Vitronectin is initially synthesized as a single-chain glycoprotein. One major allelic form of vitronectin readily undergoes proteolytic cleavage to yield a two-chain form. In plasma vitronectin appears as a characteristic mixture of 65- and 75-kDa forms. The major site of vitronectin synthesis is liver but it is also produced by platelets and macrophages. In addition to plasma and other body fluids vitronectin can be found in the connective tissue where it probably has an important function in cell-matri interactions. Like clusterin, vitronectin binds to the nascent C5b-7, C5b-8 and C5b-9 complexes and prevents their incorportion into cell membranes.
The three-dimensional structure of an N-terminal fragment comprising the first 51 amino acids from human plasma vitronectin, the somatomedin B (SMB) domain, has been determined by two-dimensional NMR approaches. An average structure was calculated, representing the overall fold from a set of 20 minimized structures. The core residues overlay with a root mean square deviation of 2.29 ± 0.62 Å. The N- and C-terminal segments exhibit higher root mean square deviations, reflecting more flexibility in solution and/or fewer long-range NOEs for these regions. Residues 26-30 form a unique single-turn α-helix, the locus where plasminogen activator inhibitor type-1 (PAI-1) is bound. This structure of this helix is highly homologous with that of a recombinant SMB domain solved in a co-crystal with PAI-1, although the remainder of the structure differs. Significantly, the pattern of disulfide cross-links observed in this material isolated from human plasma is altogether different from the disulfides proposed for recombinant forms. The NMR structure reveals the relative orientation of binding sites for cell surface receptors, including an integrin-binding site at residues 45-47, which was disordered and did not diffract in the co-crystal, and a site for the urokinase receptor, which overlaps with the PAI-1-binding site.
The N-terminal somatomedin B domain (SMB) of vitronectin binds PAI-1 and the urokinase receptor with high affinity and regulates tumor cell adhesion and migration. PAI-1 is an atypical member of the serpin family of serine protease inhibitors; it has an inherently brief half-life because of the spontaneous insertion of its intact reactive center loop into the main β-sheet of the molecule,which render the molecule inactive. This transition to the inactive latent form is slowed by binding to vitronectin, a glyoprotein of 70 kDa present in both plasma and the extracellular matrix. Binding to vitronectin stabilizes active PAI-1 and localizes it to its site of action. Expansion of the main β-sheet of PAI-1,through either the latency transition or cleavage of the reactive center loop,results in disruption of the complex and the release of inactive PAI-1 and unliganded vitronectin. The bulk of experimental evidence suggests that high-affinity binding and stabilization of PAI-1 is mediated by the somatomedin B domain comprising the first 44 residues of vitronectin.
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