General Properties

Two different lectins can be purified from R. communis seeds, and based on the earlier nomenclature they are termed RCA-I and RCA-II. RCA-I is an agglutinin but a very weak toxin. The other lectin RCA-II is ricin, and it is both an agglutinin and a very potent toxin. Ricin is one of the most famous lectins in the world, because it can be derived from the common castor bean and has been used by villains to kill or threaten people.

RCA-I is ∼120 kDa and ricin is ∼60 kDa (Figure 31.1). Ricin is synthesized as a single prepropolypeptide containing an A-chain domain, a 12-amino-acid linker region, and the B chain, with proteolysis generating the separate A and B chains. The A chain is an N-glycoside hydrolase that inactivates the 60S ribosome, and the B chain binds sugar. Mature ricin contains four intrachain disulfide bonds and a single disulfide bond links the A chain to the B chain. The mature A chain has 267 amino acids, and the B chain has 262 amino acids. Each B chain polypeptide is a product of gene duplications and has two CRDs, each of which is composed of an ancient 40-amino-acid galactose-binding polypeptide region.

In contrast, RCA-I is a tetramer with two ricin heterodimer-like proteins that are noncovalently associated. Each heterodimer in RCA-I contains an A-chain disulfide linked to the galactose-binding B chain. The sequences of the A chain of ricin and the A chain of RCA-I differ in 18 of 267 residues and are 93% identical, whereas the B chains differ in 41 of 262 residues and are 84% identical. All of the subunits are N-glycosylated and usually express oligomannose-type N-glycans. Because ricin is a multimeric glycoprotein, recombinant forms produced in Escherichia coli (such as ricin B chain) are usually poorly active, and eukaryotic expression systems must be used. The genome of R. communis also encodes several other proteins that have high homology with ricin, and some of these lectins have been designated ricin-A, -B, -C, -D, and -E.

The two CRDs in the B chain of ricin are about 35 Å apart. Thus, ricin binds divalently to either terminal β-linked galactose or N-acetylgalactosamine, whereas RCA-I prefers terminal β-linked galactose. Indeed, these lectins are often purified and separated from one another by differential elution from galactose-based affinity resins; ricin is eluted first with the hapten GalNAc and then RCA-I is eluted with Gal. Although the affinities of these lectins for monosaccharides are very low (K_d in the range 10⁻³ to 10⁻⁴ m), lectin binding to cells is very high affinity (K_d in the range 10⁻⁷ to 10⁻⁸ m), which arises from the multivalency and enhanced binding to multiple surface glycans terminating in the sequence Galβ1-4GlcNAc-R. In general, both ricin and RCA-I bind to glycans containing nonreducing terminal Galβ1-4GlcNAc-R or GalNAcβ1-4GlcNAc-R, although they will bind weakly to the isomer Galβ1-3GlcNAc-R. Neither ricin nor RCA-I binds well to glycoconjugates containing nonreducing terminal α-linked Gal residues. Immobilized RCA-I is commonly used for glycan isolation and characterization because it is safer and has higher avidity owing to its tetrameric nature. In contrast, ricin is often used as a toxin in cell selection for glycosylation mutants, and the A chain of ricin is often used in chimeric proteins as a toxin for specific-cell killing.

R-Type Domain

The CRD in the R-type domain of ricin and RCA-I is a carbohydrate-binding molecule (CBM) and has been placed in the CBM13 family in the Carbohydrate-Active Enzymes (CAZy) database (see Chapter 6). The R-type domain is an ancient protein fold that is found in many glycosyltransferases as well as in bacterial and fungal hydrolases. Interestingly, the R-type CRD is one of the few that is well conserved between animal, plant, and bacterial lectins.

The crystal structure of ricin (Figure 31.2) shows that the A chain has eight α-helices and eight β-strands and it is the catalytic subunit. The B chain, which contains R-type lectin domains, has two tandem CRDs that are ∼35 Å apart and have a shape resembling a barbell, with one binding domain at each end. Each R-type domain has a three-lobed organization that is a β-trefoil structure (from the Latin “trifolium” meaning “three-leaved plant,” as seen in leaves from some pea plants and clovers) (Figure 31.3). The β-trefoil structure probably arose evolutionarily through gene fusion events linking a 42-amino-acid peptide subdomain that has galactose-binding activity. The three lobes are termed α, β, and γ and are arranged around a threefold axis. Conceivably, each lobe could be an independent binding site, but in most R-type lectins only one or two of these lobes retain the conserved amino acids required for sugar binding. Sugar binding is relatively shallow in these loops and arises from aromatic amino acid stacking against the Gal/GalNAc residues and from hydrogen bonding between amino acids and hydroxyl groups of the sugar ligands. A characteristic feature of these loops in the R-type domain in ricin is the presence of (QxW)₃ repeats (where x is any amino acid), which are found in many, but not all, R-type family members. But not all proteins with a β-trefoil fold bind carbohydrate, as this structural domain can promote other types of interactions, including binding to protein and RNA. Examples of other proteins with the β-trefoil fold include members of the actin cross-linking protein fascin, interleukin 1 cytokine family, and fibroblasts growth factors (FGFs) 1 and 2.

FIGURE 31.2.

Ricin and abrin. (A) Photographs showing the leaves and seeds of Ricinus communis and the seeds of Abrus precatorius. R. communis is commonly called the castor oil plant and the seeds are castor beans. The A. precatorius seeds are sometimes called rosary (more...)

Toxicity of Ricin

Ricin is remarkably toxic and the impact of ingested ricin is severe, with symptoms initiated after a 2- to 24-hour latent period. The lethal dose (LD₅₀) of ricin may be as low as 3–5 µg/kg per kilogram of body weight, depending on the mode of exposure. Ricin is a type II ribosome-inactivating protein (RIP-II). Overall, RCA-I is less toxic than ricin, apparently the result of the weaker enzymatic activity of its A chain. Notably, type I ribosome-inactivating proteins (RIP-I) that lack a B subunit are not lectins and are much less toxic than ricin because of poor ability to enter cells. Examples of RIP-I proteins are saporin, lychnin, gelonin, and momordin-I.

Ricin binds to cells via interactions with β-linked Gal/GalNAc-containing glycans on the cell surface and is imported into endosomes (Figure 31.4). From there, the protein moves by retrograde trafficking to the endoplasmic reticulum (ER) via the trans-Golgi network and Golgi apparatus. In the ER, the A and B chains separate after reduction of the disulfide bond, perhaps through the action of the ER enzyme, protein disulfide isomerase (PDI). Some fraction of the free A chain, which may be partly denatured in the ER, may then escape from the ER by retrotranslocation through the Sec61 translocon and thereby enter the cytoplasm. This retrotranslocation may involve the quality control system in the ER known as ER-associated degradation (ERAD), which is involved in removing misfolded proteins from the ER (see Chapter 36). Some of the A chain may be ubiquitinated and degraded in the proteasome, but the A chain may not be an efficient substrate for ubiquitination because it has few lysine residues.

FIGURE 31.4.

Pathway of ricin uptake by cells and the mechanism by which the toxic activity of the A chain in the cytoplasm results in cell death.

FIGURE 31.3.

Structures of the β-trefoil R-type domains in different proteins. (Top) Cysteine-rich R-type domain of the mannose receptor (MR) in complex with 4-O-sulfated GalNAc; ricin B chain in complex with galactose; and acidic fibroblast growth factor (more...)

In the cytoplasm, just a few molecules of ricin A chain appear to be sufficient to kill cells. The A chain is an RNA N-glycosidase that cleaves one adenine residue (A4324) in the exposed GAGA tetraloop of the 28S RNA in the eukaryotic 60S ribosomal subunit. This deletion results in a loss of binding of elongation factor-2 and the inability of the ribosome to promote protein synthesis.

The Mannose Receptor Family

There are four known members of the human MR family, all of which contain an R-type lectin domain, and a fifth member FcRY in birds. The human MR family includes the MR, the phospholipase A2 (PLA2) receptor, DEC-205/MR6-gp200, and Endo180/urokinase plasminogen activator receptor-associated protein (Figure 31.1). All of these proteins are large type I transmembrane glycoproteins and they contain a single fibronectin type II domain, multiple C-type lectin domains (CTLDs) (Chapter 34), and an amino-terminal cysteine-rich domain. Some of the CTLDs in the MR and Endo180 bind glycans, but other CTLDs in these proteins do not bind glycans and have other functions. Each protein is a recycling plasma membrane receptor with a cytoplasmic domain that mediates clathrin-dependent endocytosis and uptake of extracellular glycan-containing ligands. All of the MR family members except DEC-205 recycle back to the cell surface from early endosomes, but DEC-205 recycles from late endosomes. However, each receptor has evolved to have distinct functions and distributions. These receptors are unusual among animal lectins in that they can bind ligands in either a “cis” or “trans” fashion, which means they can bind to cell-surface glycoconjugates on the same cell or to those on other cells and to soluble ligands.

The Mannose Receptor

The MR (CD206) is important in the innate and adaptive immune systems. It is expressed at high levels on hepatic endothelial cells and Kupffer cells as well as on many other endothelial and epithelial cells, macrophages, and immature dendritic cells (DCs). The MR is part of the innate immune system and it facilitates the phagocytosis of mannose-rich pathogens. It also assists leukocytes in responding appropriately to antigens by promoting trafficking to the germinal center and is also involved in antigen presentation. The receptor was originally discovered in the 1980s in rabbit alveolar macrophages as a large membrane protein (175 kDa) that bound mannose-containing ligands as well as pituitary hormones such as lutropin and thyrotropin, which have 4-O-sulfated N-acetylgalactosamine residues on N-glycans. However, although the MR is a C-type lectin with multiple CTLDs (see Chapter 34), it also contains an cysteine-rich R-type domain that binds sulfated glycans on pituitary hormones and causes their clearance from the circulation.

Interestingly, the MR and other members of the MR family are among the few mammalian GBPs that have two separate lectin motifs (C-type and R-type) in the same polypeptide. This group is also unusual in that it is the only known lectin group in mammals with more than two CTLDs in the same molecule. Only CTLDs 4 and 5 of the MR have been shown to bind glycans in a Ca⁺⁺-dependent manner and to bind mannose, N-acetylglucosamine, and fucose.

The R-type domain of the MR binds other sulfated glycans and also N-glycans on pituitary glycoprotein hormones containing 4-SO₄-GalNAcβ1-4GlcNAcβ1-2Manα1-R. Other ligands for the MR include chondroitin-4-sulfate proteoglycans on leukocytes that also contain 4-SO₄-GalNAc β1-R residues and perhaps sulfated glycans, such as those containing 3-O-sulfated galactose, 3-O-sulfated Le^x, and 3-O-sulfated Le^a. The cysteine-rich R-type domain in the MR binds with relatively low affinity to sulfated monosaccharides (K_d in the range of 10⁻³ to 10⁻⁴ m), but oligomeric forms of the protein probably display much higher avidity for glycoproteins with multiple sulfated glycans. Interestingly, although the cysteine-rich R-type domain binds sulfated glycans, the CTLDs bind unsulfated glycans and may also play a role in glycoprotein homeostasis and clearance.

The MR is also unusual in that it is the only member of the MR family that can function both in clathrin-dependent endocytosis and in the phagocytosis of nonopsinized microbes and large ligands. The MR can bind many different microorganisms, including Candida albicans, Pneumocystis carinii, Leishmania donovani, Mycobacterium tuberculosis, and Klebsiella pneumoniae. The MR may distinguish self from nonself glycans via its multivalent attachment to the mannose-rich glycoconjugates on these organisms, which are presented in unique clusters. The MR also functions in adaptive immunity through its ability to deliver antigens to major histocompatibility (MHC) class II compartments and through its cleavage and release as a soluble protein into blood. Interestingly, it is speculated that the membrane-bound MR may bind antigens through its CTLDs and then, following proteolytic cleavage, the soluble MR bound to its cargo may move to germinal centers where it may bind via its R-type domain to macrophages and DCs expressing ligands such as sialoadhesion or CD45 that may contain sulfated glycans.

The Phospholipase A2 Receptor

The PLA2 receptor (PLA2R) was discovered as a receptor for PLA2 neurotoxins in snake venoms and was referred to as the M-type PLA2 receptor to distinguish it from the neuronal or N-type PLA2R. The cDNA encoding this 180-kDa glycoprotein has homology with the MR (with 29% identity). It can occur as both a long form that is a type I transmembrane glycoprotein with a domain structure like the MR and a shorter form that is secreted. The membrane-bound form can function as an endocytic receptor to internalize PLA2 ligands. In animal cells, secreted PLA2s represent a large family of enzymes that are important in the degradation of phospholipids and the release of arachidonic acid, which is the precursor for prostaglandins, leukotrienes, thromboxanes, and prostacyclins. The murine PLA2 receptor binds to sPLA2-X enzyme, but the human PLA2R may bind to several different PLA2 isozymes. Although early studies suggested that the PLA2R might be involved in clearance of PLA2s, murine knockouts for the PLA2R had the unusual phenotype of resistance to endotoxic shock. This suggested that the PLA2R might be important in regulating production of proinflammatory cytokines by soluble PLA2s. Thus, the PLA2R might function in signal transduction mediated by PLA2 binding. Some of the CTLDs in the PLA2R function by binding PLA2 ligands, rather than binding directly to glycan ligands. The fibronectin type II domain binds to collagen, which is a feature shared by this domain in other MR family members, except for possibly DEC-205. Recently, PLA2R is an autoantigen in adult idiopathic membranous nephropathy (MN).

DEC-205

DEC-205 is a 205-kDa member of the MR family that is expressed by dermal DCs and, at a lower level, by epidermal Langerhans cells. It is also expressed on some epithelial cells, on bone marrow stroma, and by endothelial cells. It was identified originally in the 1980s by a monoclonal antibody NLDC-145, which recognized a surface antigen (SAG) on Langerhans cells. DEC-205 is important in the recognition and internalization of antigens for presentation to T cells. Upon endocytosis, DEC-205 internalizes to late endosomes/lysosomes and recycles to the surface. Expression of DEC-205 is enhanced in both types of cells upon cell maturation induced by inflammatory stimuli. None of the 10 CTLDs in DEC-205 has the conserved amino acids known to be important in carbohydrate binding, and thus far there is no evidence that these domains bind glycans, but they bind unknown ligands on apoptic cells.

Endo180

Endo180 was found to be part of a trimolecular cell-surface complex with urokinase plasminogen activator (uPA) and its receptor (uPAR) and was termed the uPAR-associated protein or uPARAP. It was also discovered as a novel antigen on macrophages and human fibroblasts. Like the MR, Endo180 is expressed on macrophages, but Endo180 is also expressed on fibroblasts and chondrocytes, some endothelial cells, and tissues undergoing ossification. The cysteine-rich R-type domain at the amino terminus of Endo180 has been shown to be unable to bind sulfated glycans. The function of this domain is unknown. Endo180 is predicted to be important in remodeling of the extracellular matrix (ECM). It binds to the carboxy-terminal region of type I collagen, and collagens type II, IV, and V, through its FN II domain, and binds to uPA and uPAR, but whether glycan recognition is important for these interactions is not known. Endo180 (CD280) binds in a Ca⁺⁺-dependent manner to N-acetylglucosamine, mannose, and fucose through CTLD2, and this is the only domain that contains all of the conserved amino acids for binding both Ca⁺⁺ and sugar. Endo180 is an endocytic receptor for these soluble ligands and may be important in matrix turnover. In addition, cells expressing Endo180 show enhanced adhesion to matrixes, suggesting that Endo180 may also be important in cell adhesion.

UDP-GalNAc:Polypeptide α-N-Acetylgalactosaminyltransferases

Mucin-type O-glycans have the common core structure of GalNAcα1-Ser/Thr (Figure 31.5), which may be further modified by addition of Gal or GalNAc residues (Chapters 10 and 14). Remarkably, recent studies show that the vast majority (>80%) of all proteins passing through the secretory apparatus have at least one Ser or Thr residues modified with α-linked GalNAc. Modification of Ser/Thr residues is dependent on a family of UDP-GalNAc:polypeptide α-N-acetylgalactosaminyltransferases (ppGalNAcTs) that function in the Golgi apparatus. Such enzymes are found in all animals. Twenty genes encoding ppGalNAcTs have been identified in humans; there are 19 in rodents and 14 in Drosophila (Chapters 6 and 9). These ppGalNAcTs may be separated into two general classes: those that can transfer N-acetylgalactosamine from UDP-GalNAc to unmodified polypeptide acceptors, and those that prefer acceptor glycopeptides containing GalNAc-Ser/Thr (i.e., a set predicted to have a CRD). These enzymes are very large proteins and have a unique multidomain structure. The enzymes are all type II transmembrane proteins and have an amino-terminal catalytic domain and a carboxy-terminal R-type domain of about 130 amino acid residues. This R-type domain has the QxW repeat function as lectins during the catalytic process of these enzymes.

FIGURE 31.5.

Structure and function of UDP-GalNAc:polypeptide α-N-acetylgalactosaminyltransferases (ppGalNAcTs). (A) The activity of the ppGalNAcT is shown using an acceptor peptide with Ser and Thr residues and UDP-GalNAc as the donor. Some of the ppGalNAcTs (more...)

Crystal structures of members of this enzyme family show the catalytic and R-type domain function dynamically with polypeptide substrates. For example, in ppGalNAc-T2, the catalytic domain interacts with the peptide in a peptide-binding grove, whereas the R-type lectin domain interacts primarily with the GalNAc residue in modified peptides. The proximity of these two interactions helps to position the enzyme to attack special acceptor peptides. Such specificities among the multiple ppGalNAcTs allows the family to have tremendous coverage over a wide range of polypeptide substrates and confers specificity on these enzymes in terms of particular Ser/Thr acceptor sites.

Streptomyces lividans Endo-β1-4Xylanase 10A

A feature of many microbial glycosidases is the presence of both a catalytic domain and an R-type domain in CBM13 family. S. lividans endo-β1-4xylanase 10A (Xyn10A) is a good example of such an enzyme. Xyn10A catalyzes the cleavage of β1-4xylans and can bind to xylan and a variety of small soluble sugars, including galactose, lactose, and xylo- and arabino-oligosaccharides. The catalytic domain is at the amino terminus and the carboxyl terminus has an R-type β-trefoil motif. As mentioned above, the R-type domain CBM represents the CBM13 family in the CAZy database. In Xyn10A, all of the original β-trefoil sugar-binding motifs are retained, along with the conserved disulfide bridges, and evidence suggests that each of the three potential sugar-binding sites in the β-trefoil structure interact with sugars and each site may span up to four xylose residues. The binding to monosaccharides is very weak (K_d in the range of 10⁻² to 10⁻³ m), but multivalent binding avidity to polysaccharides can be very strong.

Actinomycete Longispora albida Actinohivin

Actinohivin is an actinomycete-derived lectin in the CBP module famly 13 that binds to clustered mannose residues of the high-mannose-type glycans commonly found on the viral envelope of human immunodeficiency virus 1 (HIV-1). Actinohivin is small (114 aa) and has three sugar binding pockets within its β-trefoil structure.