Cloning of cDNA encoding human placental protein 12 (PP12): binding protein for IGF I and somatomedin.

By N-terminal sequence comparison it was shown that the placental protein 12 (PP12)(1) is homologous to the IGF I (insulin-like growth factor) and somatomedin binding protein (2-4). Starting with an antibody raised against PP12 we have screened cDNA libraries from human placenta. Two cDNA clones were identified with an open reading frame encoding a hydrophobic leader peptide consisting of 25 aa and the mature protein of 234 aa (MW 25 270). The MW and the amino acids composition are in good agreement to the data of Bohn and Kraus. The protein sequence shows no N-linked glycosylation site. The cDNA encodes 18 Cys-residues, which may explain the wide discrepancy in the reported MW of the PP12 protein (25 200 to 34 000). The N-terminal protein sequence is identical to the sequence of Baxter et al., but different in residues 11 and 12 to Koistinen et al. . The 5' nontranslated region of PP12 cDNA extends 150 nt. The stop codon at position 928 is followed by 566 nt of untranslated region including the poly(A) addition site AATAAA 18 nt upstream of poly(A). The size of PP12 cDNA (1511 nt) agrees with a band of 1600 nt in northern blot analysis (not shown).