Structural aspects of the IGFBP family

To date six IGF binding proteins (IGFBP) have been characterized. Analysis of the amino acid sequence reveals that the IGFBPs are clearly distinct but are sharing regions with strong homology. Specifically the hydrophobic cysteine rich N-terminal region and to a lesser extend the C-terminal part are preserved. The alignment of the cysteine molecules is strongly conserved across the 6 IGFBPs. The middle one-third region, where no cysteines are present (except for IGFBP-4) is most divergent. IGFBP-3 and -4 are glycosylated, whereas IGFBP-1 and -2 contain an Arg-Gly-Asp sequence near the carboxyl terminus. Determination of the number of free-SH groups of IGFBP-1 and -3 has revealed that most likely all cysteine residues are involved in disulfide bond formation. All members of the IGFBP family bind IGF-I and IGF-II with about equal affinity. Studies involving deletion mutation and site-directed mutagenesis of IGFBP-1 and -3 have suggested that the three-dimensional structure of the protein plays an important role in IGF binding. However at present it is unclear whether the IGFBPs share one or more specific IGF binding domain. The predominant function of the IGFBPs is to allocate IGF in the various body fluid compartments and tissues and to modulate IGF binding to receptors. For this purpose there exists a very sophisticated control of the routing of circulating IGF both from and to the cell. There is mounting evidence that the structure of the IGFBP proteins plays a key role in the regulation of IGF bioavailability, by modulating its molecular size, capillary membrane permeability, target tissue specificity, cell membrane adherence and IGF affinity.