Interaction between β-Purothionin and Dimyristoylphosphatidylglycerol:A 31P-NMR and Infrared Spectroscopic Study

Abstract

The interaction of β-purothionin, a small basic and antimicrobial protein from the endosperm of wheat seeds, with multilamellar vesicles of dimyristoylphosphatidylglycerol (DMPG) was investigated by 31P solid-state NMR and infrared spectroscopy. NMR was used to study the organization and dynamics of DMPG in the absence and presence of β-purothionin. The results indicate that β-purothionin does not induce the formation of nonlamellar phases in DMPG. Two-dimensional exchange spectroscopy shows that β-purothionin decreases the lateral diffusion of DMPG in the fluid phase. Infrared spectroscopy was used to investigate the perturbations, induced by β-purothionin, of the polar and nonpolar regions of the phospholipid bilayers. At low concentration of β-purothionin, the temperature of the gel-to-fluid phase transition of DMPG increases from 24°C to ∼33°C, in agreement with the formation of electrostatic interactions between the cationic protein and the anionic phospholipid. At higher protein concentration, the lipid transition is slightly shifted toward lower temperature and a second transition is observed below 20°C, suggesting an insertion of the protein in the hydrophobic core of the lipid bilayer. The results also suggest that the presence of β-purothionin significantly modifies the lipid packing at the surface of the bilayer to increase the accessibility of water molecules in the interfacial region. Finally, orientation measurements indicate that the α-helices and the β-sheet of β-purothionin have tilt angles of ∼60° and 30°, respectively, relative to the normal of the ATR crystal.