Restoration of lectin activity to an inactive abrin B chain by substitution and mutation of the 2 gamma subdomain

Ebulin-RP, a novel member of the Ebulin gene family with low cytotoxicity as a result of deficient sugar binding domains

BACKGROUND

Sambucus ebulus is a rich source of ribosome-inactivating proteins (RIPs) and RIP-related lectins generated from multiple genes. These proteins differ in their structure, enzymatic activity and sugar binding specificity.

METHODS

We have purified and characterized ebulin-RP from S. ebulus leaves and determined the amino acid sequence by cDNA cloning. Cytotoxicity was studied in a variety of cancer cells and a comparative study of the ability of ebulin-RP to bind sugars using "in vitro" and "in silico" approaches was performed.

RESULTS

Ebulin-RP is a novel heterodimeric type 2 RIP present in S. ebulus leaves together with the type 2 RIP ebulin l, which displayed rRNA N-glycosidase activity but unlike ebulin l, lacked functional sugar binding domains. As a consequence of changes in its B-chain, ebulin-RP displayed lower cytotoxicity than ebulin l towards cancer cells and induced apoptosis as the predominant pattern of cell death.

CONCLUSIONS

Ebulin-RP is a novel member of the ebulin gene family with low cytotoxicity as a result of deficient sugar binding domains. Type 2 RIP genes from Sambucus have evolved to render proteins with different sugar affinities that may be related to different biological activities and could result in an advantage for the plant.

GENERAL SIGNIFICANCE

The ebulin family of RIPs and lectins can serve as a good model for studying the evolutionary process which may have occurred in RIPs. The lack of cytotoxicity of ebulin-RP makes it a good candidate as a toxic moiety in the construction of immunotoxins and conjugates directed against specific targets.

Transient occurrence of an ebulin-related D-galactose-lectin in shoots of Sambucus ebulus L

Young shoots of Sambucus ebulus L. contain a monomeric d-galactose binding lectin (SELlm), which disappears upon shoot development, and was previously undetected since it co-purifies with the non-toxic type 2 ribosome-inactivating protein ebulin l and the dimeric lectin SELld. Molecular cloning of cDNA coding for SELlm and mass spectrometry analysis revealed a protein with a molecular mass of 34,239 Da, which displays 80%, 77% and 45% of amino acid sequence identity with the ebulin l-B chain, SELld and ricin-B chain, respectively. Furthermore, the cloned precursor, with respect to the ebulin l precursor is truncated and contains the signal peptide, a piece of the A chain, a piece of the connecting peptide and the B chain. Further processing yields the lectin protein, which contains only the B chain. Despite the fact that SELlm displays the same d-galactose-binding sites than ricin, it was found that the lectin has different binding properties to D-galactose-containing matrix than ricin. Notably, and unlike ricin, the binding of SELlm and other Sambucus lectins to such matrix was maximum in range of 0-10 degrees C and abolished at 20 degrees C.

Insights into the Role of the Aromatic Residue in Galactose-Binding Sites: MP2/6-311G++** Study on Galactose− and Glucose−Aromatic Residue Analogue Complexes

The presence of an aromatic residue (Trp, Phe, Tyr) facing the nonpolar face of galactose is a common feature of galactose-specific lectins. The interactions such as those between the C-H groups of galactose and the pi-electron cloud of aromatic residues have been characterized as weak hydrogen bonds between soft acids and soft bases, largely governed by dispersive and charge transfer interactions. An analysis of the binding sites of several galactose-specific lectins revealed that the spatial position-orientation of galactose relative to the binding site aromatic residue varies substantially. The effect of variations in position-orientations of galactose on the interaction energies of galactose-aromatic residue complexes has not been determined so far. In view of this, MP2/6-311G++** calculations were performed on galactose- and glucose-aromatic residue analogue complexes in eight position-orientations. The results show that the strength of the C-H...pi interactions in galactose-aromatic residue complexes is comparable to that of a hydrogen bond. Rather than the type of aromatic residue, the position-orientation of the saccharide appears to be more critical in determining the strength of their interactions. Earlier studies have found the binding site aromatic residue to be critical, but its role was not clear. This study shows that the aromatic residue is important for discriminating galactose from glucose, in addition to its contribution to binding energy.

Characterization of the sugar-binding specificity of the toxic lectins isolated from Abrus pulchellus seeds

The sugar-binding specificity of the toxic lectins from Abrus pulchellus seeds was investigated by combination of affinity chromatography of glycopeptides and oligosaccharides of well-defined structures on a lectin-Sepharose column and measurement of the kinetic interactions in real time towards immobilized glycoproteins. The lectins showed strong affinity for a series of bi- and triantennary N-acetyllactosamine type glycans. The related asialo-oligosaccharides interact more strongly with the lectins. The best recognized structures were asialo-glycopeptides from fetuin. Accordingly, the kinetic interaction with immobilized asialofetuin was by far the most pronounced. Human and bovine lactotransferrins and human serotransferrin interacted to a lesser extent. The interaction with asialofetuin was inhibited by galactose in a dose dependent manner. Lactose, N-acetyllactosamine and lacto-N-biose exhibited similar degree of inhibition while N-acetylgalactosamine was a poor inhibitor. These results suggested that the carbohydrate-binding site of the Abrus pulchellus lectins was specific for galactose and possess a remarkable affinity for the sequences lactose [beta-D-Gal-(1-->4)-D-Glc], N-acetyllactosamine [beta-D-Gal-(1-->4)-D-GlcNAc] and lacto-N-biose [beta-D-Gal-(1-->3)-D-GlcNAc].