Acylation of proteins of the archaebacteria Halobacterium cutirubrum and Methanobacterium thermoautotrophicum.
Although the membrane lipids of extremely halophilic archaebacteria are exclusively derived from diphytanylglycerol diether, which is non-acylated, small amounts of fatty acids have been detected in these organisms. These fatty acids are formed by the action of a fatty acid synthase (FAS), shown to be present in the extreme halophile Halobacterium cutirubrum, despite the fact that only a fraction of the activity of FAS remains at the high salt concentration (> 4 M) present in the cytoplasm. It has now been demonstrated that fatty acids do not occur in lipid-bound form but largely in the form of acylated proteins in the red membrane of H. cutirubrum. In contrast, the bacteriorhodopsin of the purple membrane of this extreme halophile does not appear to be acylated. The thermophilic methanogen, Methanobacterium thermoautotrophicum had a much higher fatty acid synthase activity than the extreme halophile, and the synthase activity of the methanogen was optimal under its normal (anaerobic) growth conditions. The methanogen also utilized the resulting fatty acids to acylate its membrane proteins. The major fatty acids in both organisms were palmitic and stearic acids with small amounts of myristic and 18:1 acids, and these were bound to protein through both ester and amide linkages.