Antimicrobial peptides that attack biological membranes, and thus cell integrity, are synthesised by animals, plants, fungi and bacteria to fight other organisms, but what determines specificity of interactions between particular peptides and target membranes is little known. Trichoderma is a soil living fungus, which is usually benefical to plants. It is therefore used for biocontrol of plant pathogens. Trichoderma cells excrete antimicrobial peptides that attack and create pores in biological membranes. Surprisingly, we have observed that sterile-grown plant cells are susceptible to permeabilisation by the major Trichoderma viride peptide alamethicin (Matic et al. 2005). However, if plant cells are first treated with cellulase excreted by Trichoderma viride, they become resistant to the antimicrobial peptide from the same fungus. Plasma membranes from resistant cells contained less sterols and phosphatidylserine, which probably affects the channel formation activity of the peptide (Aidemark et al. 2010). As a consequence, the plant becomes resistant to alamethicin, whereas the pathogens may remain sensitive, adding to the biocontrolling function of Trichoderma. Present studies are aimed at elucidating the molecular mechanisms involved in this interaction, and to evidence if this has direct importance for biocontrol and for peptide-membrane interactions in other organisms.
In the cell, metabolic processes take place in a relatively crowded environment including weak protein interactions that are lost upon extraction of the proteins. We introduced alamethicin, a hydrophobic peptide secreted by Trichoderma viride that creates channels in membranes, to study cytosolic, mitochondrial and plasma membrane enzyme complexes inside otherwise intact organelles and cells (Johansson et al. 2004; Matic et al. 2005). Using this technology, mitochondrial electron transport enzymes were found to have other properties than when observed by disruptive methods. Likewise, intact cells have been used for measuring synthesis of cell wall components and study how this synthesis responds to destabilisation of microtubules (Aidemark et al. 2010). Thus, alamethicin permeabilisation allows characterisations of complex soluble and membrane-located enzyme systems in their natural crowded cellular environment under conditions of minimal perturbance of cellular integrity.