NLP proteins are a protein family that occurs in phytopathogenic bacteria, fungi and oomycetes. They act as elicitors of the plant immune system and have a cytotoxic effect on plant cells. The first steps of the interaction of NLPs with the plasma membrane are known, they bind to the sphingolipid glycosyl inositol phosphorylceramide, a common plant membrane lipid. Recently, the mechanism of action of NLPs has been elucidated: they act on the redistribution of the sphingolipid receptor, leading to small pores in the membrane which cause leakage and necrosis. In the present thesis we wanted to define some properties of NLPPya cysteine mutants from oomycete Pythium aphanidermatum. Cysteine mutants are located at the C-terminal end on the α-helix, for the purpose of binding the fluorescent probe to the mutated thiol group. The fluorescent probe IANBD amide helps us to understand the environmental changes of the protein when an interaction with a model membrane containing GIPC occur. By testing the mutant infiltration into tobacco leaves, measuring the release of ions from the infiltrated area, and the GIPC multilamellar vesicle cosedimentation test, we examined the conservation of mutant cytotoxicity in comparison with the wild-type. The introduction of these mutations do not affect protein folding, as mutants have a similar melting point as the wild-type. Fluorimetric measurements indicate the incorporation of the fluorescent probe into the hydrophobic range, which means that the C-terminal part interacts with the membrane
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