Luteinizing hormone receptors are self-associated in slowly diffusing complexes during receptor desensitization

We have previously shown that rat LH receptors (LHRs) occupied by human CG (hCG) exhibit slow receptor lateral diffusion and are self-associated. Here we have examined whether LHRs become self-associated and enter slowly diff using structures in response to hormone binding and whether these receptors retain this organization while in the desensitized state. Before hormone e xposure, wild-type rat LHRs coupled at the C terminus to enhanced green flu orescent protein (GFP-LHR-wt) exhibited fast lateral diffusion, as assessed by fluorescent photobleaching recovery (FPR) methods, and most receptors w ere laterally mobile. After 30 min exposure to hCG and subsequent removal o f hormone by low pH wash, hormone challenge at any time within the next 4 h produced no increase in cellular cAMP levels. During this time, LHRs were either laterally immobile or exhibited slower lateral diffusion. When LHRs were again responsive to binding of hormone, the rate of receptor lateral d iffusion had become significantly faster and the fraction of mobile recepto rs was again large. Desensitized LHRs were also self-associated and present in microscopically visible clusters on the plasma membrane. Fluorescence e nergy transfer (FET) methods were used to measure the extent of interaction between receptors coupled to either GFP or to yellow fluorescent protein ( YFP). Before hormone treatment, there was essentially no energy transfer be tween LHRs. After desensitization of the receptors by 30 min exposure to hC G, energy transfer efficiency increased to 18%. Values for FET efficiency b etween desensitized receptors decreased over time, and receptors were respo nsive to hormone only after measurable energy transfer had completely disap peared. Together these results suggest that desensitized LHRs exist in larg e, slowly diffusing structures containing self-associated receptors and tha t these structures must dissipate before the receptor can again respond to hormone.