Interaction of lactogenic hormones with purified recombinant extracellular domain of rabbit prolactin receptor expressed in insect cells.

The extracellular domain of rabbit prolactin receptor (rbPRLR-ECD) expressed in an insect/baculovirus expression system was purified by affinity chromatography on immobilized PRL followed by gel filtration. The purified protein was over 90% homogeneous as indicated by SDS-PAGE in the presence or absence of reducing agent, and by chromatography on a Superdex column. Its molecular mass determined by SDS-PAGE was 32 kDa, and by gel filtration, 27 kDa. Both values are higher than the 22.8 kDa deduced from the cDNA sequence, indicating extensive glycosylation. The Ka value for interaction with ovine (o) PRL was 25.4 nM-1, but even at high rbPRLR-ECD:hormone molar ratios, the stoichiometry of interaction with oPRL or human growth hormone indicated formation of only 1:1 complexes, in contrast to human growth hormone (hGH)-ECD which forms 2:1 complexes with hGH.