Acid phospholipid vesicles produce conformational changes on the antitumour protein alpha-sarcin

The antitumour protein alpha-sarcin interacts at neutral pH with acid phospholipid vesicles promoting their aggregation and fusion. This interaction produces conformational changes on the protein molecule. Circular dichroism and infrared spectroscopy have been used to analyze the secondary structure of the protein molecule. The obtained results show an increased alpha-helix content upon interaction with the lipid vesicles. Detergents and halogenated alcohols have also been considered as an approach to the study of the conformational changes produced upon alpha-sarcin-phospholipid vesicles interaction. SDS treatment as well as trifluoroethanol also increase the helical content of alpha-sarcin. Intrinsic fluorescence of the protein has also been measured for the analysis of the conformational changes produced. The above helicogenic treatments produce a decrease on the structural quenching in alpha-sarcin which is consistent with the existence of hydrophobic protein-lipid interactions. The observed conformational changes are interpreted in terms of a shielding from polar groups caused by the lipids, which promotes intrachain hydrogen bonding and decreased static quenching. The reported conformational changes are discussed in terms of electrostatic and hydrophobic interactions involved in the fusion of lipid vesicles promoted by alpha-sarcin, and potentially in the passage of the protein across membrane cells.