Hordothionins inhibit protein synthesis at the level of initiation in the wheat-germ system

The inhibitory effect of pure alpha and beta hordothionins on protein synthesis directed by pea mRNA has been studied in the wheat-germ translation system. It is demonstrated that a component of the wheat germ counteracts the thionin effect. Formation of polysomes in vitro in the presence of thionin was inhibited to the same extent as the total translation system while run-off translation of isolated polysomes from pea plants was not affected by thionin. These data are consistent with an effect of thionin on the initiation reaction. Analyses of the formation of initiation complexes in the presence and absence of mRNA support this view and show that thionin interferes with the formation of the 43S complex. In accordance with this observation and in contrast to earlier studies no evidence has been obtained for a direct interaction between mRNAs and thionins. The analysis of the translation products also gave no indication for preferential translation of individual mRNAs by the thionin-inhibited translation system. Compared to translation in vitro, exposure of barley protoplasts to thionins showed a less dramatic effect on protein synthesis as measured by incorporation of [35S]methionine into proteins. These data are discussed with respect to the effects of thionins on the plasma membranes as shown previously with animal cell cultures. It is concluded that at least in barley such effects would need higher concentrations of thionins than are required for the inhibition of protein synthesis.