Specific interaction between anionic phospholipids and milk bovine component PP3 and its 119–135 C-terminal fragment

The behaviour of component PP3, a bovine milk protein with emulsifying properties, was investigated at the air–water interface and in a lipidic environment using the monolayer technique. The amphipathic 119–135 C-terminal fragment of PP3 was also tested since we proposed, on the basis of structural analysis, that this region is probably responsible for the surface-active properties of the protein. This hypothesis was confirmed by the tensiometric measurements at the air–water interface in which the addition of the C-terminal peptide increased the surface pressure with a similar amplitude as the whole protein. Penetration measurements into lipidic monolayers indicated that the insertion of component PP3 and its C-terminal peptide is the highest with anionic phospholipids in a gel state. Moreover, the electrostatic attractions provided by anionic phospholipids are essential for the peptide interaction. We also showed by Fourier transform infrared spectra study, that the peptide displays a β-type conformational state in aqueous solution and in the presence of solvant or anionic phospholipid (DPPG). In contrast, the protein adopts in aqueous solution an helical conformation which remains the dominant conformational state in the presence of DPPG although the apparition of β-structure is detected.