Membrane behavior during dehydration: Implications for desiccation tolerance

Because water is essential in the formation and maintenance of the bilayer structure, dehydration has a major impact on the conformation of membranes. Dehydration leads to increases in the gel-to-liquid crystalline transition temperature (T-m) and, depending on the phospholipid species involved, to the formation of nonbilayer phases. Di- and oligosaccharides, which are abu ndant in desiccation-tolerant organisms, are particularly effective at supp ressing the dehydration-induced increase in T-m, by interacting with the po lar headgroups. Moreover, they prevent membrane fusion by the timely format ion of a glassy matrix. Both properties are crucial in the protection of li posomes from dehydration stress. Dry, desiccation-tolerant organisms lack n on-bilayer phases, but gel phase has been encountered. T-m of membranes in dried organisms is fewer than that of dried isolated membranes, probably as a result of the interaction of the sugars with the headgroups and the part itioning of amphiphilic (fluidizing) compounds into the membranes in situ. Depression of T-m in situ may be beneficial in postponing or preventing a p hase change with drying and the associated loss of solutes. It may also be beneficial in preventing lateral phase separations when the interacting sug ars partake in a glassy matrix at a low moisture content. Injury can ensue when dry cells having gel phase phospholipids are plunged into (cold) water . Melting the gel phase lipid before imbibition, e.g., by exposure to humid air or heating, reduces leakage and preserves viability. We suggest that d epression of T-m, the interaction of sugars with the polar headgroups, as w ell as the formation of a glassy matrix, are required for the irt situ pres ervation of membranes in a dry state.