Membrane Lipids: What Membrane Physical Properties are Conserve during Physiochemically-Induced Membrane Restructuring?

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Abstract
SYNOPSIS. Current theories assert that organisms finely adjust the order, or fluidity, of their cellular membranes in response to changes in their physiochemical environment (e.g., pressure, temperature, salinity, etc.). However, membrane order may not be the only property that is conserved. The most commonly observed alterations in cell membrane composition under conditions of altered physiochemical environment, namely changes in the phosphatidylethanolamine/phosphatidylcholine (PE/PC) ratio and the content of highly unsaturated acyl chains, are difficult to fully reconcile with the conservation of membrane order alone. This report reviews the literature concerning two properties of membranes that may play vital roles in the adaptation of cellular membranes to changing environments: a) the tendency of membranes to relax into the reversed hexagonal phase and b) the occurrence and structure of lipid-driven domains within the membrane. The tendency of a membrane to form the reversed hexagonal phase is a property central to a variety of important cellular events. This tendency is tightly regulated by variation of the ratio of hexagonal phase-forming lipids to lamellar phase-forming lipids in the membrane. In most animal cells, this corresponds to the PE/PC ratio. Highly unsaturated acyl chains, in conjunction with cholesterol, modulate the occurrence and structure of lipid-driven membrane domains. These membrane domains are also critically involved in a number of key cellular processes. The changes in membrane lipid composition that occur during adaptation to the environment may be required for the preservation of the tendency to form nonlamellar phases and of the occurrence and specific structure of domains within the membrane, in addition to overall membrane order.

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