Relative localization of the prolactin receptor binding sites for lactogenic hormones

A monoclonal antibody termed MAb R7B4, directed to an epitope present in prolactin receptors (PRLRs), was used as a tool to map the receptor binding sites for human growth hormone (hGH), ovine prolactin (oPRL) and human placental lactogen (hPL). Although the three hormones completely inhibited the binding of each other to Nb2 cells or rat liver receptors, MAb R7B4 behaviour was different depending on the hormone tested and the receptor source. According to the MAb effects, PRLR from Nb2 cells would locate both hGH and oPRL close to R7B4 epitope, whereas hPL would bind far from the MAb binding site. On the other hand, PRLR from rat liver should bind hGH close to the R7B4 epitope but oPRL and hPL would be recognized by a separate region of the same receptor. Thus, results presented in this paper suggest that PRLR binding sites for hGH, oPRL and hPL do not exactly overlap in spite of full competition between ligands.