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Ordered and Disordered Phases Coexist in Plasma Membrane Vesicles of RBL-2H3 Mast Cells. An ESR Study

Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853

Correspondence: Address reprint request to Dr. Jack H. Freed, Dept. of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY 14853-1301. E-mail: jhf{at}ccmr.cornell.edu.

Four chain spin labels and a spin-labeled cholestane were used to study the dynamic structure of plasma membrane vesicles (PMV) prepared from RBL-2H3 mast cells at temperatures ranging from 22°C to 45°C. Analysis shows that the spectra from most labels consist of two components. The abundant spectral components exhibit substantial ordering that is intermediate between that of a liquid-ordered (Lo) phase, and that of a liquid-crystalline (Lc) phase as represented by model membranes. Also, rotational diffusion rates of the spin labels are comparable to those in the Lo phase. In contrast, the ordering for the less abundant components is much lower. These results indicate that a Lo-like region or phase (the abundant component) and an Lc-like region or phase (the less abundant component) coexist in the PMV. In contrast, membranes reconstituted from extracted lipids exhibit the more ordered phase only. This suggests that membrane-associated proteins are important for the coexistence of Lo-like and Lc-like regions in the plasma membrane. In addition, binding of the myristoylated protein, ARF6 to PMV, leads to a new spectral component for a headgroup lipid spin label that indicates the formation of plasma membrane defects by this low molecular weight GTPase.

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