THIONINS - PROPERTIES, POSSIBLE BIOLOGICAL ROLES AND MECHANISMS OF ACTION

Thionins are low-molecular-weight proteins (M(r) ca. 5000) occurring i n seeds, stems, roots and leaves of a number of plant species. The dif ferent members of this family of plant proteins show both sequence and structural homology, and are toxic to bacteria, fungi, yeasts and var ious naked cells in vitro. Toxicity requires an electrostatic interact ion of the positively charged thionin with the negatively charged phos pholipids making up the membrane, followed by either pore formation or a specific interaction with a certain lipid domain. This domain might be composed of phosphoinositides, which mediate transduction of envir onmental signals in eukaryotes. Their in vitro toxicity to plant patho genic bacteria and fungi could reflect a direct role in plant defence, although, in view of the many divergent activities displayed by thion ins both in vitro and in vivo, a biological role other than inhibition of microbial growth is equally plausible.