Isolation and characterization of cDNAs encoding viscotoxins of mistletoe (Viscum album)

Viscotoxins have been isolated from leaf homogenates of European mistletoe (Viscum album L.) and purified to apparent homogeneity. Antisera raised against these polypeptides were used to screen a cDNA expression library in lambda gt11. Two positive clones have been isolated, one encoding a full-length preprotein of viscotoxin A3 and the other encoding the precursor of viscotoxin B. Besides the viscotoxin domain the precursor contained a signal sequence and an acidic polypeptide domain. Similar higher molecular mass precursor proteins have been described for thionins of leaves and seeds of barley. Even though the acidic part of the viscotoxin precursor is much shorter than the corresponding domain of the precursors of the leaf and seed thionins of barley, both the negative charge and the number and the relative position of cysteine residues have been conserved within the acidic domain. This result is consistent with our proposal that the acidic domain of the thionin precursor may play an important role in keeping the thionin inactive within the plant cell.