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 gr own at 20 and 55 degrees C (PE-20 and PE-55). Iso and anteiso methyl-b ranched, saturated acyl chains are predominant in B. megaterium, and t he 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 a cyl chains of PE-20 are monounsaturated. The phase equilibria differ b etween the two PE preparations: (1) PE-20 is more prone to form revers ed 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 cha in composition. When the growth temperature is raised, PE molecules wi th a reduced tendency to form nonlamellar phases are probably synthesi zed by B. megaterium in order to counteract the bilayer destabilizing effect of the temperature. The regulation of the acyl chain compositio n is not needed in order to regulate the temperature for the transitio n 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))methylhexadecan oic 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, monoglucosyldiacylglyc erol (MGlcDAG) and diglucosyldiacylglycerol (DGlcDAG), decreased, and the molar fraction of the anionic lipids increased, when the methyl br anch was moved from position 15 to position 13. Concomitantly, the ord er of the methyl branch increased in cells as well as in total lipid e xtracts. The phase equilibria of total lipid extracts (neutral lipids removed) were studied with 20 wt % of water, and H-II and I-2 phases w ere formed above 63-67 degrees C. These results indicate that the regu lation of the polar head-group composition compensates for the differe nce in acyl chain packing introduced into the bilayer by the three bra nched-chain fatty acids. The regulation of the polar head-group compos ition of the A. laidlawii lipids cannot regulate the temperature for t he transition between gel/crystalline and L(alpha) phases of the lipid s, i.e. the transition to fluid acyl chains. Thus, both B. megaterium and A. laidlawii change the membrane lipid composition to adjust the b alance between lipids forming lamellar and nonlamellar phases, but the y appear to be largely insensitive to the transition between gel/cryst alline and L(alpha) phases. It is concluded that the physico-chemical properties of membrane lipids are influenced by the position of a meth yl substituent on the acyl chains and that these differences in physic o-chemical properties are sensed by living cells.