Human galectins have functionally divergent roles, although most of the members of the galectin family bind weakly to the simple disaccharide lactose (Gal␤1-4Glc). To assess the specificity of galectin-glycan interactions in more detail, we explored the binding of several important galectins (Gal-1, Gal-2, and Gal-3) using a dose-response approach toward a glycan microarray containing hundreds of structurally diverse glycans, and we compared these results to binding determinants on cells. All three galectins exhibited differences in glycan binding characteristics. On both the microarray and on cells, Gal-2 and Gal-3 exhibited higher binding than Gal-1 to fucose-containing A and B blood group antigens. Gal-2 exhibited significantly reduced binding to all sialylated glycans, whereas Gal-1 bound ␣2-3-but not ␣2-6-sialylated glycans, and Gal-3 bound to some glycans terminating in either ␣2-3-or ␣2-6-sialic acid. The effects of sialylation on Gal-1, Gal-2, and Gal-3 binding to cells also reflected differences in cellular sensitivity to Gal-1-, Gal-2-, and Gal-3-induced phosphatidylserine exposure. Each galectin exhibited higher binding for glycans with poly-N-acetyllactosamine (poly(LacNAc)) sequences (Gal␤1-4GlcNAc) n when compared with N-acetyllactosamine (LacNAc) glycans (Gal␤1-4GlcNAc). However, only Gal-3 bound internal LacNAc within poly(LacNAc). These results demonstrate that each of these galectins mechanistically differ in their binding to glycans on the microarrays and that these differences are reflected in the determinants required for cell binding and signaling. The specific glycan recognition by each galectin underscores the basis for differences in their biological activities.

L-selectin requires a threshold shear to enable leukocytes to tether to and roll on vascular surfaces. Transport mechanisms govern flow-enhanced tethering, whereas force governs flow-enhanced rolling by prolonging the lifetimes of L-selectin–ligand complexes (catch bonds). Using selectin crystal structures, molecular dynamics simulations, site-directed mutagenesis, single-molecule force and kinetics experiments, Monte Carlo modeling, and flow chamber adhesion studies, we show that eliminating a hydrogen bond to increase the flexibility of an interdomain hinge in L-selectin reduced the shear threshold for adhesion via two mechanisms. One affects the on-rate by increasing tethering through greater rotational diffusion. The other affects the off-rate by strengthening rolling through augmented catch bonds with longer lifetimes at smaller forces. By forcing open the hinge angle, ligand may slide across its interface with L-selectin to promote rebinding, thereby providing a mechanism for catch bonds. Thus, allosteric changes remote from the ligand-binding interface regulate both bond formation and dissociation.

P-selectin glycoprotein ligand-1 (PSGL-1) is a dimeric membrane mucin on leukocytes that binds selectins. The molecular features of PSGL-1 that determine this high affinity binding are unclear. Here we demonstrate the in vitro synthesis of a novel glycosulfopeptide (GSP-6) modeled after the extreme N terminus of PSGL-1, which has been predicted to be important for P-selectin binding. GSP-6 contains three tyrosine sulfate (TyrSO 3 ) residues and a monosialylated, core 2-based O-glycan with a sialyl Lewis x (C2-O-sLe x ) motif at a specific Thr residue. GSP-6 binds tightly to immobilized P-selectin, whereas glycopeptides lacking either TyrSO 3 or C2-O-sLe x do not detectably bind. Remarkably, an isomeric glycosulfopeptide to GSP-6, termed GSP-6, which contains sLe x on an extended core 1-based O-glycan, does not bind immobilized P-selectin. Equilibrium gel filtration analysis revealed that GSP-6 binds to soluble P-selectin with a K d of ϳ350 nM. GSP-6 (<5 M) substantially inhibits neutrophil adhesion to P-selectin in vitro, whereas free sLe x (5 mM) only slightly inhibits adhesion. In contrast to the inherent heterogeneity of post-translational modifications of recombinant proteins, glycosulfopeptides permit the placement of sulfate groups and glycans of precise structure at defined positions on a polypeptide. This approach should expedite the probing of structure-function relationships in sulfated and glycosylated proteins, and may facilitate development of novel drugs to treat inflammatory diseases involving P-selectin-mediated leukocyte adhesion.The interactions between selectins and their carbohydratebased ligands initiate adhesion of leukocytes to the vascular wall during inflammation. Although L-, E-, and P-selectin can bind a simple glycan containing sialyl Lewis x (sLe x ) 1 (NeuAc␣233Gal␤134[Fuc␣133]GlcNAc␤13 R) in a Ca 2ϩ -dependent manner, each selectin binds with higher affinity to a limited number of macromolecular ligands expressing sialylated and fucosylated glycans (1-4). P-selectin, which is expressed by activated platelets and endothelial cells, demonstrates the most discriminating ligand specificity of any selectin. It interacts predominantly with a disulfide-bonded dimeric mucin on leukocytes termed P-selectin glycoprotein ligand-1 (PSGL-1) (subunit mass ϳ120 kDa) (5).Each 120-kDa subunit of human PSGL-1 contains numerous sialic acids and approximately 70 extracellular Ser and Thr residues, which are potential sites for O-glycosylation, plus three potential sites for N-glycosylation (6, 7) (Fig. 1). These features suggested that the large amount of carbohydrate on the mucin might promote high avidity binding to P-selectin. However, indirect evidence suggests that the extreme N-terminal extracellular region of mature PSGL-1, which begins at residue 42, is important for high affinity binding to P-selectin (reviewed in Ref. 3). Specifically, tyrosine sulfate residues and O-glycans within that region have been considered essential for binding (Fig. 1). A monoclonal antibody directed to a peptide ep...

The dendritic cell specific C-type lectin dendritic cell specific ICAM-3 grabbing non-integrin (DC-SIGN) binds to ''self'' glycan ligands found on human cells and to ''foreign'' glycans of bacterial or parasitic pathogens. Here, we investigated the binding properties of DC-SIGN to a large array of potential ligands in a glycan array format. Our data indicate that DC-SIGN binds with K d < 2 lM to a neoglycoconjugate in which Galb1-4(Fuca1-3)GlcNAc (Le x ) trisaccharides are expressed multivalently. A lower selective binding was observed to oligomannose-type N-glycans, diantennary N-glycans expressing Le x and GalNAcb1-4(Fuca1-3)GlcNAc (LacdiNAc-fucose), whereas no binding was observed to N-glycans expressing corefucose linked either a1-6 or a1-3 to the Asn-linked GlcNAc of N-glycans. These results demonstrate that DC-SIGN is selective in its recognition of specific types of fucosylated glycans and subsets of oligomannose-and complex-type N-glycans.

x and 6-sulfo-sLe x did not support any Siglec-8 binding at the highest concentration tested (300 pmol/spot). We conclude that Siglec-8 binds preferentially to the sLe x structure bearing an additional sulfate ester on the galactose 6-hydroxyl.

Leukocytes use the cell-surface mucin P-selectin glycoprotein ligand-1 (PSGL-1) to tether to and roll on Pselectin on activated endothelial cells and platelets. By using surface plasmon resonance, we measured the affinity and kinetics of binding of soluble monomeric human P-selectin to immobilized PSGL-1 from human neutrophils. Binding was specific, as documented by its Ca 2؉ -dependence, its inhibition by specific monoclonal antibodies to P-selectin and PSGL-1, and its abrogation by treating PSGL-1 with sialidase. Similar binding was observed for soluble P-selectin that contained the lectin and epidermal growth factor domains plus all nine consensus repeats, and for a soluble construct that contained only the lectin and epidermal growth factor domains. Soluble P-selectin bound saturably to a single class of sites on PSGL-1 with a dissociation constant (K d ) of 320 ؎ 20 nM. The measured k off was 1.4 ؎ 0.1 s ؊1 , and the calculated k on was 4.4 ؋ 10 6 M ؊1 s ؊1 . We conclude that monomeric P-selectin binds to PSGL-1 with fast association and dissociation rates and relatively high affinity. These features may be important for efficient tethering and rolling of leukocytes at physiologic densities of PSGL-1 and P-selectin.Leukocyte trafficking into lymphoid tissues or into sites of infection or injury is a multistep process that involves several classes of adhesion molecules (1). Binding of selectins to cellsurface glycoconjugates mediates the initial tethering and rolling adhesion of leukocytes on endothelial cells, platelets, or other leukocytes under the shear stresses in the microcirculation (2, 3). Most leukocytes express L-selectin, whereas activated endothelial cells and/or platelets express P-and E-selectin. Each selectin has an N-terminal C-type lectin domain, followed by an epidermal growth factor (EGF) 1 -like domain, a series of short consensus repeats, a transmembrane domain, and a cytoplasmic tail.Selectins are postulated to mediate leukocyte tethering and rolling because they bind their ligands with very fast association and dissociation rates and high tensile strength (4, 5). Consistent with this hypothesis, flowing leukocytes form transient tethers on very low densities of immobilized selectins, suggesting the presence of fast dissociation rates for selectinligand bonds. The unstressed cellular k off has been estimated to be 1 s Ϫ1 for P-selectin, 0.7 s Ϫ1 for E-selectin, and 7 s Ϫ1 for L-selectin (5, 6). First-order dissociation kinetics suggest that each transient tether represents a quantal unit that corresponds to a single selectin-ligand bond, although the data do not exclude the possibility that a tether represents a small number of bonds (5, 6). Furthermore, the cellular experiments do not yield the association rates or the equilibrium affinities of selectin-ligand interactions. Therefore, biochemical measurements of the kinetics and affinity of binding of selectins to biologically relevant ligands are needed to extend the results of cellular assays.The selectins bind with low affinity to s...

Surface presentation of adhesion receptors influences cell adhesion, although the mechanisms underlying these effects are not well understood. We used a micropipette adhesion frequency assay to quantify how the molecular orientation and length of adhesion receptors on the cell membrane affected two-dimensional kinetic rates of interactions with surface ligands. Interactions of P-selectin, E-selectin, and CD16A with their respective ligands or antibody were used to demonstrate such effects. Randomizing the orientation of the adhesion receptor or lowering its ligand-and antibody-binding domain above the cell membrane lowered two-dimensional affinities of the molecular interactions by reducing the forward rates but not the reverse rates. In contrast, the soluble antibody bound with similar three-dimensional affinities to cell-bound P-selectin constructs regardless of their orientation and length. These results demonstrate that the orientation and length of an adhesion receptor influences its rate of encountering and binding a surface ligand but does not subsequently affect the stability of binding.

Galectin-1 (Gal-1) regulates leukocyte turnover by inducing the cell surface exposure of phosphatidylserine (PS), a ligand that targets cells for phagocytic removal, in the absence of apoptosis. Gal-1 monomer-dimer equilibrium appears to modulate Gal-1-induced PS exposure, although the mechanism underlying this regulation remains unclear. Here we show that monomer-dimer equilibrium regulates Gal-1 sensitivity to oxidation. A mutant form of Gal-1, containing C2S and V5D mutations (mGal-1), exhibits impaired dimerization and fails to induce cell surface PS exposure while retaining the ability to recognize carbohydrates and signal Ca 2؉ flux in leukocytes. mGal-1 also displayed enhanced sensitivity to oxidation, whereas ligand, which partially protected Gal-1 from oxidation, enhanced Gal-1 dimerization. Continual incubation of leukocytes with Gal-1 resulted in gradual oxidative inactivation with concomitant loss of cell surface PS, whereas rapid oxidation prevented mGal-1 from inducing PS exposure. Stabilization of Gal-1 or mGal-1 with iodoacetamide fully protected Gal-1 and mGal-1 from oxidation. Alkylation-induced stabilization allowed Gal-1 to signal sustained PS exposure in leukocytes and mGal-1 to signal both Ca 2؉ flux and PS exposure. Taken together, these results demonstrate that monomer-dimer equilibrium regulates Gal-1 sensitivity to oxidative inactivation and provides a mechanism whereby ligand partially protects Gal-1 from oxidation.Immunological homeostasis relies on efficient contraction of activated leukocytes following an inflammatory episode. Several factors, including members of the galectin and tumor necrosis factor families (1, 2), regulate leukocyte turnover by inducing apoptotic cell death. In contrast, several galectin family members, in particular galectin-1 (Gal-1), 2 uniquely regulate neutrophil turnover by inducing phosphatidylserine (PS) exposure, which normally sensitizes apoptotic cells to phagocytic removal (3, 4), independent of apoptosis, a process recently termed preaparesis (5).Previous studies suggested that dimerization may be required for Gal-1-induced PS exposure, as a mutant form of Gal-1 (mGal-1) containing two point mutations within the dimer interface, C2S and V5D (C2S,V5D), displays impaired Gal-1 dimerization and fails to induce PS exposure (6). However, the manner in which monomer-dimer equilibrium regulates Gal-1 signaling remains unclear. Previous studies suggest that dimerization may be required for efficient cross-linking of functional receptors or the formation of signaling lattices (7-9). Consistent with this, monomeric mutants of several other galectins fail to induce PS exposure or signal leukocytes (4, 8). Gal-1 signaling of PS exposure requires initial signaling events, such as mobilization of intracellular Ca 2ϩ followed by sustained receptor engagement (10). Although mGal-1 fails to induce PS exposure (6), whether mGal-1 can induce these initial signaling events remains unknown (10).In addition to directly regulating signaling, monomer-dimer equilibrium may ...