A novel mechanism for binding of galactose-terminated glycans by the C-type carbohydrate recognition domain in blood dendritic cell antigen 2

Blood dendritic cell antigen 2 (BDCA-2; also designated CLEC4C or CD303) is uniquely expressed on plasmacytoid dendritic cells. Stimulation of BDCA-2 with antibodies leads to an anti-inflammatory response in these cells, but the natural ligands for the receptor are not known. The C-type carbohydrate recognition domain in the extracellular portion of BDCA-2 contains a signature motif typical of C-type animal lectins that bind mannose, glucose, or GlcNAc, yet it has been reported that BDCA-2 binds selectively to galactose-terminated, biantennary N-linked glycans. A combination of glycan array analysis and binding competition studies with monosaccharides and natural and synthetic oligosaccharides have been used to define the binding epitope for BDCA-2 as the trisaccharide Galβ1–3/4GlcNAcβ1–2Man. X-ray crystallography and mutagenesis studies show that mannose is ligated to the conserved Ca2+ in the primary binding site that is characteristic of C-type carbohydrate recognition domains, and the GlcNAc and galactose residues make additional interactions in a wide, shallow groove adjacent to the primary binding site. As predicted from these studies, BDCA-2 binds to IgG, which bears galactose-terminated glycans that are not commonly found attached to other serum glycoproteins. Thus, BDCA-2 has the potential to serve as a previously unrecognized immunoglobulin Fc receptor

Common Polymorphisms in Human Langerin Change Specificity for Glycan Ligands

Langerin, a C-type lectin on Langerhans cells, mediates carbohydrate-dependent uptake of pathogens in the first step of antigen presentation to the adaptive immune system. Langerin binds a diverse range of carbohydrates including high mannose structures, fucosylated blood group antigens, and glycans with terminal 6-sulfated galactose. Mutagenesis and quantitative binding assays indicate that salt bridges between the sulfate group and two lysine residues compensate for the nonoptimal binding of galactose at the primary Ca(2+) site. A commonly occurring single nucleotide polymorphism (SNP) in human langerin results in change of one of these lysine residues, Lys-313, to isoleucine. Glycan array screening reveals that this amino acid change abolishes binding to oligosaccharides with terminal 6SO(4)-Gal and enhances binding to oligosaccharides with terminal GlcNAc residues. Structural analysis shows that enhanced binding to GlcNAc may result from Ile-313 packing against the N-acetyl group. The K313I polymorphism is tightly linked to another SNP that results in the change N288D, which reduces affinity for glycan ligands by destabilizing the Ca(2+)-binding site. Langerin with Asp-288 and Ile-313 shows no binding to 6SO(4)-Gal-terminated glycans and increased binding to GlcNAc-terminated structures, but overall decreased binding to glycans. Altered langerin function in individuals with the linked N288D and K313I polymorphisms may affect susceptibility to infection by microorganisms

Insights into Interactions of Mycobacteria with the Host Innate Immune System from a Novel Array of Synthetic Mycobacterial Glycans.

An array of homogeneous glycans representing all the major carbohydrate structures present in the cell wall of the human pathogen Mycobacterium tuberculosis and other mycobacteria has been probed with a panel of glycan-binding receptors expressed on cells of the mammalian innate immune system. The results provide an overview of interactions between mycobacterial glycans and receptors that mediate uptake and survival in macrophages, dendritic cells, and sinusoidal endothelial cells. A subset of the wide variety of glycan structures present on mycobacterial surfaces interact with cells of the innate immune system through the receptors tested. Endocytic receptors, including the mannose receptor, DC-SIGN, langerin, and DC-SIGNR (L-SIGN), interact predominantly with mannose-containing caps found on the mycobacterial polysaccharide lipoarabinomannan. Some of these receptors also interact with phosphatidyl-myo-inositol mannosides and mannose-containing phenolic glycolipids. Many glycans are ligands for overlapping sets of receptors, suggesting multiple, redundant routes by which mycobacteria can enter cells. Receptors with signaling capability interact with two distinct sets of mycobacterial glycans: targets for dectin-2 overlap with ligands for the mannose-binding endocytic receptors, while mincle binds exclusively to trehalose-containing structures such as trehalose dimycolate. None of the receptors surveyed bind furanose residues, which often form part of the epitopes recognized by antibodies to mycobacteria. Thus, the innate and adaptive immune systems can target different sets of mycobacterial glycans. This array, the first of its kind, represents an important new tool for probing, at a molecular level, biological roles of a broad range of mycobacterial glycans, a task that has not previously been possible

CD23 is a glycan-binding receptor in some mammalian species

CD23, the low affinity IgE receptor found on B lymphocytes and other cells, contains a C-terminal lectin-like domain that resembles C-type carbohydrate-recognition domains (CRDs) found in many glycan-binding receptors. In most mammalian species, the CD23 residues required to form a sugar-binding site are present, although binding of CD23 to IgE does not involve sugars. Solid-phase binding competition assays, glycoprotein blotting experiments and glycan array analysis employing the lectin-like domains of cow and mouse CD23 demonstrate that they bind to mannose, N-acetylglucosamine, glucose, and fucose and to glycoproteins that bear these sugars in nonreducing terminal positions. Crystal structures of the cow CRD in the presence of α-methyl mannoside and GlcNAcβ1-2Man reveal that a range of oligosaccharide ligands can be accommodated in an open binding site in which most interactions are with a single terminal sugar residue. Although mouse CD23 shows a pattern of monosaccharide and glycoprotein binding similar to cow CD23, the binding is weaker. In contrast, no sugar binding was observed in similar experiments with human CD23. The absence of sugar-binding activity correlates with accumulation of mutations in the CD23 gene in the primate lineage leading to humans, resulting in loss of key sugar-binding residues. These results are consistent with a role for CD23 in many species as a receptor for potentially pathogenic micro-organisms as well as IgE. However, the ability of CD23 to bind several different ligands varies between species, suggesting that it has distinct functions in different organisms

A new type of lectin discovered in a fish, flathead (Platycephalus indicus), suggests an alternative functional role for mammalian plasma kallikrein*

A skin mucus lectin exhibiting a homodimeric structure and an S–S bond between subunits of ∼40 kDa was purified from flathead Platycephalus indicus (Scorpaeniformes). This lectin, named FHL (FlatHead Lectin), exhibited mannose-specific activity in a Ca2+-dependent manner. Although FHL showed no homology to any previously reported lectins, it did exhibit ∼20% identity to previously discovered plasma kallikreins and coagulation factor XIs of mammals and Xenopus laevis. These known proteins are serine proteases and play pivotal roles in the kinin-generating system or the blood coagulation pathway. However, alignment analysis revealed that while FHL lacked a serine protease domain, it was homologous to the heavy-chain domain of plasma kallikreins and coagulation factor XI therefore suggesting that FHL is not an enzyme but rather a novel animal lectin. On the basis of this finding, we investigated the lectin activity of human plasma kallikrein and revealed that it could indeed act as a lectin. Other genes homologous to FHL were also found in the genome databases of some fish species, but not in mammals. In contrast, plasma kallikreins and coagulation factor XI have yet to be identified in fish. The present findings suggest that these mammalian enzymes may have originally emerged as a lectin and may have evolved into molecules with protease activity after separation from common ancestors

Tunicate cytostatic factor TC14-3 induces a polycomb group gene and histone modification through Ca2+ binding and protein dimerization

<p>Abstract</p> <p>Background</p> <p>As many invertebrate species have multipotent cells that undergo cell growth and differentiation during regeneration and budding, many unique and interesting homeostatic factors are expected to exist in those animals. However, our understanding of such factors and global mechanisms remains very poor. Single zooids of the tunicate, <it>Polyandrocarpa </it><it>misakiensis</it>, can give off as many as 40 buds during the life span. Bud development proceeds by means of transdifferentiation of very limited number of cells and tissues. TC14-3 is one of several different but closely related polypeptides isolated from <it>P. misakiensis</it>. It acts as a cytostatic factor that regulates proliferation, adhesion, and differentiation of multipotent cells, although the molecular mechanism remains uncertain. The Polycomb group (PcG) genes are involved in epigenetic control of genomic activity in mammals. In invertebrates except <it>Drosophila</it>, PcG and histone methylation have not been studied so extensively, and genome-wide gene regulation is poorly understood.</p> <p>Results</p> <p>When Phe⁶⁵of TC14-3 was mutated to an acidic amino acid, the resultant mutant protein failed to dimerize. The replacement of Thr⁶⁹with Arg⁶⁹made dimers unstable. When Glu¹⁰⁶was changed to Gly¹⁰⁶, the resultant mutant protein completely lost Ca²⁺binding. All these mutant proteins lacked cytostatic activity, indicating the requirement of protein dimerization and calcium for the activity. <it>Polyandrocarpa </it><it>Eed</it>, a component of PcG, is highly expressed during budding, like TC14-3. When wild-type and mutant TC14-3s were applied in vivo and in vitro to <it>Polyandrocarpa </it>cells, only wild-type TC14-3 could induce <it>Eed </it>without affecting histone methyltransferase gene expression. Eed-expressing cells underwent trimethylation of histone H3 lysine27. <it>PmEed </it>knockdown by RNA interference rescued cultured cells from the growth-inhibitory effects of TC14-3.</p> <p>Conclusion</p> <p>These results show that in <it>P. misakiensis</it>, the cytostatic activity of TC14-3 is mediated by <it>PmEed </it>and resultant histone modification, and that the gene expression requires both the protein dimerization and Ca²⁺-binding of TC14-3. This system consisting of a humoral factor, PcG, and histone methylation would contribute to the homeostatic regulation of cell growth and terminal differentiation of invertebrate multipotent cells.</p

Do female association preferences predict the likelihood of reproduction?

Sexual selection acting on male traits through female mate choice is commonly inferred from female association preferences in dichotomous mate choice experiments. However, there are surprisingly few empirical demonstrations that such association preferences predict the likelihood of females reproducing with a particular male. This information is essential to confirm association preferences as good predictors of mate choice. We used green swordtails (&lt;i&gt;Xiphophorus helleri&lt;/i&gt;) to test whether association preferences predict the likelihood of a female reproducing with a male. Females were tested for a preference for long- or short-sworded males in a standard dichotomous choice experiment and then allowed free access to either their preferred or non-preferred male. If females subsequently failed to produce fry, they were provided a second unfamiliar male with similar sword length to the first male. Females were more likely to reproduce with preferred than non-preferred males, but for those that reproduced, neither the status (preferred/non-preferred) nor the sword length (long/short) of the male had an effect on brood size or relative investment in growth by the female. There was no overall preference based on sword length in this study, but male sword length did affect likelihood of reproduction, with females more likely to reproduce with long- than short-sworded males (independent of preference for such males in earlier choice tests). These results suggest that female association preferences are good indicators of female mate choice but that ornament characteristics of the male are also important

Quasi-one-dimensional antiferromagnetism and multiferroicity in CuCrO4_4

The bulk magnetic properties of the new quasi-one-dimensional Heisenberg antiferromagnet, CuCrO$_4$, were characterized by magnetic susceptibility, heat capacity, optical spectroscopy, EPR and dielectric capacitance measurements and density functional evaluations of the intra- and interchain spin exchange interactions. We found type-II multiferroicity below the N\'{e}el temperature of 8.2(5) K, arising from competing antiferromagnetic nearest-neighbor ($J_{\rm nn}$) and next-nearest-neighbor ($J_{\rm nnn}$) intra-chain spin exchange interactions. Experimental and theoretical results indicate that the ratio $J_{\rm nn}/J_{\rm nnn}$ is close to 2, putting CuCrO$_4$ in the vicinity of the Majumdar-Ghosh point.Comment: 9 pages, 8 figures, submitted to PR