Specific endometrial binding sites for bovine placental lactogen are antigenically similar to the growth hormone receptor

Bovine liver growth hormone receptors bind both bovine growth hormone (bGH) and bovine placental lactogen (bPL) with high affinity (Kd = 1.4 x 10(-11) M and 3.0 x 10(-11) M, respectively). By contrast, the uterine endometrium of pregnant cattle has high-affinity (Kd = 8.0 x 10(-11) M) binding sites for bPL but, displays negligible binding of bGH. A polyclonal antiserum raised against the extracellular domain of the bGH receptor, was used to determine if there was antigenic similarity between the liver bGH receptor and the endometrial bPL binding site(s). On Western blots, this antiserum displayed cross-reactivity with a 180,000-mol wt protein (nonreducing conditions) in detergent-solubilized extracts of microsomal membranes from both tissues. However, different detergents (Triton X-100 for endometrium and 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonate [CHAPS] for liver) were required to solubilize the cross-reacting protein in the two tissues. The purified immunoglobulin fraction from this same antiserum also blocked binding of [125I]bPL to microsomal membrane preparations from both liver and endometrium. These results indicate that the endometrial binding site for bPL is antigenically similar to the bGH receptor and raise the possibility that it may be a modified GH receptor.