Interaction of bacterial surface layer proteins with lipid membranes: synergysm between surface charge density and chain packing.

S-layer proteins from Lactobacillus kefir and Lactobacillus brevis are able to adsorb on the surface of positively charged liposomes composed by Soybean lecithin, cholesterol and stearylamine. The different K values for S-layer proteins isolated from L. kefir and L. brevis (4.22 x 10(-3) and 2.45 x 10(2) microM(-1) respectively) indicates that the affinity of the glycosylated protein isolated from L. kefir is higher than the non-glycosylated one. The attachment of S-layer proteins counteracts the electrostatic charge repulsion between stearylamine molecules in the membrane surface, producing an increase in the rigidity in the acyl chains as measured by DPH anisotropy. Laurdan generalized polarization (GP) shows that glycosylated causes a GP increase, attributed to a lowering in water penetration into the head groups of membrane phospholipids, with charge density reduction, while the non-glycosylated does not affect it. The octadecyl-rhodamine results indicate that S-layer coated liposomes do not show spontaneous dequenching in comparison with control liposomes without S-layer proteins, suggesting that S-layer protein avoid spontaneous liposomal fusion. It is concluded that the increase in stability of liposomes coated with S-layers proteins is due to the higher rigidity induced by the S-layer attachment by electrostatic forces.