Conformational studies of a synthetic peptide from the putative lipid-binding domain of bovine milk component PP3

In bovine milk, a glycosylated phosphoprotein, component PP3, is known for its remarkable emulsifying properties and its capability to inhibit lipolyt ic activities. The determination of its primary structure is not sufficient to explain these properties. Secondary structure predictions of component PP3 and of its homologous proteins were achieved using a combination of mul tiple predictive methods. Based on this study, the f 119-135 region of comp onent PP3 was proposed to be likely to adopt an amphipathic helical conform ation, which is a lipid-binding motif. The conformation of the synthetic pe ptide corresponding to the C-terminal f 119-135 part of bovine component PP 3 was analyzed by circular dichroism experiments using various media. The c ircular dichroism data indicated that the peptide was able to form an amphi pathic alpha-helix structure in trifluoroethanol as well as in the presence of sodium dodecyl sulfate or acidic and neutral lipids, but not in water. Moreover, the conformation of this peptide is solvent dependent because it was found to adopt a beta-sheet structure for low concentrations of sodium dodecyl sulfate or a low molar ratio of acidic lipid to peptide. Tensiometr ic measurements showed that the amphipathic C-terminal region of component PP3 is highly tensioactive and, thus, must be responsible for the particula r behavior of the protein in emulsions.