Model system for heat-induced translocation of cytoplasmic β-galactosidase across phospholipid bilayer membrane

The possibility of the translocation of the enzyme across the phospholipid bilayer membrane was investigated by using the liposomes prepared by 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine (POPC) in which β-galactosidase (β-gal) was entrapped. Exposing the POPC liposomes entrapping β-gal inside to heat treatment (40-50°C, 1-60 min) was found to induce its translocation across the liposome membrane. The translocated activity of β-gal from inner to outer aqueous phase of liposomes indicated the maximal value when the liposomes entrapping β-gal were heated at 45°C for 30 min. The gel permeation profiles of the liposomes before and after heat treatment (45°C, 30 min) also supported the translocation of β-gal across the liposome membrane. The membrane fluidity of liposomes was found to be increased with increasing temperature, so that the hydrophobicity of liposome membrane was also increased. The local hydrophobicity of β-gal was maximized at the temperature of 40-50 °C. The mechanisms of β-gal translocation have been suggested to be triggered by the enhancement of hydrophobic interaction between the liposome surface and β-gal molecules. Finally, a minimal scheme of possible mechanism on the heat-induced translocation of β-gal has been presented on the basis of the hydrophobic interaction between the liposome and the proteins. The experimental data on the heat-induced translocation of β-gal were well corresponding to those from model calculation.