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Biophys. J. BioFAST: First Published March 24, 2006. doi:10.1529/biophysj.105.070839
© 2006 by the Biophysical Society.

A more recent version of this article appeared on June 15, 2006.
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MEMBRANES

COEXISTING DOMAINS IN THE PLASMA MEMBRANES OF LIVE CELLS CHARACTERIZED BY SPIN LABEL ESR SPECTROSCOPY

Musti J. Swamy 1, Laura Ciani 2, Mingtao Ge 3, David A. Holowka 3, Barbara A. Baird 3 and Jack H. Freed 4*

1 University of Hyderabad
2 University of Florence
3 Cornell University
4 ACERT

* To whom correspondence should be addressed. E-mail: jhf{at}ccmr.cornell.edu.

Submitted on July 14, 2005
Revised on September 14, 2005
Accepted on 28 February 2006


  Abstract
The importance of membrane-based compartmentalization in eukaryotic cell function has become broadly appreciated, and a number of studies indicate that these eukaryotic cell membranes contain coexisting liquid-ordered (Lo) and liquid-disordered (Ld) lipid domains. However, the current evidence for such phase separation is indirect, and so far there has been no direct demonstration of differences in the ordering and dynamics for the lipids in these two types of regions or their relative population in the plasma membranes of live cells. In this study, we provide direct evidence for the presence of two different types of lipid populations in the plasma membranes of live cells from four different cell lines by electron spin resonance. Analysis of the ESR spectra recorded over a range of temperatures, from 5°C to 37°C, shows that the spin-labeled phospholipids incorporated experience two types of environments, Lo-like and Ld, with distinct order parameters and rotational diffusion coefficients, but with some differences amongst the four cell lines. These results suggest that coexistence of lipid domains that differ significantly in their dynamic order in the plasma membrane is a general phenomenon. The Lo-like region is typically found to be a major component in contrast to a model in which small liquid-ordered lipid rafts exist in a "sea" of disordered lipids. The results on ordering and dynamics for the live cells are also compared with those from model membranes exhibiting coexisting Lo and Ld phases.

Key Words: liquid disordered phase, liquid ordered phase, order parameter, plasma membranes, rotational diffusion coefficient, spin label




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Copyright © 2006 by the Biophysical Society.