FREEZE-FRACTURE PLANES OF METHANOGEN MEMBRANES CORRELATE WITH THE CONTENT OF TETRAETHER LIPIDS

Methanospirillum hungatei GPI contained 50% of its ether core lipids (polar lipids less head groups) as tetraether lipids, and its plasma membrane failed to fracture along its hydrophobic domain during freeze-etching. The membrane of Methanosaeta (''Methanothrix'') concilii did not contain tetraether lipids and easily fractured to reveal typical intramembranous particles. Methanococcus jannaschii grown at 50-degrees-C contained 20% tetraether core lipids, which increased to 45% when cells were grown at 70-degrees-C. The frequency of membrane fracture was reduced as the membrane-spanning tetraether lipids approached 45%. As the tetraether lipid content increased, and while fracture was still possible, the particle density in the membrane increased; these added particles could be tetraether lipid complexes torn from the opposing membrane face. The diether membrane (no tetraether lipid) of Methanococcus voltae easily fractured, and the intramembranous particle density was low. Protein-free liposomes containing tetraether core lipids (ca. 45%) also did not fracture, whereas those made up exclusively of diether lipids did split, indicating that tetraether lipids add considerable vertical stability to the membrane. At tetraether lipid concentrations below 45%, liposome bilayers fractured to reveal small intramembranous particles which we interpret to be tetraether lipid complexes.