Isolation of Bovine Serum Albumin Fragment P-9 and P-9-Mediated Fusion of Small Unilamellar Vesicles

Abstract

Fusion peptide P-9 was isolated from bovine serum albumin by controlled pepsin degradation in the presence of caprylic acid, followed by Sephadex G-75 gel filtration and ion-exchange chromatography of CM-cellulose. By this procedure, P-9 could be strictly separated from peptic fragment P-Phe, which has a molecular weight close to that of P-9. P-Phe has no fusogenic activity. The addition of P-9 to phosphatidylcholine (PC) liposomes containing cholesterol (Chol) gave rise to an increase of absorption intensity at aroung pH 4.0. The increase of turbidity by P-9 addition did not decrease with increasing pH, indicating P-9-mediated fusion of PC liposomes. The extent of the fusion of PC liposomes was strongly dependent on the PC chain length and temperature. The membrane fluidity close to the polar head groups of the fatty acyl chains of PC affected markedly the extent of P-9-mediated liposome fusion. However, there was no correlation between membrane fluidity near the hydrophobic end of the fatty acyl chains and the extent of liposome fusion. The rate of liposome fusion was dependent on both lipid composition and PC chain length. These results suggest that a contact or an interaction of P-9 with liposomal membrane occurs in the rigid regions. The character of the membrane-water interface region in the liposome controls a triggering effect for P-9-mediated fusion.