as well as Tyr27 contribute to the
bilayers, demonstrating that their interaction is interactions with IL-8, suggesting that the hydro-
strong enough to immobilize and align IL-8 along phobic characteristics of these residues play roles in
with the receptor at a unique orientation in the binding to the N-terminal domain of CXCR1. 26
magnetically aligned bilayers (Fig. 4a and b). As a Many studies of chemokines and their interactions
control, no IL-8 signals could be observed in OS with receptors have concluded that one or more of
solid-state NMR experiments in a sample containing the extracellular loops of the receptors are involved.
labeled IL-8 and an unlabeled NT39–350 (Fig. 4c). In particular, alanine-scanning experiments have
Since binding to the receptor is necessary to shown that the third and fourth extracellular loops
immobilize and align the IL-8, this suggests that of CXCR1 are involved in the binding to IL-8. 19 An
the binding site is predominantly located in the overall broadening of solution NMR signals of IL-
N-terminal region of the receptor. 8 in the presence of 1TM1–72 (data not shown) and
NT39–350 (Fig. 2b) at a molar ratio of 1:1 was
observed, but in both cases, the signals were less
Discussion affected than those of IL-8 in the presence of the full-
length receptor (Fig. 3). Two possible reasons for this
Comparisons between the solution NMR and difference are that the binding of IL-8 to 1TM1–72 is
solid-state NMR spectra of ND1–38 alone and not as tight as for the full-length receptor or that the
bound to IL-8 provide information about the binding is as tight as full-length receptor, but the
influence of the lipid bilayer on interactions of the smaller size of the IL-8 and 1TM1–72 complex
N-terminal domain of CXCR1 and IL-8. The N- (∼ 18 kDa) reorients faster than IL-8 and the full-
terminal region of CXCR1 determines the specificity length complex (∼ 52 kDa) in isotropic q = 0.1
and affinity for IL-8. 22,23 Recently, a 34-residue bicelles. In the case of the N-terminal truncated
peptide with a sequence corresponding to the N- receptor, the molecular mass of NT39–350 is reduced
terminal residues of rabbit CXCR1 was shown to by only 10% compared to the full-length receptor;
interact with the membrane surface by monitoring thus, the reduction in rotational correlation time is
fluorescence of two tryptophan residues of the unlikely to be sufficient to account for the spectral
peptide. 24 Both human and rabbit CXCR1 receptors changes. It may be that the changes are a manifes-
have similar affinity and specificity for human IL- tation of weak interactions of IL-8 to extracellular

7. 200 Interactions of Interleukin-8 with CXCR1
loop regions of the receptor without the contribu- peripheral membrane protein, interacts transiently
tions from the missing residues in the N-terminal with the membrane surface and adopts a rela-
domain of the receptor. tively well-defined yet still flexible structure that
The role of dimerization of IL-8 in binding CXCR1 may contribute to receptor selectivity. Our NMR
is not fully understood, but recent studies have data on the N-terminal domain of CXCR1 in the
shown that the IL-8 monomer binds to the N- absence and presence of phospholipid bicelles
terminal domain of CXCR1 with higher affinity than clearly demonstrate the significant effects of the
the IL-8 dimer. 27,28 We used only the monomeric membrane environment on the structure and
form of CXCR1, and in all of our experiments, the dynamics of this domain (Fig. 1). In particular,
spectral changes stopped when an approximately the Trp10 side chain is likely to be embedded in
equimolar concentration of CXCR1 monomer to the the bilayer.
IL-8 monomer was achieved. These results suggest In the second step, after binding to IL-8, the N-
that one molecule of CXCR1 binds to one molecule terminal domain dissociates from the membrane
of the IL-8 monomer. Since IL-8 exists as a stable surface. Upon interaction with IL-8, the solution
homodimer in an aqueous solution, it is possible NMR signals of the N-terminal domain that were
that the chemical shift perturbation of IL-8 upon completely broadened out due to the membrane
binding to CXCR1 constructs results not only from interaction (step 1) reappeared as a result of
the direct interaction between them but also from dissociation of the domain from the membrane
the dimer-to-monomer transition of IL-8. (Fig. 1d). The complementary OS solid-state NMR
It is essential to obtain atomic-resolution structural spectrum of the domain in complex did not yield
details about how IL-8 interacts with its high-affinity any signals, which also demonstrates that the
membrane-embedded receptors in order to under- complex is no longer immobilized by interactions
stand the first step of the complex signaling cascade. with the membrane (Fig. 1b).
In the meantime, we interpret the NMR results In the third step, the complex of IL-8 and the N-
discussed above in terms of a multistep series of terminal domain rearranges to engage a second
interactions between IL-8 and CXCR1 with signifi- binding site on the receptor, most likely involving
cant contributions from the phospholipid bilayers one or more extracellular loops (Fig. 2b and e). This
(Fig. 5). Thus, we propose that the ternary complex of step might be the trigger for the conformational
IL-8/CXCR1/bilayer is an essential species. changes in the receptor needed to activate secondary
In the first step, the N-terminal domain of signaling cascades. This does not exclude the
CXCR1, which has many characteristics of a possibility that IL-8 interacts simultaneously with
Fig. 5. Model of IL-8 interacting with CXCR1 in membranes. Step 1: The N-terminal domain of CXCR1 (green) is
flexible yet structured by interacting with the surface of the membrane, contributing to receptor selectivity. The first half
of the domain is mainly involved in membrane interaction, and Trp10 serves as an anchor on the extracellular side of the
membrane. Step 2: The strong interaction between the N-terminal domain and IL-8 dissociates the domain from the
membrane surface. Step 3: The N-terminal domain in complex with IL-8 is translated to the second binding site of the
extracellular loops, potentially creating a conformational change in CXCR1 for subsequent G-protein activation. A
monomer from the IL-8 dimer structure (Protein Data Bank ID 2IL8) is represented. The residues of IL-8 (12, 17, 20, 44, 48,
49, 50, 61, and 62) whose chemical shifts were perturbed significantly by interaction with the N-terminal domain of
CXCR1 are shown as red spheres.

8. Interactions of Interleukin-8 with CXCR1 201
the N-terminal domain and extracellular loops of the (NT39–350), the first transmembrane helix domain of
receptor. CXCR1 (1TM1–72), and the N-terminal extracellular do-
A two-site mechanism of chemokine receptor main of CXCR1 (ND1–38) were expressed, purified, and
interaction in which the N-terminal domain and refolded as described previously. 7,8 The amino acid
sequences of the CXCR1 constructs are shown in Support-
extracellular loop in the receptor are involved in the
ing Information. The amino acid sequence of ND1–38
ligand interaction has been proposed based on the substitutes Ser for Cys at position 30 to prevent compli-
various structure–function studies reviewed by cations due to intermolecular disulfide bond formation.
Rajagopalan and Rajarathnam. 29 Although it is not For the solution NMR experiments, the concentration of
fully understood how the two-site mechanism IL-8 and ND1–38 polypeptides was 0.1 mM, in 20 mM
mediates affinity, selectivity, and activation of the Hepes, at pH 5.5, in 400 μl of 90% H2O/10% 2H2O. The
receptor, the N-terminal residues of the receptor are protein-containing bicelle samples of IL-8 and ND1–38
shown to be essential for both binding affinity and were prepared by dissolving the lyophilized polypeptides
receptor selectivity. 22 The OS solid-state NMR data directly into premixed solutions containing DMPC and
presented here show that the N-terminal domain of DHPC phospholipids. The lipids were obtained from
Avanti Polar Lipids†. The isotropic (q = 0.1) and magnet-
CXCR1 is mainly responsible for the strong interac-
ically alignable (q = 3.2) samples contain 10% DHPC (w/v)
tion with IL-8 (Fig. 4). and 10% DMPC (w/v), respectively. The samples of the
It has been proposed that the chemokine N-terminal CXCR1 constructs, except for the soluble ND1–38 poly-
“ELR” motif interacts with the extracellular loops of peptide, were prepared from proteoliposome pellets [20%
the receptor. 30,31 Recently, the highly dynamic (w/v) lipid] in which 1 mg of the polypeptide was
N-terminus including the ELR motif of the chemokine reconstituted into a solution containing 10 mg of DMPC.
SDF-1 has been proposed to play a crucial role in the For the titration experiments, a stock solution of the
interaction with its receptor CXCR4. 32 However, we unlabeled proteins under the same buffer conditions was
do not observe experimental NMR evidence that the added to the uniformly 15N-labeled proteins so that the
N-terminal ELR motif of IL-8 interacts with full-length final molar ratios were 0.25, 0.5, and 1.0.
For the OS solid-state NMR experiments, 1 mg of the
or N-terminal truncated CXCR1. This may be due to
unbound form of uniformly 15N-labeled ND1–38 and IL-
differences between the two receptors, or it may 8 were dissolved in 200 μl of a q = 3.2 lipid mixture
require future studies of the structures and mecha- containing 20% DMPC (w/v) and 20 mM Hepes, at
nisms of GPCRs to fully sort out. pH 5.5. The complex was formed by adding 0.6 mg of
The interactions between ligands and their mem- uniformly 15N-labeled IL-8 to the unlabeled CXCR1
brane-embedded receptors, especially GPCRs, are constructs or 1 mg of labeled ND1–38 to the unlabeled IL-
the first step in initiating the complex cascades of 8 in a final molar ratio of 1:1. The pH of the IL-8: 1TM1–72
protein interactions known to regulate physiological complex was adjusted to 4.7 to increase the sample
processes in mammals. Here, we demonstrate that solubility, while the pH of the other samples was 5.5.
the interaction between IL-8 and its receptor, CXCR1,
must be analyzed in the context of the phospholipid NMR spectroscopy
bilayer environment. Solid-state NMR spectroscopy
is unique in providing atomic-resolution information The solution NMR experiments were performed at
about membrane proteins and their complexes in 40 °C on a Bruker DRX 600-MHz spectrometer equipped
phospholipid bilayers under conditions where signal with 5-mm triple-resonance cryoprobe with z-axis
transduction occurs. The resulting NMR data enable gradient. Heteronuclear solution NMR experiments were
us to propose a model for the interactions between performed on uniformly 15 N-labeled or uniformly
13
IL-8 and CXCR1 that involve the phospholipid C/ 15N-double-labeled samples with a protein concen-
tration of 0.1 mM. One-dimensional 15N-edited 1H NMR
bilayer, IL-8, the N-terminal domain of CXCR1,
spectra resulted from signal averaging of 128 transients.
and residues in inter-helical loops near the C-terminus. Two-dimensional 1H/ 15N HSQC spectra were obtained
In summary, we conclude that the membrane bilayer on uniformly and selectively 15N-labeled samples. Triple-
plays a role that is as important as the structural resonance HNCA and HNCOCA experiments were
features of the two protein components in the performed on 13C/ 15N-double-labeled IL-8 and ND1–38
interactions of IL-8 and CXCR1 in the first step of for resonance assignments. The chemical shift perturba-
transducing biological signals. tions by addition of unlabeled samples were calculated
using the equation
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðDyΗ Þ2 + ðDyN = 5Þ2
Materials and Methods Dy =
2
Sample preparation where ΔδH is the change in the backbone amide proton
chemical shift and ΔδN is the change in backbone amide
nitrogen chemical shift.
IL-8 was expressed and purified as described
previously. 22 Full-length CXCR1 and three truncated
constructs including N-terminal truncated CXCR1 † www.avantilipids.com

9. 202 Interactions of Interleukin-8 with CXCR1
The solid-state 15N NMR spectra were obtained at 40 °C 7. Park, S. H., Prytulla, S., De Angelis, A. A., Brown,
on a 700-MHz Bruker Avance spectrometer. The J. M., Kiefer, H. & Opella, S. J. (2006). High-resolution
homebuilt 1H/ 15N double-resonance probe used in the NMR spectroscopy of a GPCR in aligned bicelles.
experiments had a 5-mm-inner-diameter solenoid coil J. Am. Chem. Soc. 128, 7402–7403.
tuned to the 15N frequency and an outer MAGC (modified 8. Casagrande, F., Maier, K., Kiefer, H., Opella, S. J. &
Alderman–Grant coil) “low E” coil tuned to the 1H Park, S. H. (2011). Expression and purification of G-
frequency. 33 The one-dimensional 15N chemical shift protein coupled receptors for NMR structural studies.
NMR spectra were obtained by spin-lock cross-polariza- In Production of Membrane Proteins (Robinson, A. S.,
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an acquisition time of 10 ms. Transients (4096) were signal 9. Park, S. H., Casagrande, F., Das, B. B., Albrecht, L.,
averaged for each spectrum, and an exponential function Chu, M. & Opella, S. J. (2011). Local and global
corresponding to line broadening of 100 Hz was applied to dynamics of the G protein-coupled receptor CXCR1.
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The NMR data were processed using the programs 10. Clore, G. M., Appella, E., Yamada, M., Matsushima,
NMRPipe/NMRDraw. 34 The chemical shift frequencies K. & Gronenborn, A. M. (1989). Determination of the
were externally referenced to 15N-labeled solid ammoni- secondary structure of interleukin-8 by nuclear
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1
Acknowledgements H NMR solution structure of an active monomeric
interleukin-8. Biochemistry, 34, 12983–12990.
This research was supported by grants from the 13. Skelton, N. J., Quan, C., Reilly, D. & Lowman, H.
National Institutes of Health and utilized the Biotech- (1999). Structure of a CXC chemokine-receptor frag-
ment in complex with interleukin-8. Structure, 7,
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of Proteins at the University of California, San Diego, 14. De Angelis, A. A., Nevzorov, A. A., Park, S. H.,
which is supported by grant P41EB002031. F.C. was Howell, S. C., Mrse, A. A. & Opella, S. J. (2004). High-
supported by postdoctoral fellowships from the Swiss resolution NMR spectroscopy of membrane proteins
National Science Foundation (PBBSP3-123151) and in aligned bicelles. J. Am. Chem. Soc. 126, 15340–15341.
the Novartis Foundation, formerly the Ciba-Geigy 15. Park, S. H., De Angelis, A. A., Nevzorov, A. A., Wu, C.
Jubilee Foundation. H. & Opella, S. J. (2006). Three-dimensional structure
of the transmembrane domain of Vpu from HIV-1 in
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