Structure and expression of a chicken insulin-like growth factor I precursor.

Insulin-like growth factor I (IGF-I) is a 70 amino acid growth-promoting polypeptide whose sequence and functions have been highly conserved among mammals. As an initial step in defining the role of IGF-I in other vertebrate species, we have isolated and characterized an IGF-I cDNA from the chicken. This cDNA encodes a 153 amino acid primary translation product which resembles in structure and sequence the IGF-IA protein of mammals. There is strong amino acid conservation between chicken and mammalian IGF-I throughout the entire protein. Sixty of 70 amino acids are identical in mature IGF-I among the chicken, rat, and human peptides, with five differences being localized to the C domain, and two to the D region. A comparable degree of amino acid identity is found in the COOH-terminal extension peptide (28/35 residues). At the NH2-terminus, where there is more amino acid divergence (32/48 identities), the most 5'-AUG codon is the only methionine residue conserved among all three species, suggesting that it functions as the authentic translation initiation site, an observation supported by cell-free studies of biosynthesis and cotranslational proteolytic processing. The pattern of IGF-I gene expression appears to be simpler in chickens than in mammals, since a single predominant mRNA of 2.6 kilobases can be detected in liver polyadenylated RNA on Northern blots. In the chicken, as in rats and humans, IGF-I mRNA is synthesized in multiple tissues, including liver, brain, skeletal muscle, and heart.(ABSTRACT TRUNCATED AT 250 WORDS)