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 . coil (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. coil expression system . All four proteins were purified as monomers with > 95% homogeneity. Their affinity for various lactogenic and somatogenic hormones was det ermined 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 sum marized 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 biolo gical activity of the hormone, the ability or inability of prolactin a nd placental lactogen to form 2:1 complexes with rb or bPRLR-ECD canno t predict their biological activity. This conclusion does not preclude however, hormone- ore antibody-induced dimerization of the membrane-e mbedded receptor.