Stabilizing effect of an S-layer on liposomes towards thermal or mechanical stress

Isolated subunits of the crystalline cell surface layer (S-layer) protein o f Bacillus stearothermophilus PV72/p2 were recrystallized on positively cha rged unilamellar liposomes. Liposomes were composed of dipalmitoylphosphati dylcholine (DPPC), cholesterol and hexadecylamine (HDA) in a molar ratio of 10:5:4 and they were prepared by the dehydration-rehydration method follow ed by an extrusion procedure. The S-layer protein to DPPC ratio was 5.7 nmo l/mu mol which approximately corresponds to the theoretical Value estimated by using the areas occupied by the S-layer lattice and the lipid membrane. Coating of the positively charged liposomes with S-layer protein resulted in inversion of the zeta-potential from +29.1 mV to -27.1 mV. Covalent cros slinking of the recrystallized S-layer protein was achieved with glutaralde hyde. Chemical analysis revealed that almost all amino groups (>95%) from H DA in the liposomal membrane were involved in the reaction. To study the in fluence of an S-layer lattice on the stability of the liposomes, the hydrop hilic marker carboxyfluoresceine (CF) was encapsulated and its release was determined for plain and S-layer-coated liposomes in the course of mechanic al and thermal challenges. In comparison to plain liposomes, S-layer-coated liposomes released only half the amount of enclosed CF upon exposure to sh ear forces or ultrasonication as mechanical stress factors. Furthermore, te mperature shifts from 25 degrees C to 55 degrees C and vice versa induced c onsiderably less CF release from S-layer-coated than from plain liposomes. A similar stabilizing effect of the S-layer lattice was observed after glut araldehyde treatment of plain and S-layer-coated liposomes. (C) 1999 Elsevi er Science B.V. All rights reserved.