Why do Prokaryotes Regulate Membrane Fluidity?
While adaptation of microbial membrane lipid composition is widely observed in response to changes in temperature, and in some cases to the presence of membrane-fluidizing substances, the physiological importance of such adaptations remains largely unexplored. To shed some light on this question question we have used our ability to extensively vary the membrane fatty acid composition of Acholeplasma laidalawii B to study a variety of membrane functions is membranes whose lipids contain a single fatty acyl species whose structure and physical properties can be widely varried. Our results suggest that control of the membrane lipid phase state is considerably more important to the proper functioning of the organism than is control of the ‘fluidity’ of liquid-crystalline lipids.
A. laidlawii B can be grown to normal or near-normal yields when cultured with avidin plus any of a large number of individual fatty acids, with structures ranging from a fourteen-carbon anteisobranched species to an eighteen-carbon monounsaturated species to an eighteen-carbon isobranched species. Cells will not grow, however, if their membrane lipids are largely in the gel state. The temperature dependence of a key membrane enzyme, the osmoregulatory (Na+,Mg2+)-ATPase, is independent of the lipid fatty acid composition in the membranes of such ‘fatty acid-homogeneous’ cells, so long as the lipids remain liquid-crystalline. The thermostability of this enzyme is likewise independent of fatty acid composition, and the absolute ATPase activity shows no consistent variation with composition that could be attributed to ‘fluidity’ effects. However, when the membrane lipids enter the gel state, the ATPase is entirely inactivated. Studies of cellular permeability to ions and polyols indicate that while more ‘fluid’ membranes are more permeable to these compounds, membranes in which gel and liquid-crystalline lipids coexist are more permeable still. Therefore, the phase state of the membrane lipids affects certain key physiological properties (including growth) much more dramatically than do variations in the ‘fluidity’ of liquid-crystalline membrane lipids, and we suggest that regulation of the membrane phase state is the primary function of ‘homeoviscous adaptation’ in prokaryotic organisms.
KeywordsMembrane Lipid Lipid Phase Membrane Function Membrane Lipid Composition Brane Lipid
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