Spin-labeling studies on the Membranes of Differentiating Cells of Dictyostelium discoideum

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Abstract
Differentiating cells of D. discoideum were spin-labeled with 5-nitroxide stearic acid or phosphatidylcholine spin labels, and the ESR spectra of the cells were measured. There was no difference in the flexibility of the lipid bilayer in the membranes between preaggregation cells and disaggregated slug cells. The temperature dependence of the overall splitting value for 5-nitroxide stearic acid was also indistinguishable for the 2 cell types in the range of 4-25.degree. C. Two distinct inflection points were observed at 15.degree. C and 19.5.degree. C in the temperature dependence, suggesting the presence of a phase transition in the lipid bilayer portion of the membranes. There is apparently no change in the membrane fluidity of the cells on formation of a multicellular organization. In addition, trypsinization of slug cells, which enhances concanavalin [con] A-induced redistribution of the Con A-binding sites on the cell surface did not cause any change in the flexibility of the lipid bilayer. The increased mobility of the sites has no apparent correlation to the bulk membrane fluidity.