Molecular dynamics of luteinizing hormone receptors on rat luteal cells

To better understand the in situ organization of the luteinizing hormone receptor on rat luteal cells, we have examined the molecular motions of this receptor following binding of ovine luteinizing hormone (oLH) or human chorionic gonadotropin (hCG). Fluorescence photobleaching recovery (FPR) measurements of LH receptor lateral diffusion were performed using tetramethylrhodamine isothiocyanate (TRITC)-derivatized oLH or hCG as a probe. These experiments indicate that TRITC-oLH occupied LH receptors on luteal cells obtained from superovulated female rats have a lateral diffusion coefficient D of (1.7 +/- 0.6).10(-10) cm2s-1 at 27 degrees C with fluorescence recovery after photobleaching of 46 +/- 5%. In similar experiments, binding of TRITC-hCG caused a significant decrease in LH receptor lateral diffusion; fluorescence recovery after photobleaching was less than 20%. To determine whether hCG-occupied receptors might exist in large aggregates, we measured the rotational correlation times (RCT) of hCG and oLH bound to the LH receptor on intact cells using single cell polarized fluorescence depletion (PFD). At 4 degrees C, LH receptors occupied by eosin isothiocyanate (EITC)-hCG exhibited a slower RCT (64 microseconds) than did receptors occupied by EITC-oLH (43 microseconds). At this temperature both TRITC-oLH and TRITC-hCG occupied LH receptors were laterally immobile. These FPR and PFD results suggest that the molecular motions of the luteal cell LH receptor are significantly modulated by the subtle structural differences in various bound gonadotropins.