Generation of chicken growth hormone-binding proteins by proteolysis

A soluble protein that specifically bound growth hormone (GH) was characterized in culture medium of a COS-7 cell line transfected with the cDNA of the full-length chicken GH receptor (cGHR). Incubation of culture medium with 125I-labeled human GH resulted in the formation of a single specific complex with high affinity (KD = 0.36 nM) and apparent molecular weight of 75 kDa. The production of large quantities of GH-binding protein (GHBP) amounting to, per hour, 23% of the cell's GHR, points to the importance of partial proteolysis for GHR turnover. Considerable amounts of GHBP were also detected in a cytosolic fraction. These results strongly suggest that in chicken, as in rabbit and monkey, the GHBP is generated, at least partially, by proteolytic cleavage of the membrane-anchored GHR.