Spin-labeling studies on the Membranes of Differentiating Cells of <I>Dictyostelium discoideum</I>

Abstract

Differentiating cells of Dictyostelium discoideum were spinlabeled with 5-nitroxide stearic acid or phosphatidylcholine spin labels, and the electron spin resonance 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 two cell types in the range of 4°C to 25°C. Two distinct inflection points were observed at 15°C and 19.5°C in the temperature dependence, suggesting the presence of a phase transition in the lipid bilayer portion of the membranes. We thus concluded that there is no change in the membrane fluidity of the cells on formation of a multicellular organization. In addition, trypsinization of slug cells, which enhances Con A-induced redistribution of the Con A-binding sites on the cell surface (Kawai, in preparation), did not cause any change in the flexibility of the lipid bilayer. This suggests that the increased mobility of the sites has no apparent correlation to the bulk membrane fluidity.