From the Divisions of Cell Biology and Immunology and
§ Biological Chemistry and Molecular Microbiology, School of
Life Sciences, University of Dundee,
Dundee DD1 5EH, Scotland, United Kingdom
Received for publication, October 16, 2002, and in revised form, November 15, 2002
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
Sialic acid-binding immunoglobulin-like lectins
(Siglecs) recognize sialylated glycoconjugates and play a role in
cell-cell recognition. Siglec-7 is expressed on natural killer cells
and displays unique ligand binding properties different from other members of the Siglec family. Here we describe the high resolution structures of the N-terminal V-set Ig-like domain of Siglec-7 in two
crystal forms, at 1.75 and 1.9 Å. The latter crystal form reveals the
full structure of this domain and allows us to speculate on the
differential ligand binding properties displayed by members of the
Siglec family. A fully ordered N-linked glycan is observed, tethered by tight interactions with symmetry-related protein molecules in the crystal. Comparison of the structure with that of sialoadhesin and a model of Siglec-9 shows that the unique preference of Siglec-7 for The Siglecs1 are a
specialized subgroup of the Ig superfamily that share significant
sequence similarity and the ability to recognize sialylated
glycoconjugates (1). There are at least 11 bona fide members
in humans and potentially 8 in mice, all of which are type 1 membrane
proteins, containing an N-terminal sialic acid-binding V-set Ig
domain and varying numbers of C2-set Ig domains (2). Apart from
myelin-associated glycoprotein (MAG, Siglec-4), which is found uniquely
in the nervous system, Siglecs are expressed predominantly in the
hemopoietic and immune systems and appear to mediate both adhesive and
signaling functions (3).
Siglecs can be divided into two subgroups based on sequence similarity
in the extracellular and intracellular regions. Sialoadhesin (Siglec-1), CD22 (Siglec-2), and MAG constitute one subgroup, share
~25-30% sequence identity in the extracellular region, and have
divergent cytoplasmic tails. The second subgroup is made up of the
CD33-related Siglecs that include CD33 (Siglec-3) and the recently
discovered human Siglecs 5-11 (reviewed in Ref. 3). These proteins
share 50-80% sequence similarity and have two highly conserved
tyrosine-based motifs in their cytoplasmic tails. In humans, the
CD33-related Siglec genes are clustered on chromosome 19q13.3-4
and are separate from the genes encoding CD22, MAG (19q13.1), and
sialoadhesin (20p). Recent studies using specific monoclonal antibodies
have shown that the CD33-related Siglecs are expressed in a partially
overlapping manner on all major leukocytes of the innate immune system,
suggesting a role for these proteins in regulation of leukocyte
function. This is further supported by the finding that human
CD33-related Siglecs have two conserved immunoreceptor tyrosine-based
inhibitory motif-like sequences, which in all cases studied can
interact with the tyrosine phosphatases SHP-1 and SHP-2 following
tyrosine phosphorylation (4-8). Recent studies of CD22
(Siglec-2) have also illustrated the importance of sialic acid
recognition and specificity in triggering biological functions mediated
by Siglecs (9-11).
To date, structural information on the Siglecs is limited to the V-set
domain of sialoadhesin (Siglec-1) in complex with 3'-sialyllactose (12). A 1.8-Å crystal structure revealed the basis for
recognition of the terminal sialic acid. This mode of recognition is
likely to be common to all Siglecs. However, the structure did not give insights into the differential specificity for sialic acid linkages exhibited by the Siglecs. Furthermore, the low sequence similarity shared between sialoadhesin and the CD33-related Siglecs is an obstacle
to interpreting mutagenesis data in a structural framework. It would
therefore be desirable to obtain the structure of at least one member
of the CD33-related subgroup, which could be used as a template for
modeling other CD33-related Siglecs as well as for gaining molecular
insights into sialic acid linkage recognition in the Siglec family. In
this context, Siglec-7 is of particular interest since it is the major
Siglec expressed by natural killer cells (13, 14) and possesses a
unique preference for Siglec-7 is closely related to Siglec-9 with 80% overall sequence
identity (2, 16). However, Siglec-9 cannot bind Molecular Cloning, Overexpression, and Purification--
The
Siglec-7 V-set domain was PCR-amplified from CD33-like HDPUW68 in the
pSPORT vector (Invitrogen) (14) using the forward primer
(5'-ACTTCTAGAGCACCTCCAACCCCAGATATG-3') and reverse primer
(5'-ACTGGATCCTTATGTCACGTTCACAGAGAGCTG-3'). The amplified gene was inserted into the pDEF mammalian expression vector via XbaI and BamHI restriction sites introduced by
the primers (bold). The resulting plasmid was transformed into Chinese
hamster ovary Lec1 (CHO Lec1) cells using FuGENE (Roche
Pharmaceuticals). The CHO Lec1 cells stably expressing the Siglec-7
V-set domain were selected in the presence of hygromycin and cultivated
in Crystallization and Data Collection--
Crystallization
experiments were carried out at 20 °C using hanging drop vapor
diffusion. Crystallization conditions were found using a sparse matrix
sampling approach with Crystal Screens I and II from Hampton
Research. A drop of 0.5 µl of protein (5 mg/ml in 25 mM
Tris-HCl, pH 8.0) was mixed with 0.5 µl of reservoir solution and
equilibrated against 250 µl of reservoir solution. Small crystals
appeared after 2 days in Screen I conditions 9 (0.2 M
ammonium acetate, 0.1 M trisodium citrate, pH 5.6, 30% polyethylene glycol 4000) and 28 (0.2 M sodium acetate
trihydrate, 0.1 M sodium cacodylate, pH 6.5, 30%
polyethylene glycol 8000). The initial crystals (crystal form 1, Sig7a)
from condition 9 were cryoprotected using solution 9 from Cryo Screen
(Hampton Research) and frozen in a nitrogen stream. Subsequently, a
second crystal form (Sig7b) was found in conditions identical to
those for Sig7a. These crystals were soaked in a
cryoprotectant consisting of 30% polyethylene glycol monomethylether
5000, 0.1 M MES, pH 6.5, 0.2 M ammonium
sulfate, and 5% 2-methyl-2,4-pentanediol and then frozen in a nitrogen stream.
X-ray data for Sig7a were collected at beamline ID14-EH2 European
Synchrotron Radiation Facility (ESRF) on an ADSC Quantum4 CCD at a
temperature of 100 K (see Table I). Data for Sig7b were collected at
beamline ID29 (ESRF) on an ADSC Q210 CCD at a temperature of 100 K (see
Table I). All data were processed and scaled with the HKL suite of
programs (17)
Structure Determination and Refinement--
The Sig7a structure
was solved by molecular replacement with AMoRe (18) using the
sialoadhesin structure (Protein Data Bank entry: 1QFO (12)) as a search
model. A single solution was found with an R-factor of 0.505 (correlation coefficient = 0.337). The resulting model phases were
used as input for Arp/wARP (19), which built 82 out of 127 residues.
This was followed by refinement with CNS (20) interspersed with model
building in O (21). Five percent of the data were set aside for
calculation of Rfree (22). After several rounds
of refinement, the model was incomplete. No electron density was
observed for the B-C and the C-C' loops (residues 53-57 and 68-74,
respectively) and the N terminus (residues 18-23). This structure was
used as a search model in AMoRe against 8-4-Å diffraction data
collected on Sig7b. A single solution was found with an
R-factor of 0.386 (correlation coefficient = 0.723). This was followed by refinement in CNS and manual model building with
O. A well defined density for the complete glycan structure attached to
Asn-105 was observed (see Fig. 1). In general, the Sig7b electron
density maps, although calculated from lower resolution data, were of
higher quality than those for Sig7a and allowed building of the
flexible loops and extension of the N terminus, resulting in a model
with a final R-factor of 0.210 (Rfree = 0.255, see Table I).
A model of Siglec-9 was produced by homology modeling with WHAT IF (23)
using the Sig7b structure as a template. A model of the Siglec-7/9
interaction with disialic acid was produced through superposition of
the sialoadhesin-sialyllactose structure (Protein Data Bank entry 1QFO
(12)) of the Siglec-7/9 structures followed by superposition of
disialic acid (from Protein Data Bank entry 1FV2 (24)) using the
coordinates for the first sialic acid.
Overall Structure--
The structure of the Siglec-7 V-set
ligand-binding domain was solved by molecular replacement in two
crystal forms (Sig7a and Sig7b) and refined to 1.75 Å (r = 0.223, Rfree = 0.249) and 1.9 Å (r = 0.210, Rfree = 0.255) resolution, respectively (Table I). The Sig7a structure is incomplete,
with loops B-C, C-C', and the 6 N-terminal residues missing. The Sig7b
structure is complete from residue 18 to 144 and includes a fully
ordered N-linked glycan with a truncated structure as
defined by the CHO Lec1 cell line used (discussed below). All
analyses described here are based on the Sig7b structure, although all
figures shown here include the side chain conformation of Arg-124 (a
key ligand-binding residue) as seen in the Sig7a structure. The Sig7b
Ramachandran plot calculated with PROCHECK (25) shows that 98.3% of
non-glycine residues are in the most favorable and additionally allowed
regions. Two residues, Gln-19 and Asp-93, are in disallowed regions,
which are located in disordered regions of the map.
CHO Lec1 mammalian cells cannot synthesize complex oligosaccharides
(26) due to a point mutation in the
N-acetylglucosaminyltransferase I (GnTI), resulting in
homogenous glycosylation with the structure Man5-GlcNAc2 (27)). This is advantageous for
protein crystallography studies, which can be hampered by glycan
heterogeneity. The crystal structure of Sig7b reveals a complete
Man5-GlcNAc2 glycan N-linked to
Asn-105 that is well defined in the electron density maps (Fig. 1). This high degree of order can be
attributed to crystal packing. There are 3 hydrogen bonds between the
glycan and the protein and an additional 16 hydrogen bonds between the
glycan and symmetry-related protein molecules (Fig. 1). Thus, a
total of 19 hydrogen bonds tether the glycan within the crystal
lattice.
Comparison with Sialoadhesin--
Siglec-7, given its higher
sequence identity with other members of the Siglec family
as compared with sialoadhesin (Fig. 2), may provide a more suitable template for interpretation of
structure-function relationships in the whole Siglec family. Despite
the low sequence identity of Siglec-7 with sialoadhesin (Fig. 2), the
structures share many common features and superpose with a root mean
square deviation of 1.04 Å using 100 C The Ligand-binding Site--
Despite repeated attempts with a
range of ligands, either by soaking or by co-crystallization, a crystal
of a Siglec-7-ligand complex could not be obtained. Although the
structure of Sig7b was solved without a ligand in the binding site,
comparison of the sialoadhesin structure with 3'-sialyllactose-bound
(12) provides a model of the interactions that may occur and identifies the residues essential for specificity. The ligand-binding site lies
between strands A and G (Figs. 3 and 4).
As discussed previously, the partial opening of the Structural Basis for Differential Specificity--
Comparison of
the sialic acid-binding site of Siglec-7 and sialoadhesin show that
many of the residues important in the interaction with sialic acid are
conserved (Figs. 2 and 4). An arginine (Arg-124), conserved in all
Siglecs, interacts with the carboxyl group on the terminal sialic acid
sugar (Fig. 4). Trp-132 provides a hydrophobic interaction with the
glycerol moiety. Protein backbone hydrogen bonding with the glycerol
and N-acetyl groups seen in the sialoadhesin complex are
also possible in the Siglec-7-binding site. However, there are some key
differences. Sialoadhesin Trp-2 forms a hydrophobic contact with the
N-acetyl methyl group. This tryptophan is replaced by Tyr-26
in Siglec-7, which could potentially hydrogen-bond with the
N-acetyl carbonyl. In Siglec-9, this residue is absent. In sialoadhesin, mutation of the equivalent Trp-2 revealed that the hydrophobic interaction is important for N-acetyl neuraminic
acid interactions (12). This could also explain why sialoadhesin cannot
interact with N-glycoylneuraminic acid since the additional oxygen atom in the latter form of sialic acid would be expected to
result in a steric clash (Fig. 4). Apart from sialoadhesin and MAG, all
other siglecs examined so far, including hCD22, hCD33, and Siglec-6
(29), can bind both N-acetyl and
N-glycoylneuraminic acid, suggesting that the hydrophobic
contact with the N-acetyl neuraminic acid may not be
required for sialic acid recognition in all cases. This could also
explain why Siglec-9 mediates robust sialic acid binding in the absence
of an equivalent aromatic residue (2, 15, 16).
An important aspect of Siglec function is the differential linkage
specificity displayed by members of this family. This issue was
recently addressed in a domain-swapping experiment between Siglec-7 and
-9 (15). Surprisingly, this experiment showed that a single stretch of
6 amino acids in the tip of the C-C' loop could confer Siglec-9-like
binding specificity on Siglec-7. The comparison of sialoadhesin with
Siglec-7 shows that in the latter, the C-C' loop is longer and extends
farther away from the body of the protein, offering the potential for
more interactions with additional sugars (Figs. 3 and 4). In
particular, the absence of an equivalent of sialoadhesin Tyr-44 in
Siglec-7 creates a larger cavity and exposes the basic Lys-75 (Fig. 4).
Sequence alignment shows that the C-C' loop is variable in the Siglec
family, and it is thus possible that interactions with specific side
chains that extend toward the binding site for the second sugar are
responsible for binding selectivity. For Siglec-7, these residues are
represented by Asn-70, Ile-72, and Lys-75, whereas the equivalent
residues in Siglec-9 are Ala-66, Thr-68, and Asp-71. Thus, from a
structural point of view, it is possible that the linkage specificity
displayed by Siglecs is due to the interaction of side chains in the
C-C' loop with subterminal sugars.
A Symmetry-related Loop Occupies the Binding Pocket--
Although
Siglec-7 was crystallized in the absence of any carbohydrate ligands,
the binding pocket is not empty. Expansion of the space group symmetry
shows that the pocket is occupied by the A'-B loop and C terminus of a
symmetry-related protein molecule (Fig.
5), burying a total surface area of 200 Å2. Comparison with sialic acid binding in sialoadhesin
shows that the amide of Gln-37 mimics the interaction with the sialic
acid glycerol side chain (Fig. 5). Met-40 has hydrophobic interactions with the C Conclusions--
The structure of the Siglec-7 sialic acid-binding
domain is the first such report on a member of the CD33-related Siglec
subset. Our results support previous predictions that sialic acid
recognition by Siglecs is based on a common template and provide
insights into the molecular basis of sialic acid linkage specificity
exhibited by different Siglec family members. The structure presented
here could be used as a template for the design of specific inhibitors, which could be used to dissect the precise role of CD33-related Siglecs
in the regulation of leukocyte activation. Future studies will be aimed
at elucidating the precise structural basis for sialic acid recognition
by this family of mammalian lectins.
(2,8)-linked disialic acid is likely to reside in the C-C' loop,
which is variable in the Siglec family. In the Siglec-7 structure, the
ligand-binding pocket is occupied by a loop of a symmetry-related
molecule, mimicking the interactions with sialic acid.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS AND DISCUSSION
REFERENCES
(2,8)-linked disialic acids over
(2,6)- and
(2,3)-linked sialic acids (15). When cross-linked at the cell
surface in a redirected killing assay, Siglec-7 was capable of
inhibiting the cytotoxicity toward cell targets and is therefore a
potential natural killer cell inhibitory receptor (13). Similar to CD22 on B cells, glycan recognition by Siglec-7 is likely to be directly linked to its function in modulating the activation of natural killer cells.
(2,8)-linked disialic acids and prefers
(2,3)- and
(2,6)-linked terminal sialic acids (2, 15, 16). Using protein chimeras, this difference in
sialic acid linkage preference was recently shown to reside in a
6-amino acid stretch within the C-C' loop of Siglecs-7 and -9 (15). In
the present study, we have obtained the first crystal structure of the
Siglec-7 V-set domain, in two crystal forms refined to 1.75 and 1.9 Å.
Comparison of Siglec-7 with a model of Siglec-9 shows that differences
in residues in the tip of the C-C' loop may explain the preference of
Siglec-7 for
(2,8)-linked disialic acids. The Siglec-7 structure
also reveals a fully ordered N-linked glycan and interesting
crystallographic packing interactions within the ligand-binding site.
The structure will provide a useful template for modeling other
CD33-related Siglecs.
MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS AND DISCUSSION
REFERENCES
-MEM containing 5% fetal calf serum and 1%
penicillin/streptomycin mix (Invitrogen). Secreted protein was
harvested and filtered through a 0.2-µm filter and then passed over
an 8-ml anti-Siglec-7 polyclonal antibody column. This was prepared by
coupling affinity-purified sheep anti-Siglec-7 IgG to cyanogen
bromide-activated Sepharose CL-4B (Sigma) at 5 mg/ml. Protein was
eluted from the antibody affinity column using 8-ml aliquots of 0.1 M glycine buffer, pH 2.5, followed by immediate
neutralization with 10% 1.0 M Tris-HCl, pH 8.0. Resulting
fractions were buffer-exchanged into 0.025 M Tris, pH 7.5, 0.1 M NaCl. Further purification was achieved using a
Superdex 75 16/60 column on an Akta Purifier system (Amersham Biosciences). Purity was assessed by SDS-PAGE and matrix-assisted laser
desorption ionization time-of-flight mass spectrometry. The protein was
concentrated for crystallization to 5 mg/ml using a Vivaspin 10-kDa
cutoff spin concentrator (Vivascience).
RESULTS AND DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS AND DISCUSSION
REFERENCES
Details of data collection and structure refinement for the two
Siglec-7 crystal forms
View larger version (71K):
[in a new window]
Fig. 1.
Glycan density. The structure of
the N-linked glycan attached to Asn-105 is shown. The
protein is shown as a ribbon (blue). The
molecular surfaces of symmetry-related protein molecules are shown in
gray. The glycan is represented by a stick
model. Hydrogen bonds are shown as black dashed lines.
The final 2 Fo Fc ,
calc map (contoured at 1
)
is shown in magenta.
atoms (Fig.
3). Siglec-7 shows an Ig-like fold based
on a
-sandwich formed by two
-sheets consisting of strands
A'GFCC' and ABED, respectively. The G strand is split (forming G and
G'), as is the A strand (to give A and A'). As with sialoadhesin, an
intra-sheet disulfide is present (between Cys-46 and Cys-106),
replacing the inter-sheet disulfide more commonly observed in other
Ig-like folds (28). The cysteine on strand F is replaced by Phe-123,
which lies 10 Å from Cys-46, farther than the 6-8-Å distance
observed in other Ig domains (28). This results in a widening of the
cleft between the sheets, exposing residues that may interact with
ligand. The largest differences between the sialoadhesin and Siglec-7
structures occur in the flexible loops on the outside of the protein,
namely the B-C and C-C' loops, the latter of which is different in
length (Figs. 2 and 3). The C-C' loops of both Siglec structures extend
away from the main body of the protein (Fig. 3).
View larger version (50K):
[in a new window]
Fig. 2.
Sequence alignment of the human Siglec
family. All currently known human Siglec sequences are shown in a
CLUSTAL-W (33) sequence alignment with sequence conservation indicated
by shaded boxes. Secondary structure elements are shown as
defined by the Siglec-7 structure. Residues surrounding the
ligand-binding site are indicated by open triangles (side
chain contacts) and closed triangles (backbone
contacts).
View larger version (74K):
[in a new window]
Fig. 3.
Comparison of sialoadhesin, Siglec-7, and
Siglec-9 structures. The crystal structures of sialoadhesin in
complex with sialyllactose (Protein Data Bank entry 1QFO), the crystal
structure of Siglec-7 (described here), and a model for Siglec-9 are
shown in two representations. In A, the backbones are shown
as ribbons with the side chains surrounding the
ligand-binding site shown as sticks with orange
carbons. The C-C' loop is highlighted in magenta.
The two pyranose sugars from the sialyllactose ligand are shown as
sticks with green carbons. Hydrogen
bonds with the protein are shown as black dotted
lines. B, electrostatic surfaces calculated with GRASP
(34) contoured at 6 kiloteslas (red) and +6 kiloteslas
(blue). For sialoadhesin, the two pyranose sugars of the
ligand are shown in a sticks representation. The
surface area corresponding to the C-C' loop is highlighted
by dashed circles.
-sandwich,
caused by the absence of the inter-sheet disulfide, provides a large
flat surface onto which the ligand binds in the sialoadhesin structure
(12). Compared with sialoadhesin, this surface has a more basic
character in Sig7b, resulting from the additional basic residues
Arg-23, Arg-120, and Lys-135 (Fig. 3). The additional positive charge
presented by the binding face of Siglec-7 may provide a further docking site for a negatively charged sugar, possibly explaining the Siglec-7 preference for disialylated ligands.
View larger version (72K):
[in a new window]
Fig. 4.
Comparison of the ligand-binding sites.
The backbone structures of sialoadhesin, Siglec-7, and Siglec-9 are
shown as ribbons. Key residues interacting with the terminal
pyranose sugars are shown for sialoadhesin (sticks with
orange carbons) and compared with the equivalent residues in
the Siglec-7 structure and the Siglec-9 model. The terminal sialic acid
of the sialoadhesin sialyllactose ligand is shown as sticks
with green carbons. Models for (2,8)-linked disialic acid
in Siglec-7 and Siglec-9 are shown as sticks with green
carbons. Hydrogen bonds with the protein are shown as black
dotted lines.
of Lys-131 and C
of Trp-132. In addition, there are several water-mediated hydrogen bonds (Fig. 5). Although the
interactions with the symmetry-related loop are not extensive, this is
one of the few examples of a peptide occupying a carbohydrate-binding pocket (30) and perhaps represents a first step toward a peptide-based Siglec-7 inhibitor. Interesting examples of a such peptide inhibitors have been described recently for family 18 chitinases (31) and concanavalin A (32).
View larger version (94K):
[in a new window]
Fig. 5.
A crystal contact in the ligand-binding
site. The molecular surface of the Siglec-7 structure is shown in
gray. The A'-B loop of a symmetry-related molecule is shown
in magenta. Residues on this loop pointing into the
ligand-binding site are shown as sticks with
orange carbons. Hydrogen bonds are shown as green
dashed lines. The terminal sialic acid as observed in the
sialoadhesin-sialyllactose complex is shown as a stick model
with blue carbons. Water molecules involved in hydrogen
bonding to both the protein and the symmetry-related A'-B loop are
shown as cyan spheres.
![]() |
ACKNOWLEDGEMENTS |
---|
We thank the European Synchrotron Radiation Facility, Grenoble, France, for the time at beamline ID14-4. We also thank Rod McEver for providing the CHO Lec1 mutant cells and for helpful discussions on their use in protein expression. We also thank Yvonne Jones for helpful comments.
![]() |
FOOTNOTES |
---|
* This work was funded by Biotechnology and Biological Sciences Research Council Grant 94/B14010.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The atomic coordinates and the structure factors (code 1O7S and 1O7V) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).
¶ Supported by a Wellcome Trust Senior Research Fellowship.
Supported by a Wellcome Trust Career Development Research
Fellowship. To whom correspondence should be addressed. Fax:
44-1382-345764; E-mail: dava@davapc1.bioch.dundee.ac.uk.
Published, JBC Papers in Press, November 15, 2002, DOI 10.1074/jbc.M210602200
![]() |
ABBREVIATIONS |
---|
The abbreviations used are: Siglec, sialic acid-binding immunoglobulin-like lectin; CHO, Chinese hamster ovary; MES, 4-morpholineethanesulfonic acid.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() |
---|
1. | Crocker, P. R., Clark, E. A., Filbin, M., Gordon, S., Jones, Y., Kehrl, J. H., Kelm, S., Le, Douarin, N., Powell, L., Roder, J., Schnaar, R. L., Sgroi, D. C., Stamenkovic, K., Schauer, R., Schachner, M., van den Berg, T. K., van der Merwe, P. A., Watt, S. M., and Varki, A. (1998) Glycobiology 8, v[Medline] [Order article via Infotrieve] |
2. |
Angata, T.,
Hingorani, R.,
Varki, N. M.,
and Varki, A.
(2001)
J. Biol. Chem.
276,
45128-45136 |
3. | Crocker, P. R. (2002) Curr. Opin. Struct. Biol. 12, 609-615[CrossRef][Medline] [Order article via Infotrieve] |
4. |
Paul, S. P.,
Taylor, L. S.,
Stansbury, E. K.,
and McVicar, D. W.
(2000)
Blood
96,
483-490 |
5. |
Ulyanova, T.,
Shah, D. D.,
and Thomas, M. L.
(2001)
J. Biol. Chem.
276,
14451-14458 |
6. |
Whitney, G.,
Wang, S. L.,
Chang, H.,
Cheng, K. Y., Lu, P.,
Zhou, X. D.,
Yang, W. P.,
McKinnon, M.,
and Longphre, M.
(2001)
Eur. J. Biochem.
268,
6083-6096 |
7. | Yu, Z. B., Lai, C. M., Maoui, M., Banville, D., and Shen, S. H. J. Biol. Chem. 276, 23816-23824 |
8. | Yu, Z. B., Maoui, M., Wu, L. T., Banville, D., and Shen, S. H. (2001) Biochem. J. 353, 483-492[CrossRef][Medline] [Order article via Infotrieve] |
9. |
Jin, L.,
McLean, P. A.,
Neel, B. G.,
and Wortis, H. H.
(2002)
J. Exp. Med.
195,
1199-1205 |
10. |
Kelm, S.,
Gerlach, J.,
Brossmer, R.,
Danzer, C. P.,
and Nitschke, L.
(2002)
J. Exp. Med.
195,
1207-1213 |
11. | Lanoue, A., Batista, F. D., Stewart, M., and Neuberger, M. S. (2002) Eur. J. Immunol. 32, 348-355[CrossRef][Medline] [Order article via Infotrieve] |
12. | May, A. P., Robinson, R. C., Vinson, M., Crocker, P. R., and Jones, E. Y. (1998) Mol. Cell 1, 719-728[Medline] [Order article via Infotrieve] |
13. |
Falco, M.,
Biassoni, R.,
Bottino, C.,
Vitale, M.,
Sivori, S.,
Augugliaro, R.,
Moretta, L.,
and Moretta, A.
(1999)
J. Exp. Med.
190,
793-801 |
14. |
Nicoll, G., Ni, J.,
Liu, D.,
Klenerman, P.,
Munday, J.,
Dubock, S.,
Mattei, M. G.,
and Crocker, P. R.
(1999)
J. Biol. Chem.
274,
34089-34095 |
15. |
Yamaji, T.,
Teranishi, T.,
Alphey, M. S.,
Crocker, P. R.,
and Hashimoto, Y.
(2002)
J. Biol. Chem.
277,
6324-6332 |
16. |
Zhang, J. Q.,
Nicoll, G.,
Jones, C.,
and Crocker, P. R.
(2000)
J. Biol. Chem.
275,
22121-22126 |
17. | Otwinowski, Z., and Minor, W. (1997) Methods Enzymol. 276, 307-326 |
18. | Navaza, J. (1994) Acta Crystallogr. Sect. A 50, 157-163[CrossRef] |
19. | Perrakis, A., Morris, R., and Lamzin, V. S. (1999) Nat. Struct. Biol. 6, 458-463[CrossRef][Medline] [Order article via Infotrieve] |
20. | Brunger, A. T., Adams, P. D., Clore, G. M., Gros, P., Grosse-Kunstleve, R. W., Jiang, J.-S., Kuszewski, J., Nilges, M., Pannu, N. S., Read, R. J., Rice, L. M., Simonson, T., and Warren, G. L. (1998) Acta Crystallogr. Sect. D Biol. Crystallogr. 54, 905-921[CrossRef][Medline] [Order article via Infotrieve] |
21. | Jones, T. A., Zou, J. Y., Cowan, S. W., and Kjeldgaard, M. (1991) Acta Crystallogr. Sect. A 47, 110-119[CrossRef][Medline] [Order article via Infotrieve] |
22. | Brunger, A. T. (1992) Nature 355, 472-474[CrossRef] |
23. | Vriend, G. (1990) J. Mol. Graph. 8, 52-56[CrossRef][Medline] [Order article via Infotrieve] |
24. |
Fotinou, C.,
Emsley, P.,
Black, I.,
Ando, H.,
Ishida, H.,
Kiso, M.,
Sinha, K. A.,
Fairweather, N. F.,
and Isaacs, N. W.
(2001)
J. Biol. Chem.
276,
32274-32281 |
25. | Laskowski, R. A., McArthur, M. W., Moss, D. S., and Thornton, J. M. (1993) J. Appl. Cryst. 26, 283-291 |
26. | Stanley, P., Narasimhan, S., Siminovitch, L., and Schachter, H. (1975) Proc. Natl. Acad. Sci. U. S. A. 72, 3323-3327[Abstract] |
27. |
Puthalakath, H.,
Burke, J.,
and Gleeson, P. A.
(1996)
J. Biol. Chem.
271,
27818-27822 |
28. | Richardson, J. S. (1981) Adv. Protein Chem. 34, 167-339[Medline] [Order article via Infotrieve] |
29. |
Brinkman-Van der Linden, E. C. M.,
Sjoberg, E. R.,
Juneja, L. R.,
Crocker, P. R.,
Varki, N.,
and Varki, A.
(2000)
J. Biol. Chem.
275,
8633-8640 |
30. | Johnson, M. A., and Pinto, B. M. (2002) Aust. J. Chem. 55, 13-25[CrossRef] |
31. |
Houston, D. R.,
Shiomi, K.,
Arai, N.,
Omura, S.,
Peter, M. G.,
Turberg, A.,
Synstad, B.,
Eijsink, V. G. H.,
and van Aalten, D. M. F.
(2002)
Proc. Natl. Acad. Sci. U. S. A.
99,
9127-9132 |
32. |
Jain, D.,
Kaur, K.,
Sundaravadivel, B.,
and Salunke, D. M.
(2000)
J. Biol. Chem.
275,
16098-16102 |
33. | Thompson, J. D., Higgins, D. G., and Gibson, T. J. (1994) Nucleic Acids Res. 22, 4673-4680[Abstract] |
34. | Nicholls, A., Sharp, K., and Honig, B. (1991) Proteins 11, 281-296[Medline] [Order article via Infotrieve] |