Lectins are important tools in histochemistry (Brooks et al., 1997), structural analysis of oligosaccharide chains, detection of alteration of glycosylation, quantification of glycoconjugates, affinity purification of glycoconjugates, cytofluorimetric studies of cells, and cell separation (Rhodes and Milton, 1998). Some lectins are the mitogenic, while others are cytotoxic, or influence neuronal trafficking, pathogen-host interaction, and lectins are also carriers for drug delivery (Haltner et al., 1997). Moreover, plant lectins, widely distributed in food and feed, play an important role in nutrition and health science (Brooks et al., 1997; Haltner et al., 1997; Pusztai and Bardocz, 1991).

Within the large group of plant lectins, a number of lectins from non-related species belong to a family of so-called ribosome inactivating proteins (RIP). The most prominent member of this group, ricin, was recognised over hundred years ago as a very toxic and haemagglutinating protein from the Castor bean. Besides ricin and the mistletoe lectins (ML), volkensin, abrin, modeccin and ebulin II are established members of the RIP lectin family (for review see Van Damme et al., 1998a, b; Pusztai and Bardocz, 1991, 1995; Rhodes and Milton, 1998; Stirpe and Batelli, 1990; Barbieri et al., 1993).

Commonly, the RIPs are divided in two groups: type 1 RIPs are single-chain proteins which do not bind to carbohydrates, and type 2 RIPs which consist of a lectin B subunit and a toxophoric A chain which is a RNA N-glycosidase that affects tRNA, and thus block protein synthesis (Stirpe and Batelli, 1990; Barbieri et al., 1993) (Table 1). Type 1 RIPs and the A chains of type 2 RIPs show nearly the same enzymatic activity especially in cell-free systems (Barbieri et al., 1993).

The type 2 RIPs have strongly related molecular subunit architectures. The toxophoric A chain is connected with the sugar-binding B chain via a disulfid bridge. X-ray and sequence studies of MLs and other members of the toxic lectin family demonstrate a high degree of structural identity but quite different biological activities. Whereas more than 30 type 1 RIPs have been described, only a few type 2 RIPs are characterised (Table 1). Further examples of type 2 RIPs are described by Van Damme et al. (1998a,b). In this regard, the isolation of lectins with the same molecular architecture like ricin from Viscum album was also a big surprise in the seventies (Luther, 1976; Franz et al., 1977).

There is some evidence that type 2 RIPs evolved from type 1 and both from a common unknown ancestor (Van Damm et al., 1998b). Further, RIPs are suggested to be products of the fusion of two different genes—the gene for the lectin B-chain and the gene for the N-glycosidase A-chain (Stirpe and Batelli, 1990; Barbieri et al., 1993). It is not yet clear whether ML I, ML II and ML III, which have approximately similar molecular weights (about 60 kDa) and high degree of homology as compared to ricin, are products of one gene and distinctions between them are determined by post-translational changes, or they are products of several genes (Pusztai and Bardocz, 1995; Barbieri et al., 1993). Recent investigations by Western Blot analysis revealed, that eluted antibodies specific for ML I A or B chain also recognised A and B chain of ML II and B-chain of ML III, suggesting homologies of epitopes of the three ML (Stein et al., 1999d). Only the unglycosylated ML III A-chain was not detectable with these antibodies. The

European mistletoe belongs to the few plant families containing more than one class of lectins, the gal/galNAc-recognising ML I, ML II and ML III of the RIP 2 type, and one lectin with hevein basic structure, the glucNAc-oligomere binding lectin VisalbCBL.

Table 1 Characteristics of representative type 2 RIPs and schematic description of RIP structures.

type 2 RIP

Type 1 RIP

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