Thermal Adaptation in Biological Membranes: Is Homeoviscous Adaptation the Explanation?

The phase behavior and physical properties of lipids in biological membranes are exquisitely sensitive to changes in temperature (50). Because membranes (a) act as physical barriers to solute diffusion, (b) mediate the transmembrane movement of specific solutes, (c) regulate the utilization of energy stored in transmembrane ion gradients, Cd) provide an organizing matrix for the assem­ bly of multicomponent metabolic and signal transduction pathways, and (e) supply precursors for the generation of lipid-derived second messengers, tem­ perature-induced perturbations in membrane organization pose a serious chal­ lenge to the maintenance of physiological function in poikilotherms. However, poikilotherms exploit the diversity of lipid structure to fashion membranes with physical properties appropriate to their thermal circumstance and, in this way, restore membrane function following thermal challenge. Based on the finding that membrane lipids of Escherichia coli grown at 43 and 15°C dis­ played similar physical properties when compared at their respective growth temperatures, Sinensky concluded that membrane fluidity was defended as growth temperature changes and referred to this cellular homeostatic response as homeoviscous adaptation CHV A) (94). Since the original exposition of this hypothesis, HV A has emerged as the most commonly employed paradigm to assess the efficacy of thermal adaptation in biological membranes and to explain patterns of temperature-induced change in membrane lipid composi-