Membrane interaction of a beta-structure-forming synthetic peptide comprising the 116–139th sequence region of the cytotoxic protein alpha-sarcin

Abstract

alpha-Sarcin is a cytotoxic protein that strongly interacts with acid phospholipid vesicles. This interaction exhibits a hydrophobic component although alpha-sarcin is a highly polar protein. A peptide comprising the amino acid sequence corresponding to the 116-139th segment of the alpha-sarcin cytotoxin has been synthesized by a standard fluoren-9-yl-methoxycarbonyl-based solid phase method. Its primary structure is: (116)-NPGPARVIYTYPNKVFCGIIAHTK-(139). Two beta-strands have been predicted in this region of alpha-sarcin, where the less polar stretches of the protein are found. The synthetic peptide interacts with negatively charged large unilamellar vesicles of either natural or synthetic phospholipids. An apparent fragmentation of the vesicles is produced by the peptide based on electron microscopy studies. The peptide promotes leakage of the intravesicular aqueous contents and lipid mixing of bilayers. The packing of the phospholipid molecules is greatly perturbed by the peptide, as deduced from the drastic changes induced by the peptide in cooperative properties associated with the phase transition of the bilayers. At saturating peptide/phospholipid ratios, the phase transition of dimyristoylphosphatidylglycerol vesicles is abolished. All of these effects are saturated at about 0.3 peptide/lipid molar ratio. The peptide adopts a mostly random structure in aqueous solution. A conformation composed of a high proportion of antiparallel beta-sheet is induced as a consequence of the interaction with the phospholipid vesicles in opposition to trifluoroethanol that promotes alpha-helical peptide structures, as deduced from circular dichroism measurements. The obtained results are discussed in terms of the potential involvement of the region comprising residues 116-139 of alpha-sarcin in the hydrophobic interactions of this cytotoxic protein with membranes.