A complex fruit-specific type-2 ribosome-inactivating protein from elderberry (Sambucus nigra) is correctly processed and assembled in transgenic tobacco plants

Abstract

Fruits of elderberry (Sambucus nigra) express small quantities of a type-2 ribosome-inactivating protein with an exclusive specificity towards the NeuAc(α2,6)Gal/GalNAc disaccharide and a unique molecular structure typified by the occurrence of a disulfide bridge between the B-chains of two adjacent protomers. A cDNA clone encoding this so-called Sambucus nigra fruit specific agglutinin I (SNA-If) was isolated and expressed in tobacco (Samsun NN) under the control of the 35S cauliflower mosaic virus promoter. Characterization of the purified protein indicated that the recombinant SNA-If from tobacco leaves has the same molecular structure and biological activities as native SNA-If from elderberry fruits, demonstrating that transgenic tobacco plants are fully capable of expressing and correctly processing and assembling a type-2 ribosome-inactivating protein with a complex molecular structure. None of the transformants showed a phenotypic effect, indicating that the ectopically expressed SNA-If does not affect the viability of the tobacco cells. Bioassays further demonstrated that none of the transgenic lines exhibited a decreased sensitivity to infection with tobacco mosaic virus suggesting that the elderberry type-2 RIP SNA-If does not act as an antiviral agent in planta.