Constitutively-active human LH receptors are self-associated and located in rafts

Several naturally occurring mutations in human luteinizing hormone receptors (LHR) at position 578 are associated with constitutive activation of the receptor. To determine whether human LHRs that signal in the absence of ligand are self-associated, fluorescence resonance energy transfer (FRET) between receptors was evaluated. Values for FRET between wild type LHR in the absence of ligand were less than 1% and increased significantly to over 11% after exposure to hCG. Constitutively active receptors exhibited 11-15% FRET efficiency in the absence of hormone and these values did not change with hCG treatment. A large fraction of constitutively active LHR-D578H receptors were also associated with so-called plasma membrane rafts. Disruption of these membrane microdomains reduced FRET efficiency but did not affect signalling through cAMP. Thus, in the absence of ligand, constitutively active receptors are self-associated and located in high buoyancy membrane fractions, both characteristics of the hormone-treated wild type receptor.