Mutational analysis of the carbohydrate-binding activity of the NeuAc(α-2,6)Gal/GalNAc-specific type 2 ribosome-inactivating protein from elderberry (Sambucus nigra) fruits

Sambucus nigra agglutinin I (SNA-I) is a type 2 ribosome-inactivating protein. Site-directed mutagenesis was used to mimic the conversion of the highly active B-chain of fruit-specific SNA (SNA-If) into the completely inactive B-chain of the closely related and naturally occurring loss-of-activity mutant called S. nigra agglutinin lectin-related protein. In the first mutant SNA-If-M1 the high-affinity site 2 of SNA-If was disrupted by replacing the presumed critical residue Asp²³¹ with Glu²³¹. In the double mutant SNA-If-M2, site 1 of SNA-If-M1 was also disrupted by substituting the presumed critical residue Asn⁴⁸ with Ser⁴⁸. The parent type 2 ribosome-inactivating protein and both mutants were expressed in Nicotiana tabacum Samsun NN and the recombinant proteins were purified and analysed. Recombinant SNA-If agglutinated rabbit erythrocytes equally well as SNA-If, but both mutants were completely inactive in this test. Binding assays to immobilized galactose and fetuin revealed that the mutation Asp²³¹→Glu²³¹ reduces the affinity of the B-chain for galactose and fetuin by more than 50%. Furthermore, the introduction of the second mutation Asn⁴⁸→Ser⁴⁸ reduces the binding activity to less than 20% of the original activity.