Rotational Dynamics of Luteinizing Hormone Receptors on Bovine and Ovine Luteal Cell Plasma Membranes1

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Abstract
To determine whether LH receptor rotational diffusion is similar in closely related species, we compared the rotational correlation times of LH receptors on bovine CL membranes with those of LH receptors on sheep small luteal cells and luteal cell plasma membranes using time-resolved phosphorescence anisotropy techniques. After binding of erythrosin isothiocyanate (ErITC)-derived bovine LH (bLH), ErITC-ovine LH (oLH), or ErITC-hCG, there was no difference in the initial and final anisotropy at 4 degrees C, 15 degrees C, 25 degrees C, and 37 degrees C, indicating that the bLH receptor was rotationally immobile on the time scale of our experiments. On these same membrane preparations, the epidermal growth factor (EGF) receptor occupied by ErITC-murine EGF exhibited temperature-dependent rotational correlation times of 80 +/- 5 microseconds, 111 +/- 7 microseconds, 254 +/- 4 microseconds, and > 1000 microseconds at 4 degrees C, 15 degrees C, 25 degrees C, and 37 degrees C, respectively. Slower rotational times for EGF receptor observed at higher temperatures suggested the occurrence of temperature-dependent receptor aggregation. Like the bLH receptor, the oLH receptor on intact cells and on CL plasma membranes was rotationally immobile on the time scale of our experiments when occupied by ErITC-hCG. However, the oLH-occupied receptors on small luteal cells and on luteal cell membranes had comparable rotational correlation times at 37 degrees C. These results suggest that bLH receptors are present in large, rotationally immobile structures, whereas the receptor-containing structure formed on ovine luteal cells depends on whether that receptor is occupied by hCG or oLH. Also, despite the similarities between reproductive function in these species, the LH-occupied receptor appears to be organized differently in the plasma membranes of these hormone-responsive luteal cells.

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