INTRODUCTION Current opinion is that biological mem- branes posses a functional lipid domain struc- ture, often spoken of as “rafts” (for recent reviews see refs. 1–9). These domains are usu- ally small in cell membranes at physiological temperatures—on the scale of nanometers— and consist of phase separated ordered lipids, surrounded by fluid phase domains (hence the term rafts) (1–9). Although the discussion con- cerning the origins (10) and biological signifi- cance (11) of rafts in native membranes continues, there is abundant evidence that they Lipid Phase Behavior and Stabilization of Domains in Membranes of Platelets Chad Leidy, 1,2 Karine Gousset, 1,3 Josette Ricker, 1,2 Willem F. Wolkers, 1,2 Nelly M. Tsvetkova, 1,2 Fern Tablin, 1,3 and John H. Crowe 1,2,* 1 Center for Biostabilization, 2 Section of Molecular and Cellular Biology, and 3 Department of Anatomy, Physiology, and Cell Biology, School of Veterinary Medicine, University of California, Davis, CA 95616 Abstract Lipid domains are acquiring increasing importance in our understanding of the regulation of several key functions in living cells. We present here a discussion of the physical mechanisms dri- ving the phase separation of membrane lipid components that make up these domains, including phase behavior of the lipids and the role of cholesterol. In addition, we discuss phenomena that regulate domain geometry and dimensions. We present evidence that these mechanisms apply to the regulation of domains in intact cells. For example, the observation that physiologically func- tional microdomains present at 37˚C aggregate into macrodomains in human blood platelets when they are chilled below membrane lipid phase transition temperatures is predictable from the known behavior of the constituent lipids in vitro. Finally, we show that the principles developed from studies on these lipids in model systems can be used to develop techniques to stabilize the physiological, resting microdomain structure of platelets during freeze-drying. These latter find- ings have immediate applications in clinical medicine for the development of methods for storing platelets for therapeutic use. Index Entries: Rafts; membranes; cholesterol; lipid phase transitions; freeze-drying; trehalose. *Author to whom all correspondence and reprint requests should be addressed. E-mail: jhcrowe@ucdavis. edu REVIEW ARTICLE © Copyright 2004 by Humana Press Inc. All rights of any nature whatsoever reserved. 1085-9195/04/40/123–148/$25.00 Cell Biochemistry and Biophysics 123 Volume 40, 2004