Legume lectin

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Legume lectin domain (or L-type lectin domain)
PDB 1lem EBI.jpg
Structure of the monosaccharide binding site of lentil lectin. [1]
Identifiers
SymbolLectin_legB
Pfam PF00139
Pfam clan CL0004
InterPro IPR001220
PROSITE PDOC00278
SCOP2 1lem / SCOPe / SUPFAM

The legume lectins (or L-type lectins) are a family of sugar-binding proteins or lectins found in the seeds and, in smaller amounts, in the roots, stems, leaves and bark of plants of the family Fabaceae. [2] [3] The exact function of the legume lectins in vivo is unknown but they are probably involved in the defense of plants against predators. Related proteins in other plant families and in animals have also been found. They have been used for decades as a model system for the study of protein-carbohydrate interactions, because they show an amazing variety of binding specificities and are easy to obtain and purify. Over the years, a quite impressive amount of structural data has been gathered. [3] Well-studied members of this protein family include phytohemagglutinin and concanavalin A.

Contents

Sugar binding by legume lectins

The legume lectins use an ingenious framework for binding specific sugars. This framework consists of a conserved monosaccharide binding site in which four conserved residues from four separate regions in the protein confer affinity (see figure), a variable loop that confers monosaccharide specificity and a number of subsites around the monosaccharide binding site that harbour additional sugar residues or hydrophobic groups. [3]

Quaternary structure

The legume lectins are also interesting from the point of view of protein structure. Despite the conserved structure of the legume lectin subunit, they can adopt a wide range of quaternary structures. [4] [5] The reason behind this remarkable variability is probably to be found in the interaction with multivalent ligands. [6]

A typical legume lectin monomer (lentil lectin), complexed with a sugar (glucose). The four sugar-binding loops are shown in different colours. The variable loop that confers monosaccharide specificity is shown in orange. Legume lectin monomer.jpg
A typical legume lectin monomer (lentil lectin), complexed with a sugar (glucose). The four sugar-binding loops are shown in different colours. The variable loop that confers monosaccharide specificity is shown in orange.
Quaternary structures of some legume lectins. One of the subunits is in the same orientation in all structures for ease of comparison. Legume lectin quat.jpg
Quaternary structures of some legume lectins. One of the subunits is in the same orientation in all structures for ease of comparison.

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Protein kinase Enzyme that adds phosphate groups to other proteins

A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them (phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a functional change of the target protein (substrate) by changing enzyme activity, cellular location, or association with other proteins. The human genome contains about 500 protein kinase genes and they constitute about 2% of all human genes. There are two main types of protein kinase. The great majority are serine/threonine kinases, which phosphorylate the hydroxyl groups of serines and threonines in their targets and most of the others are tyrosine kinases, although additional types exist. Protein kinases are also found in bacteria and plants. Up to 30% of all human proteins may be modified by kinase activity, and kinases are known to regulate the majority of cellular pathways, especially those involved in signal transduction.

Glycome

The glycome is the entire complement of sugars, whether free or present in more complex molecules, of an organism. An alternative definition is the entirety of carbohydrates in a cell. The glycome may in fact be one of the most complex entities in nature. "Glycomics, analogous to genomics and proteomics, is the systematic study of all glycan structures of a given cell type or organism" and is a subset of glycobiology.

Lectin Carbohydrate-binding protein

Lectins are carbohydrate-binding proteins that are highly specific for sugar groups that are part of other molecules, so cause agglutination of particular cells or precipitation of glycoconjugates and polysaccharides. Lectins have a role in recognition at the cellular and molecular level and play numerous roles in biological recognition phenomena involving cells, carbohydrates, and proteins. Lectins also mediate attachment and binding of bacteria, viruses, and fungi to their intended targets.

An oligosaccharide is a saccharide polymer containing a small number of monosaccharides. Oligosaccharides can have many functions including cell recognition and cell binding. For example, glycolipids have an important role in the immune response.

Glycolipid Class of chemical compounds

Glycolipids are lipids with a carbohydrate attached by a glycosidic (covalent) bond. Their role is to maintain the stability of the cell membrane and to facilitate cellular recognition, which is crucial to the immune response and in the connections that allow cells to connect to one another to form tissues. Glycolipids are found on the surface of all eukaryotic cell membranes, where they extend from the phospholipid bilayer into the extracellular environment.

Concanavalin A Lectin (carbohydrate-binding protein) originally extracted from the jack-bean

Concanavalin A (ConA) is a lectin originally extracted from the jack-bean. It is a member of the legume lectin family. It binds specifically to certain structures found in various sugars, glycoproteins, and glycolipids, mainly internal and nonreducing terminal α-D-mannosyl and α-D-glucosyl groups. ConA is a plant mitogen, and is known for its ability to stimulate mouse T-cell subsets giving rise to four functionally distinct T cell populations, including precursors to regulatory T cells; a subset of human suppressor T-cells is also sensitive to ConA. ConA was the first lectin to be available on a commercial basis, and is widely used in biology and biochemistry to characterize glycoproteins and other sugar-containing entities on the surface of various cells. It is also used to purify glycosylated macromolecules in lectin affinity chromatography, as well as to study immune regulation by various immune cells.

Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed, mainly, by cells of the innate immune system, such as dendritic cells, macrophages, monocytes, neutrophils and epithelial cells, to identify two classes of molecules: pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens, and damage-associated molecular patterns (DAMPs), which are associated with components of host's cells that are released during cell damage or death. They are also called primitive pattern recognition receptors because they evolved before other parts of the immune system, particularly before adaptive immunity. PRRs also mediate the initiation of antigen-specific adaptive immune response and release of inflammatory cytokines.

Selectin Transmembrane proteins with a lectin-like domain, an epidermal growth factor-like domain, and a variable number of domains homologous to complement regulatory proteins

The selectins are a family of cell adhesion molecules. All selectins are single-chain transmembrane glycoproteins that share similar properties to C-type lectins due to a related amino terminus and calcium-dependent binding. Selectins bind to sugar moieties and so are considered to be a type of lectin, cell adhesion proteins that bind sugar polymers.

The terms glycan and polysaccharide are defined by IUPAC as synonyms meaning "compounds consisting of a large number of monosaccharides linked glycosidically". However, in practice the term glycan may also be used to refer to the carbohydrate portion of a glycoconjugate, such as a glycoprotein, glycolipid, or a proteoglycan, even if the carbohydrate is only an oligosaccharide. Glycans usually consist solely of O-glycosidic linkages of monosaccharides. For example, cellulose is a glycan composed of β-1,4-linked D-glucose, and chitin is a glycan composed of β-1,4-linked N-acetyl-D-glucosamine. Glycans can be homo- or heteropolymers of monosaccharide residues, and can be linear or branched.

Siglecs(Sialic acid-binding immunoglobulin-type lectins) are cell surface proteins that bind sialic acid. They are found primarily on the surface of immune cells and are a subset of the I-type lectins. There are 14 different mammalian Siglecs, providing an array of different functions based on cell surface receptor-ligand interactions.

The mannose receptor is a C-type lectin primarily present on the surface of macrophages, immature dendritic cells and liver sinusoidal endothelial cells, but is also expressed on the surface of skin cells such as human dermal fibroblasts and keratinocytes. It is the first member of a family of endocytic receptors that includes Endo180 (CD280), M-type PLA2R, and DEC-205 (CD205).

Peanut agglutinin

Peanut agglutinin (PNA) is plant lectin protein derived from the fruits of Arachis hypogaea. Peanut agglutinin may also be referred to as Arachis hypogaea lectin. Lectins recognise and bind particular sugar sequences in carbohydrates; peanut agglutinin binds the carbohydrate sequence Gal-β(1-3)-GalNAc. The name "peanut agglutinin" originates from its ability to stick together (agglutinate) cells, such as neuraminidase-treated erythrocytes, which have glycoproteins or glycolipids on their surface which include the Gal-β(1-3)-GalNAc carbohydrate sequence.

Ribosome-inactivating protein Protein synthesis inhibitor

A ribosome-inactivating protein (RIP) is a protein synthesis inhibitor that acts at the eukaryotic ribosome. This protein family describes a large family of such proteins that work by acting as rRNA N-glycosylase. They inactivate 60S ribosomal subunits by an N-glycosidic cleavage, which releases a specific adenine base from the sugar-phosphate backbone of 28S rRNA. RIPs exist in bacteria and plants.

BanLec is a lectin from the jacalin-related lectin family isolated from the fruit of the bananas Musa acuminata and Musa balbisiana. BanLec is one of the predominant proteins in the pulp of ripe bananas and has binding specificity for mannose and mannose-containing oligosaccharides. A 2010 study reported that BanLec was a potent inhibitor of HIV replication.

Mamannamana Vijayan is an Indian structural biologist. His main area of research is protein structures. His contributions have been towards the structure and carbohydrate specificity of lectins and protein hydration. He has also contributed towards the area of structure and interactions of mycobacterial proteins and supramolecular association with reference to chemical evolution and origin of life. Vijayan did biological macromolecular crystallography in India.

Carbohydrate-binding module

In molecular biology, a carbohydrate-binding module (CBM) is a protein domain found in carbohydrate-active enzymes. The majority of these domains have carbohydrate-binding activity. Some of these domains are found on cellulosomal scaffoldin proteins. CBMs were previously known as cellulose-binding domains. CBMs are classified into numerous families, based on amino acid sequence similarity. There are currently 64 families of CBM in the CAZy database.

Leguminous lectin family

In molecular biology, the leguminous lectin family is a family of lectin proteins.

Avadhesha Surolia is a Glycobiologist at Indian Institute of Science (IISc), Bangalore. Presently, he is an Honorary Professor at the Molecular Biophysics Unit (MBU), IISc and holds the Bhatnagar fellowship of the Council of Scientific and Industrial Research (CSIR), India. He is known for his work on lectin structure and interactions, orientation and dynamics of cell surface carbohydrate receptors and protein folding, diabetes, anti-malarials and anti-cancer agents based on curcumin, flavonoids, etc. In addition, neuropathic pain, neurodegenerative disorders and the link between immunity and obsessive compulsive disorder are areas of his current interest

Intelectin

Intelectins are lectins expressed in humans and other chordates. Humans express two types of intelectins encoded by ITLN1 and ITLN2 genes respectively. Several intelectins bind microbe-specific carbohydrate residues. Therefore, intelectins have been proposed to function as immune lectins. Even though intelectins contain fibrinogen-like domain found in the ficolins family of immune lectins, there is significant structural divergence. Thus, intelectins may not function through the same lectin-complement pathway. Most intelectins are still poorly characterized and they may have diverse biological roles. Human intelectin-1 (hIntL-1) has also been shown to bind lactoferrin, but the functional consequence has yet to be elucidated. Additionally, hIntL-1 is a major component of asthmatic mucus and may be involved in insulin physiology as well.

Glycan-protein interactions Class of biological intermolecular interactions

Glycan-Protein interactions represent a class of biomolecular interactions that occur between free or protein-bound glycans and their cognate binding partners. Intramolecular glycan-protein (protein-glycan) interactions occur between glycans and proteins that they are covalently attached to. Together with protein-protein interactions, they form a mechanistic basis for many essential cell processes, especially for cell-cell interactions and host-cell interactions. For instance, SARS-CoV-2, the causative agent of COVID-19, employs its extensively glycosylated spike (S) protein to bind to the ACE2 receptor, allowing it to enter host cells. The spike protein is a trimeric structure, with each subunit containing 22 N-glycosylation sites, making it an attractive target for vaccine search.

References

  1. ↑ Loris R, Casset F, Bouckaert J, et al. (December 1994). "The monosaccharide binding site of lentil lectin: an X-ray and molecular modelling study". Glycoconj. J. 11 (6): 507–17. doi:10.1007/bf00731301. PMID   7696853. S2CID   20037257.
  2. ↑ Sharon N, Lis H (November 1990). "Legume lectins--a large family of homologous proteins". FASEB J. 4 (14): 3198–208. doi:10.1096/fasebj.4.14.2227211. PMID   2227211. S2CID   23310019.
  3. 1 2 3 Loris R, Hamelryck T, Bouckaert J, Wyns L (March 1998). "Legume lectin structure". Biochim. Biophys. Acta. 1383 (1): 9–36. doi:10.1016/S0167-4838(97)00182-9. PMID   9546043.
  4. ↑ Manoj N, Suguna K (October 2001). "Signature of quaternary structure in the sequences of legume lectins". Protein Eng. 14 (10): 735–45. doi: 10.1093/protein/14.10.735 . PMID   11739891.
  5. ↑ PDBe Browser for legume lectin assemblies
  6. ↑ Hamelryck TW, Moore JG, Chrispeels MJ, Loris R, Wyns L (June 2000). "The role of weak protein-protein interactions in multivalent lectin-carbohydrate binding: crystal structure of cross-linked FRIL". J. Mol. Biol. 299 (4): 875–83. doi:10.1006/jmbi.2000.3785. PMID   10843844.
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