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 la rge amounts of phosphatidylglycerol and cardiolipin and has an absolut e requirement for certain divalent cations. Cardiolipin was isolated f rom this mutant strain and its interaction with divalent cations was s tudied by various biophysical techniques. Monolayer measurements showe d that the cations decrease the molecular surface area of cardiolipin in the order Ca2+ approximate to Mg2+ > Sr2+ > Ba2+. P-31-NMR and X-ra y diffraction measurements demonstrated a comparable sequence for the ability of the cations to promote H-II phase formation in dispersions of the E. coli cardiolipin: Ca2+ and Mg2+ induced H-II phase formation at 50 degrees C, Sr2+ at 75 degrees C, while Ba2+ was found to be una ble to promote H-II phase formation in the temperature range measured. Furthermore, all divalent cations were found to increase the temperat ure at which the transition to the liquid-crystalline phase takes plac e, which was below 5 degrees C for the lipid in the absence of divalen t cations. In the presence of Sr2+, Mg2+ and Ba2+ and at 25 degrees C two lamellar phases were observed, one corresponding to a liquid-cryst alline 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 ph ase could be observed. The ability of the different cations to promote H-II 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 repl ace the role of phosphatidylethanolamine in the mutant strain, and tha t this role involves the preference of these lipids for organization i n non-bilayer lipid structures.