Molecular cloning of the acid-labile subunit of the rat insulin-like growth factor binding protein complex.

The insulin-like growth factors, IGF-I and IGF-II, circulate in both humans and rats as part of a 125-150 kDa complex comprising IGFs, the IGF binding protein IGFBP-3, and an acid-labile subunit. Clones encoding rat acid-labile subunit have been isolated from a rat liver cDNA library probed with a human acid-labile subunit cDNA. Two overlapping clones encode a leucine-rich protein of 576 amino acids preceded by a 27-residue signal sequence, with 78% homology to the human acid-labile subunit. Northern analysis of mRNA from adult rat brain, kidney, heart, lung, spleen, muscle and liver shows a major species of about 4.4 kb and minor bands of about 2 kb, 1.4 kb and 1 kb. The tissue distribution of this protein may therefore be wider than previously recognized.