THE ROTATIONAL DIFFUSION OF LH RECEPTORS DIFFERS WHEN RECEPTORS ARE OCCUPIED BY HCG VERSUS LH AND IS INCREASED BY CYTOCHALASIN-D

We have examined the rotational diffusion of the luteinizing hormone ( LH) receptors binding human chorionic gonadotropin (hCG) or ovine lute inizing hormone (oLH) in MA-IO Leydig tumor cells using time-resolved phosphor escence anisotropy techniques. LH receptors binding erythrosi n isothiocyanate (ErITC)-derivatized oLH were rotationally mobile with rotational correlation times of 62 mu s, 48 mu s, 38 mu s, and 29 mu s at 4 degrees C, 15 degrees C, 25 degrees C, and 37 degrees C, respec tively. ErITC-hCG bound to the LH receptor was rotationally immobile, showing no anisotropy decay at 4 degrees C, 15 degrees C, 25 degrees C , and 37 degrees C. To determine whether cytoskeletal components influ enced the rotational diffusion of LH receptors, we measured rotational diffusion of LH receptors on MA-10 cells treated with 20 mu g/ml cyto chalasin D and on plasma membrane preparations. Following 1 h exposure to cytochalasin D, the relational correlation times for hCG-occupied LH receptors were typically 11 mu s at 37 degrees C compared to > 1000 mu s on untreated cells. Treatment of MA-10 cells with cytochalasin B or colchicine had no affect on LH receptor rotational diffusion. Rota tional correlation times for LH-occupied receptors decreased from 29 m u s to 12 mu s at 37 degrees C following cytochalasin D treatment. The rotational diffusion of LH receptors on plasma membrane preparations was similar to that observed for LH- and hCG-occupied receptors on int act cells treated with cytochalasin D. These various results indicate that there are differential effects of LH and hCG binding on the inter actions of LH receptors with plasma membrane proteins and that microfi laments anchor the hCG- and LH-occupied receptors.