Mistletoe Lectins The Hevein Domain Lectin VisalbCBL

Beside the type 2 RIPs from Viscum album L., Peumans et al. (1996) isolated a structurally non-related chitin-binding lectin from European mistletoe, designated VisalbCBL (Peumans et al., 1996, 1998). This lectin is a homodimer of two identical subunits of 10.8 kDa. VisalbCBL shares a high degree of sequence similarity at its N-terminus with the hevein domain, and also the amino acid composition is similar to that of other chitin-binding hololectins.

Following the schematic representation of molecular structures (Figure 6), VisalbCBL consists of two subunit-forming hevein repeats forming a dimer with four chitin-binding sites (Peumans et al., 1998). Basic properties of VisalbCBL are described in Table 4.

Figure 6 Structural classification of lectins (according to Peumans et al., 1998) and positions of the four mistletoe lectins, ML I, ML II, ML III and dimeric VisalbCBL (upper box). The lower box shows a possibility to classify lectins via their primary monosaccharide target. Class I contains fucose-recognising lectins, class II contains those recognising galactose/galNAc (such as ML, ricin, jacalin, galectins), and class III those binding to glucose/mannose/glucNAc (such as concanavalin A and VisalbCBL). Class IV is not yet occupied. * ML I forms in most cases a dimer.

Figure 6 Structural classification of lectins (according to Peumans et al., 1998) and positions of the four mistletoe lectins, ML I, ML II, ML III and dimeric VisalbCBL (upper box). The lower box shows a possibility to classify lectins via their primary monosaccharide target. Class I contains fucose-recognising lectins, class II contains those recognising galactose/galNAc (such as ML, ricin, jacalin, galectins), and class III those binding to glucose/mannose/glucNAc (such as concanavalin A and VisalbCBL). Class IV is not yet occupied. * ML I forms in most cases a dimer.

A hypothetical model of the molecular evolution of chitin-binding lectins including VisalbCBL is described by Van Damme et al. (1998b). The physiological role of VisalbCBL remains unclear. One may suggest that this lectin is part of the plants defence system against bacteria, fungi or insects as already described for other chitin-binding lectins (Van Damme et al., 1998a, b). The direct and indirect contribution of

Table 4 Basic properties and data of the chitin-binding lectin VisalbCBL.

structure specificity titer glycosylation cytotoxicity antibody cross reactivity occurrence green tissues, up to 0.001% in deciduous trees and < 0,00001% in pine tree

(P-P)2, molecular mass of protomer P: 11 kD

1:4 (1 mg/ml, pooled human erythrocytes)

no cross reactivity with polyclonal and monoclonal antibodies against ML I, ML II and ML III

VisalbCBL which is detected in considerable amounts in most of commercially available mistletoe preparations (Pfüller et al., unpublished results) to efficacy of these extracts is not yet investigated.

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