Interaction of lactogenic hormones with the soluble extracellular domain of prolactin receptors.

Two variants of rabbit prolactin receptor extracellular domain (rbPRLR-ECD) were prepared using insect/baculovirus (amino acids 1-198) and E. coli (amino acids 4-210) expression systems. Bovine PRLR-ECD (bPRPL-ECD amino acids 1-210) and human growth hormone receptor ECD (hGHR-ECD amino acids 1-246) were also prepared using E. coli expression system. All four proteins were purified as monomers with > 95% homogeneity. Their affinity for various lactogenic and somatogenic hormones was determined by binding assays. The stoichiometry of complex formation with these hormones was studied by gel filtration on a Superdex 75 column, and bioactivity was determined by in vitro bioassays. The results summarized in this paper indicate that, in contrast to hGHR-ECD, in which the ability to form a 2:1 complex with hGH is indicative of the biological activity of the hormone, the ability or inability of prolactin and placental lactogen to form 2:1 complexes with rb or bPRLR-ECD cannot predict their biological activity. This conclusion does not preclude however, hormone- or antibody-induced dimerization of the membrane-embedded receptor.