Insulin-like Growth Factor-Binding Protein
Insulin-like growth factor-binding protein refers to proteins that are secreted in vertebrates and have the affinity for binding to IGF-1. These proteins have high affinity for binding IGFs and they modulate the IGFs’ biological actions.
There are six different sub-groups of insulin-like growth factor-binding proteins that range from IGFBP-1 to IGFBP-6. Their categorization is based on gene organization, IGFs binding affinity and structural similarity.
Insulin-like growth factor-binding protein is simply called IGFBP and it acts as an insulin-like growth factor I carrier protein. About 98 percent of insulin growth factor 1 is bound to one of the six binding proteins. Insulin-like growth factor-binding protein 3 is found in the body in high amounts and it accounts for 80 percent of IGF binding.
The molar ratio of IGF-1 to IGFBP-3 is 1:1. Insulin-like growth factor-binding protein also binds to the IGF-1 in the liver. This allows the growth hormone to act on the liver so that it can produce IGF-1 continuously. IGFBPs also have inhibitor domain homologues. These are related to the family of MEROPS protease inhibitors, clan IX and equistatin.
The domain architecture of all insulin-like growth factor-binding proteins is the same. These proteins are molecules that are unusually pleiotropic. Just like the other binding proteins, insulin-like growth factor-binding protein can prolong IGFs’ half-life through their high affinity for ligands’ binding. IGFBPs share a 50 percent homology with one another. The order for their binding affinity for IGF-II and IGF-I is the same in terms of magnitude and ligands’ affinity for IGF-IR.
Individual insulin-like growth factor-binding proteins can act in a way that attenuates or enhances their similarities. 
However, there are cases when they exhibit different characteristics such as chromosomal location, site for RGD recognition, heparin binding domains, phosphorylation differences and glycosylation among others. Interaction of insulin-like growth factor-binding protein with the membranes of cellular basement can be affected by some of these differences.
Apart from functioning as protein carrier, insulin-like growth factor-binding proteins in serum form can regulate the actions of paracrine/autocrine and endocrine actions of IGF. They do this by modulating the availability of IGF to bind onto the signaling receptors of IGF-1.
Additionally, insulin-like growth factor-binding protein can act as a growth modulator without depending on IGFs. For instance, IGFBP-5 stimulates bone makers in the osteoblasts that lack functional IGFs. IGFBP binding to putative receptor on a cell membrane can also stimulate pathway signaling that is independent of the IGF receptor. This mediates IGFBPs’ effects on specific types of cells.
IGFBP-2 and IGFBP-1 have characteristics that enable them to stimulate the migration of CHO cell as well as human trophoblast cell via an alpha-5-beta 1 integrin-mediated action. Finally, when IGFBPs are transported into a nucleus through a nuclear localization signal they can exert effects that are independent of IGF via a genes’ transcriptional activation. Several studies have confirmed the ability of insulin-like growth factor-binding protein to influence cell growth both independently and by depending on IGFs.
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Sources
http://press.endocrine.org/doi/full/10.1210/edrv.20.6.0382
http://en.wikipedia.org/wiki/Insulin-like_growth_factor-binding_protein
http://www.ebi.ac.uk/interpro/entry/IPR000867
http://www.mayomedicallaboratories.com/interpretive-guide/?alpha=I&unit_code=35100