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Lectins

Lectins are sugar-binding proteins that agglutinate cells and/or precipitate glycoconjugates (= molecules with a carbohydrate portion like polysaccharides, glycoproteins, glycolipids and others).

Since they were originally only isolated from plant extracts and were used for the agglutination of blood cells (erythrocytes), they were at first called phytohaemagglutines. Later, it was found that they can also be obtained from animal organs, especially those of invertebrates and that they do not all bind to erythrocytes. W.C.BOYD and E. SLAPEIGH did therefore coin the term lectin in 1954 (from lat. legere = to choose). Lectins have at least two sugar binding sites otherwise, their agglutination/precipitation behaviour cannot be explained. Actually most of them are composed from two, four or more usually identical subunits. Their specificity is defined by the mono-or oligosaccharide that inhibits the agglutination competitively. A choice of the best-known lectins is presented in a table.

The affinity a lectin displays for cells or macromolecules (ligands) is several orders higher than that for single sugars. This led to the assumption that it were not only the carbohydrate parts that were important for the binding but that certain, non-specific protein-protein interactions (weak interactions) were stabilizing the complex. A number of lectins, for example RCA, PNA, SBA, are known to have an affinity to beta-D-galactosyl residues, but they show a greatly varying binding ability for certain cells or glycoproteins. The steric organization of the carbohydrates at the cell's or molecule's surface have a great influence, they have to be accessible for the lectin. The lectin-binding molecules are usually referred to as lectin receptors due to the structural complexity of the ligands. Although certain insecurities concerning their chemical characterization exist, lectins are increasingly used in medicinal pure research. They are used for the characterization of certain cell types or fragments (like different membranes), to detect cells in different states of development, to distinguish normal from tumour cells, to mark the different states of the cell cycle and to separate different cell types by affinity chromatography.

By linkage to fluorescence dyes (fluorescent markers), suitable tags for the localization of glycoconjugates in cells or at cell surfaces were gained. A corresponding tagging with electron-dense substances like ferredoxin or colloidal gold produced good helps for electron microscopy. Though until now not many of such studies have been performed with plant cells.

Although some of the mentioned properties are also shared by antibodies, they do not have very much in common with lectins:

Favin that occurs in Vicia faber, shows some surprising correspondences with ConA. It has two polypeptide chains of unequal length, alpha and beta. The alpha-chain is homologous to section 70-119 of ConA, the beta-chain to sections 120-237 and 1-69. These agreements are caused by a so-called 'cyclic permutation' of polypeptide sections. But as long as no further data is available, it cannot be decided whether the ConA-sequence is older and the flavin developed by taking out of the middle section and an exchange of the two terminal fragments or whether ConA was generated by fusion of the two polypeptide chains of flavin.

The polypeptide chain of WGA has 164 amino acid residues, among them numerous cysteine residues that are able to form disulfide bonds.

The amino acid chain folds into a tertiary structure of four domains: A, B, C and D. As in many enzymes, there is a cleft between A and B and between C and D. But this is not the site of sugar-binding, that takes place at the outside of the molecule. Could this be the reason why WGA does not act as an enzyme? There are no 'special conditions' at the molecules' outside that are characteristic for the active centre of an enzyme.

The comparison of WGA's and ConA's tertiary structures shows that no similarities are shared. But the domains A, B, C and D of WGA are all very similar. It may thus be assumed that the gene for WGA developed by two consecutive duplications of the original gene.

Serological cross-reactions showed that different gramineae contain lectins that are related serologically and most likely also structurally with WGA. This is obviously a protein family. The lectins of leguminoses are all metalloproteins. Whether this also points to a common phylogenetic family remains unclear.

What importance do lectins have for the plant? It is well-known that sugar-residues can be found at cell surfaces and that single cell types and their different developmental stages differ in their carbohydrate pattern.

It was therefore not far-fetched to find out whether lectins take part in recognition processes. The role of certain lectins was proven:

Many lectins are localized within the cell where they may be present in considerable amounts but their role is largely unknown. They react with storage proteins and their affinity to the species own storage proteins is far greater than that to foreign proteins. This led to the assumption (S.-K. BASHA and R. M. ROBERTS, 1981) that they complex proteins and thus transform them into a compact, insoluble state that makes them easier to store than a soluble state would.

Many lectins are toxic and may therefore protect the plant from being eaten away. The lectin of pea (Phaseolus vulgaris), for example, is deadly for the bug Calosobruchus maculatus. The agglutinin of Ricinus communis (RCA) contains a component that is highly poisonous for animals and man: ricin. Animal experiments showed that some lectins support cell division at low concentrations (mitogenes). If this is of importance for the plant remains unclear.