Probing the stability of S-layer-supported planar lipid membranes

Abstract

Isolated protein subunits of the crystalline bacterial cell surface layer (S-layer) of Bacillus coagulans E38-66 have been recrystallized on one side of planar black lipid membranes (BLMs) and their influence on the electrical properties, rupture kinetics and mechanical stability of the BLM was investigated. The effect on the boundary potential, the capacitance or the conductance of the membrane was negligible whereas the mechanical properties were considerably changed. The mechanical stability was characterized by applying voltage pulses or ramps to induce irreversible rupture. The amplitude of the voltage pulse leading to rupture allows conclusions on the ability of membranes to resist external forces. Surprisingly, these amplitudes were significantly lower for composite S-layer/lipid membranes compared to undecorated BLMs. In contrast, the delay time between the voltage pulse and the appearance of the initial defect was found to be drastically longer for the S-layer-supported lipid bilayer. Furthermore, the kinetics of the rupture process was recorded. Undecorated membranes show a fast linear increase of the pore conductance in time, indicating an inertia-limited defect growth. The attachment of an S-layer causes a slow exponential increase in the conductance during rupture, indicating a viscosity-determined widening of the pore. In addition, the mechanical properties on a longer time scale were investigated by applying a hydrostatic pressure across the BLMs. This causes the BLM to bulge, as monitored by an increase in capacitance. Compared to undecorated BLMs, a significantly higher pressure gradient has to be applied on the S-layer face of the composite BLMs to observe any change in capacitance.

Are S-layers exoskeletons? The basic function of protein surface layers revisited

“…Similar to the expected situation in Grampositive bacteria, the S-layer has an impact on the physico-chemical characteristics of the underlying cell envelope component. Although experimental data on natural systems are rare (Gerbl-Rieger et al, 1992), measurements with artificial S-layer-membrane (phospholipid) assemblies indicate possible effects (Diederich et al, 1996;Hirn et al, 1999;Küpcü et al, 1998;Mader et al, 1999;Schuster et al, 1998Schuster et al, , 1999Schuster and Sleytr, 2002;Sleytr et al, 2001). Here, S-layers from Gram-positive bacteria were recrystallised on homogeneous lipid monolayers or bilayers.…”

Mechanism of osmoprotection by archaeal S-layers: A theoretical study

“…The glycosylated surface proteins of H. salinarum and H. volcanii are modified by lipids, which presumably contribute to membrane anchoring in addition (Kikuchi et al, 1999;Konrad and Eichler, 2002). The mere non-covalent interaction of archaeal S-layers with the head groups of membrane lipids is still hypothetical but could contribute to membrane stability in Euryarchaeota (Schuster et al, 1999;Schuster and Sleytr, 2002;Engelhardt, 2007). The mutual effects between the S-layer and other components of the cell envelope remain to be elucidated.…”

How did cells get their size?

“…When multivalent ions, DNA, and charged peptides (e.g., bacterial S-layer proteins, poly-l-lysine) bind to bilayers, they can mechanically stabilize them via induced lipid ordering, rigidification, and decreased transversal compliance effects (Sackmann, 1994;Dan, 1996;Murray et al, 1999;Schuster et al, 1999;Norris et al, 1999;Ermakov et al, 2001). Say RPP was a polyionic peptide that bound to counterionic membrane lipids, enhancing liposome stability, and simultaneously (to make this a best-case scenario) increasing the rate of replicase self-copying and/or replicase-induced peptide formation.…”