A SINGLE AMINO-ACID SUBSTITUTION IN THE EXOPLASMIC DOMAIN OF THE HUMAN GROWTH-HORMONE (GH) RECEPTOR CONFERS FAMILIAL GH RESISTANCE (LARON-SYNDROME) WITH POSITIVE GH-BINDING ACTIVITY BY ABOLISHING RECEPTOR HOMODIMERIZATION

Growth hormone (GH) elicits a variety of biological activities mainly mediated by the GH receptor (GHR), a transmembrane protein that, based on in vitro studies, seemed to function as a homodimer. To test this hypothesis directly, we investigated patients displaying the classic f eatures of Laron syndrome (familial GH resistance characterized by sev ere dwarfism and metabolic dysfunction), except for the presence of no rmal binding activity of the plasma GH-binding protein, a molecule tha t derives from the exoplasmic-coding domain of the GHR gene. In two un related families, the same GHR mutation was identified, resulting in t he substitution of a highly conserved aspartate residue by histidine a t position 152 (D152H) of the exoplasmic domain, within the postulated interface sequence involved in homodimerization. The recombinant muta ted receptor protein was correctly expressed at the plasma membrane. I t displayed subnormal GH-binding activity, a finding in agreement with the X-ray crystal structure data inferring this aspartate residue out side the GH-binding domain. However, mAb-based studies suggested the c ritical role of aspartate 152 in the proper folding of the interface a rea. We show that a recombinant soluble form of the mutant receptor is unable to dimerize, the D152H substitution also preventing the format ion of heterodimers of wild-type and mutant molecules. These results p rovide in vivo evidence that monomeric receptors are inactive and that receptor dimerization is involved in the primary signalling of the GH -associated growth-promoting and metabolic actions.