REGULATION AND PHASE-EQUILIBRIA OF MEMBRANE-LIPIDS FROM BACILLUS-MEGATERIUM AND ACHOLEPLASMA-LAIDLAWII STRAIN A CONTAINING METHYL-BRANCHED ACYL CHAINS

Phosphatidylethanolamine (PE) was isolated from Bacillus megaterium grown at 20 and 55 degrees C (PE-20 and PE-55). Iso and anteiso methyl-branched, saturated acyl chains are predominant in B. megaterium, and the value of the molar ratio of iso/anteiso acyl chains is more than 20-fold higher in PE-55 than in PE-20. Moreover, about 21 mol % of the acyl chains of PE-20 are monounsaturated. The phase equilibria differ between the two PE preparations: (1) PE-20 is more prone to form reversed nonlamellar phases than PE-55; (2) PE-20 forms both reversed cubic (I-2) and reversed hexagonal (H-II) phases while PE-55 forms only an H-II phase; and (3) the lamellar liquid-crystalline (L(alpha)) phase of PE-20 takes up about 70% more water than the L(alpha) phase of PE-55. These differences can be explained by the differences in the acyl chain composition. When the growth temperature is raised, PE molecules with a reduced tendency to form nonlamellar phases are probably synthesized by B. megaterium in order to counteract the bilayer destabilizing effect of the temperature. The regulation of the acyl chain composition is not needed in order to regulate the temperature for the transition between gel/crystalline and L(alpha) phases of the membrane lipids. Acholeplasma laidlawii strain A-EF22 was grown at 37 degrees C on 15-(1,1,1-H-2(3))methylhexadecanoic acid, 14-(1,1,1-H-2(3))methylhexadecanoic acid or 13-(1,1,1-H-2(3))methylhexadecanoic acid, and these acids constituted 84-89 mol % of the acyl chains in the membrane lipids. The molar ratio between the two dominating lipids, monoglucosyldiacylglycerol (MGlcDAG) and diglucosyldiacylglycerol (DGlcDAG), decreased, and the molar fraction of the anionic lipids increased, when the methyl branch was moved from position 15 to position 13. Concomitantly, the order of the methyl branch increased in cells as well as in total lipid extracts. The phase equilibria of total lipid extracts (neutral lipids removed) were studied with 20 wt % of water, and H-II and I-2 phases were formed above 63-67 degrees C. These results indicate that the regulation of the polar head-group composition compensates for the difference in acyl chain packing introduced into the bilayer by the three branched-chain fatty acids. The regulation of the polar head-group composition of the A. laidlawii lipids cannot regulate the temperature for the transition between gel/crystalline and L(alpha) phases of the lipids, i.e. the transition to fluid acyl chains. Thus, both B. megaterium and A. laidlawii change the membrane lipid composition to adjust the balance between lipids forming lamellar and nonlamellar phases, but they appear to be largely insensitive to the transition between gel/crystalline and L(alpha) phases. It is concluded that the physico-chemical properties of membrane lipids are influenced by the position of a methyl substituent on the acyl chains and that these differences in physico-chemical properties are sensed by living cells.