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PDBsum entry 2jhm
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Sugar binding protein
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PDB id
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2jhm
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References listed in PDB file
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Key reference
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Title
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Structural basis for innate immune sensing by m-Ficolin and its control by a ph-Dependent conformational switch.
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Authors
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V.Garlatti,
L.Martin,
E.Gout,
J.B.Reiser,
T.Fujita,
G.J.Arlaud,
N.M.Thielens,
C.Gaboriaud.
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Ref.
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J Biol Chem, 2007,
282,
35814-35820.
[DOI no: ]
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PubMed id
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Abstract
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Ficolins are soluble oligomeric proteins with lectin-like activity, assembled
from collagen fibers prolonged by fibrinogen-like recognition domains. They act
as innate immune sensors by recognizing conserved molecular markers exposed on
microbial surfaces and thereby triggering effector mechanisms such as enhanced
phagocytosis and inflammation. In humans, L- and H-ficolins have been
characterized in plasma, whereas a third species, M-ficolin, is secreted by
monocytes and macrophages. To decipher the molecular mechanisms underlying their
recognition properties, we previously solved the structures of the recognition
domains of L- and H-ficolins, in complex with various model ligands (Garlatti,
V., Belloy, N., Martin, L., Lacroix, M., Matsushita, M., Endo, Y., Fujita, T.,
Fontecilla-Camps, J. C., Arlaud, G. J., Thielens, N. M., and Gaboriaud, C.
(2007) EMBO J. 24, 623-633). We now report the ligand-bound crystal structures
of the recognition domain of M-ficolin, determined at high resolution (1.75-1.8
A), which provides the first structural insights into its binding properties.
Interaction with acetylated carbohydrates differs from the one previously
described for L-ficolin. This study also reveals the structural determinants for
binding to sialylated compounds, a property restricted to human M-ficolin and
its mouse counterpart, ficolin B. Finally, comparison between the ligand-bound
structures obtained at neutral pH and nonbinding conformations observed at pH
5.6 reveals how the ligand binding site is dislocated at acidic pH. This means
that the binding function of M-ficolin is subject to a pH-sensitive
conformational switch. Considering that the homologous ficolin B is found in the
lysosomes of activated macrophages (Runza, V. L., Hehlgans, T., Echtenacher, B.,
Zahringer, U., Schwaeble, W. J., and Mannel, D. N. (2006) J. Endotoxin Res. 12,
120-126), we propose that this switch could play a physiological role in such
acidic compartments.
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Figure 2.
FIGURE 2. The S1 ligand binding site in M-ficolin, TL5A,
and L-ficolin. A–C, detailed views of the interactions of
GalNAc, GlcNAc, and Neu5Ac in site S1 of M-ficolin. D,
superposition of the ligand-free and three ligand-bound
structures of M-ficolin showing that Tyr^271 is the only mobile
component in site S1. E, interaction of GlcNAc in the homologous
S1 binding site of tachylectin 5A (14). F, superposition of the
S1 binding sites of M-ficolin (magenta) and L-ficolin (green).
In L-ficolin, steric hindrance (as shown by black lines)
prevents accommodation of large ligands such as Neu5Ac.
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Figure 3.
FIGURE 3. The pH-dependent conformational switch observed
in M-ficolin. A, the active binding conformation of site S1
observed at neutral pH involves residues contributed by four
external loops. B, nonbinding conformation of site S1 observed
at pH 5.6. The structure determined in this study (cyan, lacking
the disordered segment 278–285) and subunit C of the structure
reported previously by Tanio et al. (18) (orange) are
superposed. The acidic pH destabilizes the four loops and
dislocates the S1 site. C, superposition of the binding
(magenta) and nonbinding (cyan) conformations determined in this
study, illustrating the essential histidine-mediated
stabilizations of the binding conformation (magenta) that are
lost at acidic pH. The red dashed lines represent hydrogen bonds.
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The above figures are
reprinted
by permission from the ASBMB:
J Biol Chem
(2007,
282,
35814-35820)
copyright 2007.
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