Effect of divalent cations on lipid organization of cardiolipin isolated from Escherichia coli strain AH930.

Escherichia coli strain AH930 is a lipid biosynthetic mutant, which is unable to synthesize phosphatidylethanolamine. Instead it produces large amounts of phosphatidylglycerol and cardiolipin and has an absolute requirement for certain divalent cations. Cardiolipin was isolated from this mutant strain and its interaction with divalent cations was studied by various biophysical techniques. Monolayer measurements showed that the cations decrease the molecular surface area of cardiolipin in the order Ca2+ approximately Mg2+ > Sr2+ > Ba2+. 31P-NMR and X-ray diffraction measurements demonstrated a comparable sequence for the ability of the cations to promote HII phase formation in dispersions of the E. coli cardiolipin: Ca2+ and Mg2+ induced HII phase formation at 50 degrees C, Sr2+ at 75 degrees C, while Ba2+ was found to be unable to promote HII phase formation in the temperature range measured. Furthermore, all divalent cations were found to increase the temperature at which the transition to the liquid-crystalline phase takes place, which was below 5 degrees C for the lipid in the absence of divalent cations. In the presence of Sr2+, Mg2+ and Ba2+ and at 25 degrees C two lamellar phases were observed, one corresponding to a liquid-crystalline phase, the other to either a gel or a crystalline phase. In the presence of Ca2+ at 25 degrees C and even at 45 degrees C no evidence for a liquid-crystalline phase was obtained and only a crystalline phase could be observed. The ability of the different cations to promote HII phase formation in the isolated E. coli cardiolipin was found to correlate with their ability to support growth of the mutant strain (De Chavigny, A., Heacock, P.N., Dowhan, W. (1991) J. Biol. Chem. 266, 5323-5332), suggesting that cardiolipin with divalent cations can replace the role of phosphatidylethanolamine in the mutant strain, and that this role involves the preference of these lipids for organization in non-bilayer lipid structures.