Cloning and Expression of the Growth Hormone-Dependent Insulin-Like Growth Factor-Binding Protein

N-terminal as well as internal amino acid sequence data were obtained from the GH dependent, insulin-like growth factor (IGF) binding protein, BP-53, purified from human plasma. Based on these sequence data, full-length cDNA clones of BP-53 have been isolated, and the complete deduced sequence of BP-53 determined. This sequence contains a 27 amino acid putative signal sequence followed by a mature protein of 264 amino acids containing 18 cysteine residues clustered near the N- and C-terminus. The deduced protein sequence of BP-53 has 33% amino acid identity including conservation of all 18 cysteine residues with the recently cloned BP-28, a smaller human IGF-binding protein identified in amniotic fluid and also secreted by the cell line HEP G2. Expression of the cloned BP-53 cDNA in mammalian tissue culture cells results in secretion of the protein into the culture medium. This expressed protein is identical to plasma-derived BP-53 in its immunoreactivity, high affinity binding of IGF-I and IGF-II, and mobility on sodium dodecyl sulfate gel electrophoresis.