Vitronectin is a large glycoprotein found in blood and the extracellular matrix (ECM). The gene for Vitronectin encodes a 19 amino acid (aa) signal peptide and a 459 aa protein. The amino terminal 130 aa’s of Vitronectin contains multiple binding sites for a variety of structures. Included is a site for binding to plasminogen activator inhibitor-1 (PAI-1) and urokinase receptor, an (RGD) sequence that binds alpha v beta 3, alpha v beta 5, alpha v beta 1, alpha IIb beta 3, alpha v beta 6, and alpha v beta 8 integrins, a stretch of acidic amino acids that includes two sulfated tyrosine residues that bind thrombin‑anti-thrombin III complexes, and a collagen binding site. The major part of the Vitronectin molecule (aa 132‑459) contains six hemopexin repeats.* The carboxyl‑terminal end of Vitronectin has multiple sites and fucntions. It contains a stretch of basic amino acids that binds the acidic amino acids of the amino‑terminal region, thereby stabilizing Vitronectin’s three dimensional structure. The carboxyl‑terminal end also contains a plaminogen binding site, a heparin binding site that binds complement factor C7, C8 or C9, a glycosaminoglycan binding site, and a second PAI-1 binding site (aa 348‑370). Vitronectin also contains an endogenous cleavage site, plus cleavage sites for elastase, thrombin and plasmin. Vitronectin has also been shown to bind IGF-2 and TGF-beta. The apparent molecular weight of human Vitronectin is 75 kDa, with ~30% of its molecular mass being attributed to glycosylation at 3 different sites. In blood and plasma, Vitronectin is found predominantly as a single chain monomer. It can also be found as a dimer after endogenous cleavage. The dimer is composed of a 65 kDa and 10 kDa component held together by a disulfide bond. Binding of thrombin‑anti-thrombin II complex or complement leads to an unfolding of Vitronectin. Unfolding of Vitronectin generates disulfide-linked multimers that are found in platelet secretions and extracellular matrix. Vitronectin is produced at high levels by the liver and many tumors. As might be expected by its structure, Vitronectin is involved in a number of biological activities including cell adhesion, cell spreading and migration, cell proliferation, extracellular anchoring, fibrinolysis, hemostasis, and complement mediated immune defense.
*Hemopexin domains are associated with enzyme and protein binding.
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