Ligand-specific dimerization of the extracellular domain of the bovine growth hormone receptor.

The bovine growth hormone (bGH) receptor and its extracellular domain (bGHBP) bind two protein hormones with high affinity; bGH and bovine placental lactogen (bPL). However, each of these hormones bind with a different stoichiometry. bGH binds to the bGHBP in a 1:2 ratio while bPL binds in a 1:1 ratio. Scatchard analysis of saturation binding yields similar apparent dissociation constants (Kd) for bGH and bPL of 1.4 x 10(-11) M and 3.0 x 10(-11) M, respectively, for the membrane receptor and 6.8 x 10(-11) M and 4.2 x 10(-11) M, respectively, for bGHBP. In competition experiments using either liver membranes or bGHBP, bGH is 2-3-fold more effective than bPL in competing for 125I-bGH-binding sites. In similar experiments using 125I-bPL, bPL is 50-fold more effective than bGH in competing for binding sites. A rat monoclonal antibody raised against bGHBP competes effectively for 125I-bGH-binding sites, but not for 125I-bPL-binding sites. Since bPL cannot dimerize the bGHBP and yet it acts in part as a somatogen in vivo, homodimerization of the growth hormone receptor is apparently not essential for some biological responses signaled through this receptor.