Primary structure of human insulin‐like growth factor‐binding protein/placental protein 12 and tissue‐specific expression of its mRNA

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Abstract
The low‐molecular‐mass insulin‐like growth factor‐binding protein (IGF‐BP) and placental protein 12 (PP12) are identical proteins that are present in human serum, amniotic fluid, secretory endometrium and decidua. IGF‐BP/PP12 is believed to act as an autocrine or paracrine regulator of cell growth. A cDNA clone encompassing the entire protein coding region of this protein was isolated from a human decidual cDNA library. The authenticity of the cDNA was verified by in vitro transcription/translation experiments and by the identity of the 10 N‐terminal amino acids deduced for the mature peptide with those obtained by direct protein sequencing. The amino acid sequence indicates that pre‐IGF‐BP/PP12 consists of 259 amino acid residues. The putative signal peptide is 25 residues long, and the mature protein thus contains 234 amino acids and has a molecular mass of 25 293 Da. The sequence is very cysteine‐rich at the N‐terminus after which there are regions of clustered Pro, Glu, Ser and Thr residues (so‐called PEST regions), which exist in proteins with short half‐lives. The amino acid sequence also includes an Arg‐Gly‐Asp tripeptide that may function as a cell recognition signal. The IGF‐BP/PP12 gene encodes a single 1.6 kb mRNA species that is expressed in decidua, secretory endometrium, liver and a human hepatoma cell line (HepG2). Southern blot analysis suggests that there is a single IGF‐BP/PP12 gene in the human genome.