Luteinizing hormone receptors are self-associated in the plasma membrane

We have evaluated rat LH receptor self-association and lateral dynamics for functional and nonfunctional receptors after binding of hormone. We demons trate, for the first time, that grouped receptors observed in electron or l ight microscopy represent actual receptor dimers or oligomers rather than s imply the concentration of receptors within membrane microdomains. Fringe f luorescence photobleaching recovery methods showed that, after binding of e ither LH or human CG (hCG), functional wild-type LH receptors, expressed on 293 cells (LHR-wt cells), have mobilities that are 25% lower than those of nonfunctional LH receptors containing an arginine substitution for lysine at position 583 (LHR-K583R cells). Because lateral diffusion coefficients i n two dimensions depend only on the logarithm of the molecular size of the diffusing species, this result implies that functional receptors exist in s ubstantially larger membrane complexes than do nonfunctional receptors. In single-cell measurements of fluorescence energy transfer after hormone bind ing, functional LH receptors were also characterized by receptor self-aggre gation, Values for fluorescence resonant energy transfer efficiency were 13 +/- 2% and 17 +/- 3% between fluorophore-conjugated LH or hCG, respectivel y bound to receptors on LHR-wt cells. However, there was little or no energ y transfer between receptors on LHR-K583R cells. These results suggest that receptor functionality invalues receptor-receptor interactions and that th e extent of such receptor self-association depends on whether LH or hCG bin ds the receptor.