We have used in situ electromigration and post-field relaxation (Poo, M.-m., 1981, Annu. Rev. Biophys. Bioeng., 10:245-276) to assess the effect of immunoglobulin E (IgE) binding on the lateral mobility of IgE-Fc receptors in the plasmalemma of rat basophilic leukemia (RBL) cells. Bound IgE sharply increased the receptor's electrokinetic mobility, whereas removal of cell surface neuraminic acids cut it to near zero. In contrast, we found only a small difference between the lateral diffusion coefficients (D) of vacant and IgE-occupied Fc receptors (D: 4 vs. 3 X 10(-10) cm2/s at 24 degrees C). This is true for monomeric rat IgE; with mouse IgE, the difference in apparent diffusion rates was slightly greater (D: 4.5 vs. 2.3 X 10(-10) cm2/s at 24 degrees C). This range of D values is close to that found in previous photobleaching studies of the IgE-Fc epsilon receptor complex in RBL cells and rat mast cells. Moreover, enzymatic depletion of cell coat components did not measurably alter the diffusion rate of IgE-occupied receptors. Thus, binding of fluorescent macromolecular probes to cell surface proteins need not severely impede lateral diffusion of the probed species. If the glycocalyx of RBL cells does limit lateral diffusion of the Fc epsilon receptor, it must act primarily on the receptor itself, rather than on receptor-bound IgE.

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