The composition of phospholipid model bacterial membranes determines their endurance to secretory phospholipase A2 attack - The role of cardiolipin

Soil bacteria are decomposer organisms crucial for the biodegradation of organic pollutants, mineralization of dead organic matter and the turnover of biogenic elements. In their environment they are constantly exposed to membrane-lytic enzymes emitted to the soil by other microorganisms competing for the same niche. Therefore, the composition and structure of their membranes is of utmost importance for survival in the harsh environment. Although soil bacteria species can be Gram-negative or Gram-positive and their membranes differ significantly, they are formed by phospholipids belonging mainly to three classes: phosphatidylethanolamines (PE), phosphatidylglycerols (PG) and cardiolipins (CL). The correlation of the membrane phospholipid composition and its susceptibility to secretory membrane-lytic enzymes is widely unknown; thus, to shed light on these phenomena we applied the Langmuir monolayer technique to construct models of soil bacteria membranes differing in the mutual proportion of the main phospholipids. To characterize the systems we studied their elasticity, mesoscopic texture, 2D crystalline structure and discussed the thermodynamics of the interactions between their components. The model membranes were exposed to secretory phospholipase A2. It turned out that in spite of the structural similarities the model membranes differed significantly in their susceptibility to s-PLA2 attack. The membranes devoid of cardiolipin were completely degraded, whereas, these containing cardiolipin were much more resistant to the enzymatic hydrolysis. It also turned out that the sole presence of cardiolipin in the model membrane did not guarantee the membrane durability and that the interplay between cardiolipin and the zwitterionic phosphatidylethanolamine was here of crucial importance.