A new gene encoding the ribosome-inactivating protein from mistletoe extracts.

Extracts from mistletoe (Viscum album L.) contain three main toxic proteins--the lectins MLI (also known as viscumin), MLII and MLIII. A catalytic subunit of the mistletoe plant toxic lectin MLIII has been cloned and expressed in Escherichia coli cells. The structure and immunochemical properties of recombinant MLIII A-subunit were investigated using a panel of monoclonal antibodies against ML-toxins. Ribosome-inactivating activity of the recombinant MLIII A-subunit was determined in a cell-free system exhibiting inhibition of endogenous protein synthesis. The comparative analysis of nucleotide and deduced amino acid sequences of the cloned MLIII A and the native MLI A-subunits was performed, revealing the main differences in the primary structure of these proteins. Antigenicity analysis of the MLIII A-subunit has revealed a new epitope D179-E184 that is not present in viscumin. The role of toxic lectins with respect to the immunological properties of mistletoe extracts is discussed.