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Varki A, Cummings R, Esko J, et al., editors. Essentials of Glycobiology. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1999.
Primary contributions to this chapter were made by R.D. Cummings (University of Oklahoma at Oklahoma City).
THE C-TYPE LECTINS REPRESENT A LARGE FAMILY of Ca++-dependent lectins that chia sẻ primary structural homology in their carbohydrate-recognition domains. This very large family, which includes many endocytic receptors, many proteoglycans, and all known collectins & selectins, is found throughout the animal kingdom. Most of the members of this family differ, however, with respect to the types of carbohydrate structures that they recognize with high affinity. The C-type lectins are involved in many immune-system functions, such as inflammation & immunity to tumor và virally infected cells, whereas the collectins are involved in innate immunity. This chapter emphasizes the diversity of the C-type lectin family and presents the current state of knowledge about the overall structures of these proteins & the breadth of their biological distribution & functions.
Historical Background and Discovery of C-type Lectins (1–4)
The first lectin identified in animals was the C-type lectin known as the hepatic asialoglycoprotein receptor or the hepatic Gal/GalNAc receptor. It was identified by Ashwell and Morell & their colleagues, who were examining the structure & function of the sialylated serum glycoprotein ceruloplasmin. On the basis of observations that most serum glycoproteins contained terminal sialic acid residues, these authors hypothesized that sialylation was essential for the proper lifetime of glycoproteins in serum. In a key experiment, they enzymatically desialylated a radioactively labeled glycoprotein & injected it into lớn rabbits; they discovered that the desialylated glycoprotein was removed much more rapidly from the circulation than the parent glycoprotein. In addition, the penultimate galactose residues exposed upon desialylation were critical for clearance, since treatment with galactose oxidase, which oxidizes the C-6 hydroxyl of galactose residues, prolonged the serum survival time. Resialylation of the desialylated glycoproteins by a preparation of sialyltransferase and CMPNeuAc restored normal circulatory lifetime. The asialoglycoproteins removed from the circulation were found to be sequestered in the liver và principally in lysosomes.
Using radioactively labeled asialoglycoproteins, a specific Ca++-dependent receptor for asialoglycoproteins was identified in hepatocyte plasma membrane fractions, và the hepatic receptor was purified by affinity chromatography on columns of asialo-orosomucoid-Sepharose. The receptor consisted of a major subunit of approximately 48 kD & a minor subunit of about 40 kD. The purified rabbit hepatocyte receptor was able lớn agglutinate desialylated human và rabbit erythrocytes and lớn induce mitogenesis in desialylated peripheral lymphocytes. Notably, this was the first demonstration that an animal lectin could have sầu such profound effects on cellular metabolism. The rabbit hepatic lectin was inhibited in its binding by both GalNAc và galactose, with the former being more potent than the latter, và it was able to lớn bind to lớn glycoproteins containing either nonreducing terminal GalNAc or galactose. A similar lectin was identified in rats & termed the rat hepatic lectin. It also contained an unusual trimeric structure composed of two subunits, which is termed rat hepatic lectin R2/3. A lectin was also identified in chicken hepatocytes, but the avian lectin was different in that it only recognized glycoproteins containing terminal GlcNAc residues. Thus, only glycoproteins that were enzymatically both desialylated và degalactosylated were rapidly cleared by the avian liver.
Definition of the C-type Lectins và Sequence Motifs (5–7)
The sequencing of several mammalian Ca++-dependent lectins, notably the chicken hepatic lectin, the rat hepatic lectin, & the hepatic mannose receptor, led lớn the finding that all of these proteins shared a common structural motif. The sequence homology was noted in the CRD, and proteins with this motif were classified as members of the C-type (Ca++-dependent) lectin family by Drickamer. The domain of the C-type lectins responsible for binding carbohydrate was originally identified by analyzing proteolytic fragments, và it was found that the CRD is contained in these fragments. The CRD of the C-type lectin family is an approximately 115–130-amino-acid segment containing a recognizable consensus sequence shown in Figure 25.1. Of particular significance is the presence và spacing of cysteine residues that are disulfide-bonded, as shown, lớn contribute to lớn the folded lectin domain name.
(A) Conserved primary structures within the CRD in C-type lectins. Invariant cysteine residues are underlined & highly conserved residues are indicated in the parentheses. The amino acid spacing between residues is indicated by Xs within parentheses. (more...)
Different Subfamilies of C-type Lectins (7)
The discovery of the CRD for the C-type lectins opened the way lớn characterize other related proteins that also displayed Ca++-dependent binding to lớn carbohydrate ligands (Table 25.1 và Figure 25.2). To date, more than 20 different proteins containing a C-type lectin CRD have sầu been identified in humans và corresponding homologs have sầu also been found in many other higher animals. In addition, C-type lectins occur in many other vertebrates, including reptiles, và in invertebrates. From the genomic sequencing of Caenorhabditis elegans, approximately 150 C-type lectin genes have sầu been identified. These many C-type lectins in higher animals are classified inkhổng lồ subfamilies, based on their function or chất lượng localization.
Organization of the CRD in different members of the C-type lectin family. (Adapted, with permission, from <7> Drickamer & Taylor 1993 <© Annual Reviews>.)
The hepatic lectins discussed above sầu represent a class of C-type lectins capable of mediating endocytosis of bound ligands (Table 25.1 and Figure 25.2). The endocytic pathway involves lectin recognition of ligands at the cell surface, internalization via coated pits, & delivery of the complex to endosomal compartments where the low pH induces dissociation of ligand and lectin. The lectins recycle lớn the cell surface and repeat the process. The mammalian asialoglycoprotein receptor is trimeric & has two different polypeptides encoded by different genes. Although one polypeptide is usually dominant, both polypeptides are essential for functional assembly. In contrast, the chicken hepatic lectin is a trimer containing a single polypeptide species. This lectin occurs in trimeric and hexameric forms that promote high affinity for specific glycoconjugate ligands. Clustering of the CRDs may determine both the specificity & affinity of the lectins, since each individual CRD can act independently lớn bind sugar. The asialoglycoprotein receptor & most endocytic receptors are type II transmembrane proteins, whereas the macrophage mannose receptor is a type I protein. The hepatic lectins have a single C-type CRD, and the macrophage mannose receptor has eight CRDs and is the only known protein with multiple independent CRDs in a single polypeptide. The adjacent CRDs in the mannose receptor may help lớn direct its binding to lớn specific multivalent, mannose-containing glycans. The macrophage mannose receptor can internalize lysosomal enzymes containing high-mannose-type N-glycans, as well as participate in the phagocytosis of several pathogens, such as yeast, Pneumocystis carinii, và Leishmania.
Although the function of the hepatic asialoglycoprotein was predicted khổng lồ contribute khổng lồ homeostasis and maintenance of serum glycoprotein levels, mice containing a null mutation in the MHL-2 subunit were viable & fertile without obvious phenotypic abnormalities & without excessive sầu accumulation of endogenous serum-containing glycoproteins. When challenged with exogenous asialoglycoproteins, however, the mutant mice are defective in hepatic uptake và clearance. These results raise the possibilities that the hepatic receptors may be required to regulate serum glycoprotein levels in periods of induced căng thẳng, which is known to cause elevation of serum glycoprotein levels, and/or to lớn be involved in specific interactions with glycoconjugates from pathogenic organisms. For example, the hepatic asialoglycoprotein receptor promotes endocytosis-dependent entry of hepatitis B virut particles inlớn liver cells. It is interesting that autoimmune hepatitis is associated with autoantibodies lớn the human hepatic ASGquảng bá, và there is evidence that hepatitis virus invasion can augment this response.
It is also possible that other domains of the C-type lectins outside the canonical CRD may also have receptor activity. For example, the cysteine-rich domain of the “mannose” receptor mediates GalNAc-4-SO4 binding and may be involved in internalization và removal from the serum of circulating pituitary hormones containing this determinant. This ability to bind multiple ligands is also exemplified by the Ca++-independent Man-6-P.. receptor (a P-type lectin), which binds Man-6-P-containing ligands via interactions of a few of its 15 repeating domains & binds IGF-II & the urokinase-type plasminogene activator receptor through interactions with other domains.
The Collectins (14–17)
The collectins are a class of C-type lectins containing a collagen-like tên miền that usually assemble in large oligomeric complexes containing 9–27 subunits (Table 25.1 and Figure 25.3). Some collectins, such as mannose-binding protein A and human surfactant SP-A, organize into lớn a “bouquet” form, whereas others, such as bovine conglutinin and surfactant SP-D, organize into a “cruciform” shape. One of the best studied serum collectins is MBPhường. Bovine collectin-43 (CL-43) is structurally one of the simplest collectins và consists of only three polypeptides, each of which contains a terminal C-type lectin domain name. Rats possess two serum MBPs designated A and C, sometimes called mannan-binding proteins. Humans appear to lớn have sầu only a single MBP. corresponding to lớn the rat MBP-A.
Subunit arrangements in the collectin family of C-type lectins. (Adapted, with permission, from <7> Drickamer và Taylor 1993 <© Annual Reviews>.)
The collectins contribute to “innate” immunity and act before induction of an antibody-mediated response. Collectins can stimulate in vitro phagocytosis by recognizing surface glycans on pathogens, chemotaxis, and production of reactive oxyren, và regulate cytokine release by immune cells. Lung surfactant lipids have sầu the ability khổng lồ suppress a number of immune cell functions such as proliferation, and this is augmented by SP-A.
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MBPhường forms a trimeric helical structure via interactions in the collagenous tails that is stabilized by disulfide bonds in the cysteine-rich amino-terminal region. These trimers can aggregate lớn generate three or six trimers in a type of “bouquet” organization. Each CRD in the trimer is separated by about 53 Å, which is critical lớn the function of the lectin; this is because each individual CRD has a relatively low affinity và relatively low specifiđô thị. The spacing between CRDs provides an interesting degree of regulation, since the far spacing lessens potential interactions of the MBL with host glycoconjugates & enhances the potential interactions with extended mannan-containing glycoconjugates, especially those on bacteria, yeast, và parasites.
Binding of MBL và other collectins to lớn a target cell can directly activate complement via the classical pathway. For human MBPhường, this activation appears to result from a novel type of C1s-like serine protease that complexes with MBPhường. and initiates the complement cascade in vivo. It is interesting that some individuals have sầu mutations in the Gly-X-Y repeat, encoded within exon 1 of the MBPhường. gen, and display MBPhường deficiency syndrome. Mutations within exon 1, which are highly variable among human populations, inhibit subunit interactions, leading khổng lồ increased risk of the individuals lớn microbial infections.
An interesting member of the collectin family is tetranectin, which is a plasminogen-binding protein that recognizes the kringle 4 region in plasminogen. Tetranectin may be involved in the packaging of molecules destined for exocytosis. Human tetranectin is a homotrimer forming a triple α-helical coiled coil, and each monomer consists of a CRD connected khổng lồ a long α-helix. Tetranectin could be classified in a distinct subfamily of the C-type lectin superfamily, but the protein has the most structural similarity khổng lồ collectins.
The Selectins (18–20)
The selectins are a class of type I membrane-bound C-type lectins expressed in vascular endothelium and in circulating leukocytes. They are involved in selective sầu cell adhesion. So far, only three selectins have sầu been identified: L-selectin, expressed on all leukocytes; E-selectin, expressed by cytokine-activated endothelial cells; & P-selectin, expressed constitutively in granules of platelets and Weibel-Palade bodies of endothelial cells & on the surface of activated platelets or cells. For a more complete discussion of selectins, see Chapter 26. The selectins are involved in leukocyte-leukocyte interactions (L-selectin) và leukocyte-endothelial cell interactions (L-, E-, & P-selectin).
Each selectin has a C-type CRD at the amino terminus, followed by a consensus EGF-lượt thích domain name & a number of complement regulatory tên miền repeats. The proteins have a single transmembrane domain name và a large cytoplasmic tên miền. Each tên miền of the protein is probably important for functional interactions with cells under vascular fluid flow. All of the selectins display modest specificity and affinity for the sialylated, fucosylated structure known as the sialyl Lewis X antiren NeuAcα2–3Galβ1–4(Fucα1–3)GlcNAcβ-1-R. However, each of the selectins has much higher-affinity interactions with specific macromolecular ligands, & in most cases, these ligands are mucins containing sialylated, fucosylated O-glycans. In addition, ligands for L-selectin (GlyCAM-1) contain Gal-6-sulfated and/or GlcNAc-6-sulfated sialyl Lewis X antigens, và the major ligand for P-selectin, termed P-selectin glycoprotein lig& (PSGL-1), has sulfated tyrosine residues adjacent lớn O-glycans containing sialyl Lewis X antigen.
The Lymphocyte Lectins (21)
A growing danh mục of proteins containing the C-type CRD has been identified on human & rodent lymphocytes (Table 25.1). For the most part, the functions of these proteins are poorly understood và their ability lớn bind carbohydrate has not been demonstrated. Most of the lymphocyte C-type lectins are type II membrane proteins with a single C-type CRD. The immunoglobulin-lượt thích receptors & C-type lectin receptors are two families of MHC class-I-specific receptors found on NK cells. In mice, the C-type lectins on NK cells are represented by the Ly49 family of receptors. Human NK cells express a distantly related molecule designated CD94. In human NK cells, CD94 forms MHC class-I-specific disulfide-linked heterodimers with NKG2 family members, which also contain a C-type lectin CRD.
The NK ren complex occurs on mouse chromosome 6 & its human homolog occurs on chromosome 12. These complexes encode type II transmembrane proteins with a C-type lectin domain that can trigger or inhibit target cell lysis by NK cells (e.g., NKR-P1, Ly49, NKG2, và CD94) or can activate various hematopoietic cells (CD69). CD69 is an early activation antigene expressed on the surfaces of activated T cells, B cells, NK cells, neutrophils, eosinophils, epidermal Langerhans cells, and platelets và can be detected within hours following mitogenic stimulation. The B-cell differentiation antigene (CD72) is expressed on human pre-B cells and B cells, but it is absent from terminally differentiated plasma cells và is associated with CD5. Another C-type lectin on lymphocytes is termed the activation-induced C-type lectin (AICL), whose expression is rapidly increased following lymphocyte stimulation. AICL is encoded by a gen that also closely maps lớn the human NK gen complex and proximal lớn the CD69 gen. The mast-cell-function-associated antiren (MCFA) is a C-type lectin, whose clustering by antigene inhibits mast cell secretory responses stimulated via Fc epsilon RI. The lymphocyte low-affinity IgE Fc receptor (CD23) binds in a Ca++-dependent fashion via its CRD lớn IgE, but this binding may be carbohydrate-independent, again indicating the potential of CRDs to function in protein-protein interactions rather than carbohydrate-protein interactions.
The Proteoglycans (22–23)
The C-type CRD has also been identified in several classic proteoglycans, recently termed lecticans, that laông xã transmembrane domains và occur in the ECM. These include aggregan, brevican, versican, & neurocan (Table 25.1 & Figure 25.2). Like the selectins, each of these core proteins contains a C-type CRD, an EGF-lượt thích domain name, & a complement-regulatory domain name repeat, but their ordering is different and they are located in the carboxyl terminus of the protein. A large region containing attachment sites for chondroitin sulfate and keratung sulfate is proximal to the lectin tên miền. For a more complete discussion of the proteoglycans, see Chapter 11. The exact functions of the lectin domain name in these proteins are not known. The CRD of aggregan binds to lớn supports containing a high density of sugar, but it displays a broad specifithành phố, with the best ligands being fucose & galactose. Some recent studies suggest that the CRD may not be directly important for carbohydrate binding in these proteins, but it may be a structural domain promoting protein-protein interactions with other ECM molecules. The C-type lectin domain of versican can bind khổng lồ tenascin-R, an ECM protein specifically expressed in the nervous system, presumably by carbohydrate-protein interaction. However, there is also evidence that the C-type lectin domain name of all the lecticans, including brevican, can bind tenascin-R in a carbohydrate-independent fashion. Thus, the C-type CRD may be a structural feature of proteins capable of promoting specific protein-protein interactions.
Other Types of C-type Lectins (24–25)
A number of proteins with C-type CRDs have been identified in the pancreas & kidney, but the importance of the CRD & carbohydrate binding lớn their functions is unclear (Table 25.1). Autosomal dominant polycystic kidney disease (ADPKD) is a commonly occurring hereditary disease accounting for approximately 10% of end-stage renal disease. PKD1, one of two recently isolated ADPKD gene products, has been implicated in cell-cell and cell-matrix interactions. The PKD1 gen encodes a novel protein named polycystin that has multiple cell-recognition domains including a single C-type CRD at its amino-terminal region. The function of PKD1 & effect of mutations on its possible activity are unclear. Interestingly, some C-type lectins can be extremely small, as exhibited by HIPhường. và PSPhường. They are virtually free C-type CRDs that are preceded by a signal sequence.
Lower vertebrates, invertebrates, and some viruses also synthekích cỡ proteins with a C-type CRD, and some of these proteins have been shown khổng lồ bind carbohydrate. For example, the galactose-specific lectin from the snake Crotalus atrox binds a variety of galactose-containing glycolipids in a Ca++-dependent fashion. A number of related venom proteins are known khổng lồ inhibit platelet function and/or the coagulation cascade. Alboaggregin A from Trimeresurus albolabris (the white-lipped pit viper) contains subunits 1–4, and subunit 1 contains a single C-type CRD in its amino-terminal domain name. The protein binds to lớn the platelet GPIb/IX receptor and stimulates agglutination, but the potential role of carbohydrate recognition in this process is presently unknown.
Tertiary/Quaternary Structures of C-type Lectins (26–30)
The crystal structure of the trimeric rat MBP-A is shown in Figure 25.4. Each CRD has a continuous segment of polypeptide folded into a series of four loops stabilized by two disulfide bonds & each contains two binding sites for Ca++. This interesting structure identified the role of the C-type CRDs, since many of the acidic residues help khổng lồ coordinate Ca++, and the common glycine & proline residues are found at critical turns. In addition, the carboxyl và amino termini of the CRD are near each other. This helps explain how the C-type CRD can be similar in both type I transmembrane proteins, such as selectins, và in type II transmembrane proteins, such as the hepatic asialoglycoprotein receptor. The CRD binds to lớn sugar ligands in a shallow surface pocket, which classifies the C-type CRD as a type-2 carbohydrate-binding domain name, as opposed to a deep-pocket type-1-binding domain, which are typically found in monosaccharide transporters & bacterial toxins such as cholera toxin (see Chapter 4). Interaction of the CRD with mannose in the rat serum MBPhường occurs through equatorial hydroxyl groups 3 and 4 of terminal mannose coordinated lớn Ca++. This complex is stabilized by H bonds lớn each hydroxyl group. The loss of affinity of C-type CRD in the endocytotic receptors upon lowering the pH is probably due to lớn loss of coordinated Ca++.
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Crystal structure of a trimeric rat mannose-binding protein A (MBP-A) complexed with mannose at 1.8 Å resolution (see reference ). Mannose is indicated in the stiông chồng figure and Ca++ and Cl are indicated by the green and blue balls, respectively. (more...)
The discovery of the C-type lectins represents the exciting fulfillment of predictions made years earlier by many investigators in the field that the carbohydrate moieties of soluble and membrane-bound glycoconjugates would be important in cell-cell và cell-matrix and other such cellular interactions. Future studies are likely to lớn concentrate on identifying more C-type lectins và defining their roles by genetic mutations in animal mã sản phẩm systems and searching for more naturally occurring mutations affecting their expression and localization. In addition, the multitude of C-type lectins in animal cells, with their vast differences in carbohydrate-binding specifithành phố for macromolecular glycoconjugates, suggests that many carbohydrate structures in the vast animal repertoire may have sầu specific binding partners. This notion is becoming more compelling as a host of other carbohydrate-binding proteins in addition lớn the C-type family have sầu also been identified in animals.
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