Thionins: Plant Peptides that Modify Membrane Permeability in Cultured Mammalian Cells

Abstract

Thionins, which are toxic high-S polypeptides present in the endosperm of wheat [Triticum aestivum] and related species, prevent growth and inhibit macromolecular synthesis in cultured mammalian cells. Baby hamster kidney BHK-21 (BHK) cells were markedly more sensitive to thionins than the other cell lines tested (monkey kidney CV1, mouse fibroblast L, human cervical carcinoma HeLa). A thionin concentration of 5 .mu.g/ml (1 .mu.M) completely blocked translation in BHK cells. Omission of both Ca and Mg ions fromthe medium strongly enhanced the inhibitory effects of thionins (BHK cells, 80% inhibition, 0.5 .mu.g/ml). Thionins may act at the membrane level. Both the 86Rb+ content and the nucleotide pool of BHK cells were drastically decreased at thionin concentrations that inhibited translation. Thionin concentrations that did not affect macromolecular synthesis in these cells, allowed inhibition of translation by antibiotics, such as hygromycin B, that are not able to cross the cell plasma membrane by themselves. The inhibition of protein, RNA and DNA synthesis in BHK cells may be a consequence of membrane leakiness induced by thionin treatment. Parallelism was found between 86Rb+ leakage and inhibition of protein synthesis by treatment with different genetic variants of thionins (.alpha.1 purothionin, .alpha.2 purothionin, .beta. purothionin from wheat; hordothionin from barley), as well as with the viscotoxins, which are homologous polypeptides from the European mistletoe [Viscum album].