Abstract
The glycosyl hydrolase 18 (GH18) family consists of active chitinases as well as chitinase like lectins/proteins (CLPs). The CLPs share significant sequence and structural similarities with active chitinases, however, do not display chitinase activity. Some of these proteins are reported to have specific functions and carbohydrate binding property. In the present study, we report a novel chitinase like lectin (TCLL) from Tamarindus indica. The crystal structures of native TCLL and its complex with N-acetyl glucosamine were determined. Similar to the other CLPs of the GH18 members, TCLL lacks chitinase activity due to mutations of key active site residues. Comparison of TCLL with chitinases and other chitin binding CLPs shows that TCLL has substitution of some chitin binding site residues and more open binding cleft due to major differences in the loop region. Interestingly, the biochemical studies suggest that TCLL is an N-acetyl glucosamine specific chi-lectin, which is further confirmed by the complex structure of TCLL with N-acetyl glucosamine complex. TCLL has two distinct N-acetyl glucosamine binding sites S1 and S2 that contain similar polar residues, although interaction pattern with N-acetyl glucosamine varies extensively among them. Moreover, TCLL structure depicts that how plants utilize existing structural scaffolds ingenuously to attain new functions. To date, this is the first structural investigation of a chi-lectin from plants that explore novel carbohydrate binding sites other than chitin binding groove observed in GH18 family members. Consequently, TCLL structure confers evidence for evolutionary link of lectins with chitinases.
Highlights
The carbohydrate binding proteins from plants are usually called lectins, agglutinins, or hemagglutinins
tamarind chitinase like lectin (TCLL) was purified from Tamarindus indica seeds by affinity chromatography and ion exchange chromatography
Gel filtration chromatographic analysis in combination with SDS-PAGE (Figure 1A) showed that it is a monomeric protein. This was further confirmed by intact molecular mass determination of native TCLL by MALDI TOF, which showed the major species at a molecular mass of 33440 Da along with the other minor glycoforms (Figure 1B)
Summary
The carbohydrate binding proteins from plants are usually called lectins, agglutinins, or hemagglutinins. Plant lectins are heterogeneous and highly diverse class of non-immune origin (glyco) proteins because of their carbohydrate-binding specificity, differences in molecular structure and biochemical properties [1]. Lectins have at least one non-catalytic domain, which reversibly bind to sugars or glycans of glycoproteins and glycolipids and do not modify the structure of carbohydrates [1,2]. Lectins show extensive structural diversity with the mutual orientations of the subunits in the tertiary folds giving rise to a variety of quaternary structures These quaternary structures produce higher order sugar specificities, oligomerization is not necessary for ligand recognition since single subunit of plant lectins were found to be able to bind carbohydrate [5]. There is no precise description for the biological function of the plant lectins because of their diverse classes and carbohydrate specificities. Some functional characteristics of particular lectins reported in literature include antifungal [7], insecticidal [8], antiviral [9], antiproliferative, apoptosis-inducing [2] and symbiosis mediating between nitrogen fixing microorganisms and legume plants [3]
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