Polymorphic regulation of membrane phospholipid composition in Escherichia coli

To investigate whether the phase preference of Escherichia coli membrane lipids is regulated by adjustment of the ratio of bilayer to non-bilayer lipids, the lipid biosynthetic mutant AD93 was used. This strain lacks the ability to synthesize phosphatidylethanolamine, a non-bilayer lipid which normally accounts for 70-80% of the phospholipids in the E. coli membrane. The lack of phosphatidylethanolamine is compensated by a large increase in the levels of phosphatidylglycerol and cardiolipin. This strain has an absolute requirement for high concentrations of divalent cations. Since divalent cations are known to induce HII phase formation in cardiolipin model systems, this suggests that cardiolipin in combination with divalent cations replaces phosphatidylethanolamine in the membrane and that it is the non-bilayer forming property of the latter which is important for membrane functioning. The growth of the mutant strain was found to be dependent on the type and concentration of divalent cations present in the growth medium. In media supplemented with MgCl2 and CaCl2, growth was maximal at concentrations of 30-50 mM. In the presence of SrCl2 a growth optimum was observed at 15 mM. The cells did not grow in the presence of BaCl2 or at high concentrations of SrCl2. Furthermore, this strain was found to adapt the lipid composition of the membrane in reaction to the type of cation present during growth. The phase behavior of dispersions of mutant and parental E. coli derived lipids was studied under a variety of conditions, using 31P NMR. In the presence of the growth-promoting cations at concentrations where there is maximal growth, a bilayer to non-bilayer transition was observed in the lipid dispersions in a narrow temperature range, approximately 10 degrees C above the growth temperature, whereas at concentrations where there is no growth, a bilayer structure was observed at all temperatures tested. This suggests a polymorphic regulation of membrane lipid composition in order to maintain a propensity toward type II structure formation in the membrane.