Lectins and the problem of phitopatogene recognition by a host plant

Under consideration are some questions concerning participation of lectins in the plant pathogenesis, including their role in the recognition of microbes and elicitors, and as a protective agent limiting pathogenic growth and displacements. "Classical" lectins also probably play an important role in these processes along with lectin-like receptor kinases. The principal features of those "classical" lectins are their relativly high concentration in the plant tissues, monosaccharide specificity, and limited number of the isolecin forms. Therefore, in supposing their participation in the biological recognition, it is needed to clarify how does a limited number of lectins with a limited number of carbohydrate groups can provide recognition of a potentially huge number of pathogens. This task can be fulfilled by recognition of carbohydrate residues peculiar to a particular microbe group by the "classical" lectins. These recognition processes are similar to acivity of the animal inherited immune system responsible for a rapid primary protection even in animals with well developed antibody system. A mechanism widening the carbohydrate specificity of the carbohydrate-binding center includes interaction with hydrophobic substituents in a carbohydrate residue, as well as lectin modular organization allowing for regulation of lectin binding with oligo- and polysaccharides. The free lectins effect on the microbe growth in both plants and animals. Such an action may be inhibiting in pathogenesis, while in the case of symbiotic relations, the lectin can bear signal that readdresses metabolism of a future symbiont. So, lectins seem to serve as natural deciphering device for information contained in the carbohydrate polymers, and reading of this information is the main lectin function in the cell.     

Bombyx mori pheromone-binding protein binding nonpheromone ligands: implications for pheromone recognition

Insect pheromone-binding proteins (PBPs) transport sex pheromones through the aqueous layer surrounding G protein-coupled receptors that initiate signaling events leading to mating. This PBP-receptor system strongly discriminates between ligands with subtle structural differences, but it has proved difficult to distinguish the degree of discrimination of the PBP from that of the G protein-coupled receptor. The three-dimensional structures of the PBP of Bombyx mori, the silkworm moth, both with and without its cognate ligand bombykol ([E,Z]-10,12-hexadecadienol), have been determined by X-ray crystallography and NMR. In this paper, the structures of the same binding protein with bound iodohexadecane and bell pepper odorant were determined at 1.9 and 2.0 A, respectively. These structures illustrate the remarkable plasticity in the ligand binding site of the PBP, but suggest the protein might still act as a filter during pheromone signal processing.     

Lectins and protein traffic early in the secretory pathway

Lectins of the early secretory pathway are involved in selective transport of newly synthesized glycoproteins from the endoplasmic reticulum (ER) to the ER-Golgi intermediate compartment (ERGIC). The most prominent cycling lectin is the mannose-binding type I membrane protein ERGIC-53 (ERGIC protein of 53 kDa), a marker for the ERGIC, which functions as a cargo receptor to facilitate export of an increasing number of glycoproteins with different characteristics from the ER. Two ERGIC-53-related proteins, VIP36 (vesicular integral membrane protein 36) and a novel ERGIC-53-like protein, ERGL, are also found in the early secretory pathway. ERGL may act as a regulator of ERGIC-53. Studies of ERGIC-53 continue to provide new insights into the organization and dynamics of the early secretory pathway. Analysis of the cycling of ERGIC-53 uncovered a complex interplay of trafficking signals and revealed novel cytoplasmic ER-export motifs that interact with COP-II coat proteins. These motifs are common to type I and polytopic membrane proteins including presenilin 1 and presenilin 2. The results support the notion that protein export from the ER is selective.     

Lectins

Lectins - carbohydrate-binding proteins involved in a variety of recognition processes - exhibit considerable structural diversity. Three new lectin folds and further elaborations of known folds have been described recently. Large variability in quaternary association resulting from small alterations in essentially the same tertiary structure is a property exhibited specially by legume lectins. The strategies used by lectins to generate carbohydrate specificity include the extensive use of water bridges, post-translational modification and oligomerization. Recent results pertaining to influenza and foot-and-mouth viruses further elaborate the role of lectins in infection.     

Molecular recognition of agonist ligands by RXRs

The nuclear receptor RXR is an obligate partner in many signal transduction pathways. We report the high-resolution structures of two complexes of the human RXRalpha ligand-binding domain specifically bound to two different and chemically unrelated agonist compounds: docosa hexaenoic acid, a natural derivative of eicosanoic acid, present in mammalian cells and recently identified as a potential endogenous RXR ligand in the mouse brain, and the synthetic ligand BMS 649. In both structures the RXR-ligand-binding domain forms homodimers and exhibits the active conformation previously observed with 9-cis-RA. Analysis of the differences in ligand-protein contacts (predominantly van der Waals forces) and binding cavity geometries and volumes for the several agonist-bound RXR structures clarifies the structural features important for ligand recognition. The L-shaped ligand-binding pocket adapts to the diverse ligands, especially at the level of residue N306, which might thus constitute a new target for drug-design. Despite its highest affinity 9-cis-RA displays the lowest number of ligand-protein contacts. These structural results support the idea that docosa hexaenoic acid and related fatty acids could be natural agonists of RXRs and question the real nature of the endogenous ligand(s) in mammalian cells.     

凝集素和毒素

总的说来,凝集素并不很毒,但是在凝集素和毒素之间却存在一些关系。ＥＢＡ类毒素可以分为两个主要部分。其中一个部分呈现糖结合活性,负责靶向。另一部分则具有不同的酶活性,决定了其毒性。从天花粉中分离得到凝集素分享了ＤＥＡ类毒素（例如蓖麻毒素）的一些特性。还有一些毒素可以在细胞膜上造成孔,形成离子通道。半夏凝集素似乎可归属于这类毒素。从蛇毒中分离得到了一此凝集素,它们和蛇毒中的其它一些成员在氨基酸序列上有     

Lectins from Loasaceae

Extracts of Caiophora coronata or Loasa vulcanica seeds contain a strong lectin for T, Tn, Cad 1, and papain-treated erythrocytes.     

Lectins of marine hydrobionts

Data from the literature and results of our research on lectins isolated from some kinds of marine hydrobionts such as clams, ascidians, sea worms, sponges, and algae are presented in this review. Results of comparative analysis of the basic physicochemical properties and biological activity of lectins isolated from various sources are discussed.     

双绕蛋白质的分类与识别

蛋白质折叠识别是蛋白质结构研究的重要内容。双绕是α/β蛋白质中结构典型的常见折叠类型。选取22个家族中序列一致性小于25%的79个典型双绕蛋白质作为训练集,以RMSD为指标进行系统聚类,并对各类建立基于结构比对的概形隐马尔科夫模型(profile-HMM)。将Astral1.65中序列一致性小于95%的9 505个样本作为检验集,整体识别敏感性为93.9%,特异性为82.1%,MCC值为0.876。结果表明:对于成员较多,无法建立统一模型的折叠类型,分类建模可以实现较高准确率的识别。     

Contributions of phenylalanine 335 to ligand recognition by human surfactant protein D: ring interactions with SP-D ligands

Surfactant protein D (SP-D) is an innate immune effector that contributes to antimicrobial host defense and immune regulation. Interactions of SP-D with microorganisms and organic antigens involve binding of glycoconjugates to the C-type lectin carbohydrate recognition domain (CRD). A trimeric fusion protein encoding the human neck+CRD bound to the aromatic glycoside p-nitrophenyl-alpha-D-maltoside with nearly a log-fold higher affinity than maltose, the prototypical competitor. Maltotriose, which has the same linkage pattern as the maltoside, bound with intermediate affinity. Site-directed substitution of leucine for phenylalanine 335 (Phe-335) decreased affinities for the maltoside and maltotriose without significantly altering the affinity for maltose or glucose, and substitution of tyrosine or tryptophan for leucine restored preferential binding to maltotriose and the maltoside. A mutant with alanine at this position failed to bind to mannan or maltose-substituted solid supports. Crystallographic analysis of the human neck+CRD complexed with maltotriose or p-nitrophenyl-maltoside showed stacking of the terminal glucose or nitrophenyl ring with the aromatic ring of Phe-335. Our studies indicate that Phe-335, which is evolutionarily conserved in all known SP-Ds, plays important, if not critical, roles in SP-D function.     

Supramolecularity creates nonstandard protein ligands

Congo red and a group of structurally related dyes long used to stain amyloid proteins are known to associate in water solutions. The self-association of some dyes belonging to this group appears particularly strong. In water solutions their molecules are arranged in ribbon-like micellar forms with liquid crystalline properties. These compounds have recently been found to form complexes with some native proteins in a non-standard way. Gaps formed by the local distribution of beta-sheets in proteins probably represent the receptor sites for these dye ligands. They may result from higher structural instability in unfolding conditions, but also may appear as long range cooperative fluctuations generated by ligand binding. Immunoglobulins G were chosen as model binding proteins to check the mechanism of binding of these dyes. The sites of structural changes generated by antigen binding in antibodies, believed to act as a signal propagated to distant parts of the molecule, were assumed to be suitable sites for the complexation of liquid-crystalline dyes. This assumption was confirmed by proving that antibodies engaged in immune complexation really do bind these dyes; as expected, this binding affects their function by significantly enhancing antigen binding and simultaneously inhibiting C1q attachment. Binding of these supramolecular dyes by some other native proteins including serpins and their natural complexes was also shown. The strict dependence of the ligation properties on strong self-assembling and the particular arrangement of dye molecules indicate that supramolecularity is the feature that creates non-standard protein ligands, with potential uses in medicine and experimental science.     

Metal ion-directed DNA recognition by chelating DNA ligands.

Novel DNA ligand bearing oxine and pyridinium group has been synthesized. The DNA binding of this ligand was regulated by coexisting Cu2+ over the range of more than three orders in magnitude of binding constant. This should be due to the metal-mediated dimerization of the ligand and concomitant emergence of cationic charges. There were definite two DNA binding modes for the ligand dimer depending on the P/L ratio.     

Lectins in the endoplasmic reticulum: the sweet side of protein quality control

In a recent study in Nature Cell Biology, Christianson et al. provide intriguing insights into the mechanisms of mammalian protein quality control in the endoplasmic reticulum. Their findings open up new perspectives on the versatility and diversity of how protein quality control sorts out defective polypeptides to prevent damage to the cell.