Identification of five different insulin-like growth factor binding proteins (IGFBPs) from adult rat serum and molecular cloning of a novel IGFBP-5 in rat and human.

Five different insulin-like growth factor binding proteins (IGFBPs) were isolated from adult rat serum using gel filtration, ligand affinity chromatography, and two steps of reversed-phase high performance liquid chromatography. Three of them were identified as IGFBP-2, -3, and -4 by their amino-terminal amino acid sequences. One of the remaining two proteins was the rat homologue of the partially characterized IGFBP isolated originally from human cerebrospinal fluid, while the other appeared to be a novel member of the IGFBP family. IGFBP-1 was not found in the adult rat serum under our experimental procedures. cDNAs encoding the novel IGFBP were isolated and characterized from a rat ovary and a human placenta library. The mature protein predicted for both species contained 252 amino acids including 18 cysteines that were located in the homologous positions as IGFBP-1, -2, -3, and -4. We propose to name this protein IGFBP-5. Northern analysis of IGFBP-5 mRNA in rat tissues demonstrated that transcription of this gene is highly active in kidney, although the mRNA was detectable in all tissues examined. Alignment of the amino acid sequences of the five rat IGFBPs revealed a 47-60% similarity, indicating that their individual genes diverged from a single ancestral gene by successive gene duplication in a short time frame during evolution. The chromosomal localizations of IGFBP-1, -2, -3, -4, and -5 genes in human have been determined using polymerase chain reaction on somatic cell hybrid DNAs of human and hamster, and the results showed that they were located on chromosomes 7, 2, 7, 17, and 5, respectively.