Acholeplasma laidlawii B membranes contain a lipid (glycerylphosphoryldiglucosyldiacylglycerol) which forms micelles rather than lamellar or reversed phases when dispersed in water.

It has been proposed that each of the lipids from the Acholeplasma laidlawii membrane prefers to form either a lamellar or a reversed cubic or hexagonal phase when dispersed in excess water at physiologically relevant temperatures and ionic strengths. In this study, we have reinvestigated the thermotropic phase behavior of all the major membrane lipids from A. laidlawii B membranes derived from cells grown in equimolar palmitic and elaidic acids. We confirm that phosphatidylglycerol (PG) and diglucosyldiacylglycerol (DGDG) from such membranes do indeed form only lamellar phases over the temperature range 5-80 degrees C. We also confirm that the monoglucosyldiacylglycerol (MGDG) and acyl polyprenyl glucoside (APG) exist in lamellar phases at lower temperatures but do form reversed phases at higher temperatures. However, we present here optical, differential scanning calorimetric, quasielastic light scattering, and 2H- and 31P-nuclear magnetic resonance spectroscopic evidence indicating that one lipid component of the A. laidlawii B membrane, namely, glycerylphosphoryldiglucosyldiacylglycerol (GPDGDG), actually forms normal micellar rather than lamellar or reversed phases when dispersed in excess water at physiological temperatures. To the best of our knowledge, this is the first demonstration of the existence of a micellar phase-preferring lipid in a prokaryotic cell membrane, and only the second demonstration of the existence of a micellar phase-forming lipid in any biological membrane. We also show that GPDGDG levels change greatly depending on the fatty acid composition of the membrane lipids of this organism.(ABSTRACT TRUNCATED AT 250 WORDS)