CLONING OF THE CHICKEN INSULIN-RECEPTOR SUBSTRATE-1 GENE

The action of insulin, IGF-1, and IGF-2 is mediated via two receptor t yrosine kinases, the insulin and IGF-1 receptors. Upon ligand binding, these receptors become active kinases, undergoing autophosphorylation and phosphorylating cellular substrates, including insulin receptor s ubstrate-1 (IRS-1). IRS-1 acts as a docking protein and mediates multi ple interactions among other proteins, resulting in transduction of th e metabolic and mitogenic signals. The IRS-1 gene has been cloned from four species (human, rat, mouse, and frog). In the present study, the chicken IRS-1 gene was cloned. Chickens, as is true of birds in gener al, have a higher fasting and fed blood glucose than do mammals. Chick en IRS-1 DNA sequence encodes a 1240 amino acid protein. The most cons erved regions were the IRS homology-2 (IH-2), the pleckstrin homology, and the she and IRS-1 NPXY-binding (SAIN) domains. Twelve of the cIRS -1 tyrosine residues are in sequence motifs that, when phosphorylated, could interact with proteins containing SH2 domains. All twelve of th ese motifs were conserved. IRS-1 mRNA is expressed during embryogenesi s in chicken and persists after hatching. In LMH cells, derived from a chicken hepatoma, two bands were tyrosine phosphorylated in an insuli n-dependent manner: IRS-1 (similar to 180 kDa) and the insulin recepto r beta subunit (similar to 95 kDa). Chicken IRS-1 is structurally and functionally similar to its human homolog, despite the difference in b lood glucose levels and the evolutionary distance between birds and ma mmals.