Lipid bilayers as osmotic response elements

The importance of osmolarity in the regulation of a wide range of cellular functions and processes is well established. However, much less is known ab out the molecular mechanisms imparting sensitivity to osmotic forces to cel ls. The findings summarized in this brief review demonstrate that the princ ipal structural element of all biomembranes, the lipid bilayer, provides a highly sensitive machinery for conveying information in the osmotic conditi ons of a cell to the relevant regulatory machineries. More specifically, os motic shrinkage, swelling, as well as applied osmotic stress all have prono unced effects on the physical state and molecular interactions in the bilay er, influencing lipid packing and dynamics, and also altering the 2-dimensi onal (lateral) ordering in the membrane into compositionally distinct micro domains. Because of the cooperative behavior of lipid bilayers integrative regulation of the functions embedded in the different organelle membranes b y the physical properties of lipids is possible. Organelle membranes should thus be understood as adaptive platforms harboring specialized metabolic p athways and functions, whose activities (physiological state) can be contro lled by the physical state of the membrane lipids. Finally it is important to keep in mind that virtually all biological macromolecules maintain a hyd ration shell. Living cells thus constitute highly complex supramolecular as semblies, their numerous components responding to osmotic forces in unison. Copyright (C) 2000 S. Karger AG, Basel.