Evidence for a specific interaction of vitronectin with arginine: effects of reducing agents on the expression of functional domains and immunoepitopes

Functional structure of the somatomedin B domain of vitronectin

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. We have shown previously in the crystal structure of the PAI-1/SMB complex that SMB, a peptide of 51 residues, is folded as a compact cysteine knot of four pairs of crossed disulfide bonds. However, the physiological significance of this structure was questioned by other groups, who disputed the disulfide bonding shown in the crystal structure (Cys5-Cys21, Cys9-Cys39, Cys19-Cys32, Cys25-Cys31), notably claiming that the first disulfide is Cys5-Cys9 rather than the Cys5-Cys21 bonding shown in the structure. To test if the claimed Cys5-Cys9 bond does exist in the SMB domain of plasma vitronectin, we purified mouse and rat plasma vitronectin that have a Met (hence cleavable by cyanogen bromide) at residue 14, and also prepared recombinant human SMB variants from insect cells with residues Asn14 or Leu24 mutated to Met. HPLC and mass spectrometry analysis showed that, after cyanogen bromide digestion, all the fragments of the SMB derived from mouse or rat vitronectin or the recombinant SMB mutants are still linked together by disulfides, and the N-terminal peptide (residue 1-14 or 1-24) can only be released when the disulfide bonds are broken. This clearly demonstrates that Cys5 and Cys9 of SMB do not form a disulfide bond in vivo, and together with other structural evidence confirms that the only functional structure of the SMB domain of plasma vitronectin is that seen in its crystallographic complex with PAI-1.

Identification of the disulfide bonds in the recombinant somatomedin B domain of human vitronectin

The NH(2)-terminal somatomedin B (SMB) domain (residues 1-44) of human vitronectin contains eight Cys residues organized into four disulfide bonds and is required for the binding of type 1 plasminogen activator inhibitor (PAI-1). In the present study, we map the four disulfide bonds in recombinant SMB (rSMB) and evaluate their functional importance. Active rSMB was purified from transformed Escherichia coli by immunoaffinity chromatography using a monoclonal antibody that recognizes a conformational epitope in SMB (monoclonal antibody 153). Plasmon surface resonance (BIAcore) and competitive enzyme-linked immunosorbent assays demonstrate that the purified rSMB domain and intact urea-activated vitronectin have similar PAI-1 binding activities. The individual disulfide linkages present in active rSMB were investigated by CNBr cleavage, partial reduction and S-alkylation, mass spectrometry, and protein sequencing. Two pairs of disulfide bonds at the NH(2)-terminal portion of active rSMB were identified as Cys(5)-Cys(9) and Cys(19)-Cys(21). Selective reduction/S-alkylation of these two disulfide linkages caused the complete loss of PAI-1 binding activity. The other two pairs of disulfide bonds in the COOH-terminal portion of rSMB were identified as Cys(25)-Cys(31) and Cys(32)-Cys(39) by protease-generated peptide mapping of partially reduced and S-alkylated rSMB. These results suggest a linear uncrossed pattern for the disulfide bond topology of rSMB that is distinct from the crossed pattern present in most small disulfide bond-rich proteins.

Model for the three-dimensional structure of vitronectin: predictions for the multi-domain protein from threading and docking

The structure of vitronectin, an adhesive protein that circulates in high concentrations in human plasma, was predicted through a combination of computational methods and experimental approaches. Fold recognition and sequence-structure alignment were performed using the threading program PROSPECT for each of three structural domains, i.e., the N-terminal somatomedin B domain (residues 1-53), the central region that folds into a four-bladed beta-propeller domain (residues 131-342), and the C-terminal heparin-binding domain (residues 347-459). The atomic structure of each domain was generated using MODELLER, based on the alignment obtained from threading. Docking experiments between the central and C-terminal domains were conducted using the program GRAMM, with limits on the degrees of freedom from a known inter-domain disulfide bridge. The docked structure has a large inter-domain contact surface and defines a putative heparin-binding groove at the inter-domain interface. We also docked heparin together with the combined structure of the central and C-terminal domains, using GRAMM. The predictions from the threading and docking experiments are consistent with experimental data on purified plasma vitronectin pertaining to protease sensitivity, ligand-binding sites, and buried cysteines.

Degradation of distinct forms of multimeric vitronectin by human fibroblasts

The plasma protein vitronectin is thought to be an important regulator of extravascular plasminogen activation. In previous studies we have shown that a disulfide stabilized multimeric form of vitronectin is endocytosed and degraded by fibroblast cells (T.S. Panetti, P.J. McKeown-Longo, J. Biol. Chem. 268 (1993) 11988-11993; P.J. McKeown-Longo, T.S. Panetti, in: K.T. Preissner, S. Rosenblatt, C. Kost, J. Wegerhoff, D.F. Mosher (Eds.), Biology of Vitronectins and their Receptors, Elsevier Science Publishers, Amsterdam, 1993, pp. 111-118). The preparation of multimeric vitronectin used in these earlier studies was in the form of high molecular weight disulfide-bonded aggregates which were stable in sodium dodecyl sulfate (SDS). To address the question of whether vitronectin needed to be in the form of disulfide stabilized multimers in order to be endocytosed, a multimeric vitronectin, which was not disulfide stabilized, was prepared from vitronectin that had been treated with reducing agent and alkylated with iodoacetamide. The resulting protein migrated as a 65/75 kDa protein on SDS gels in the absence of reducing agent, confirming that this form of vitronectin was no longer stabilized into disulfide-bonded aggregates. However, the protein was still multimeric when analyzed by native gels and could be converted to SDS stable multimers by cross-linking agents. This result demonstrated that reduced and alkylated vitronectin aggregates into multimeric forms which are not stable in SDS. Similar to disulfide stabilized multimers, alkylated multimers of vitronectin bound to sulfated proteoglycans in the extracellular matrix and were endocytosed and degraded. Degradation of both forms of vitronectin was inhibited with arginine-glycine-aspartic acid peptides, an anti-alphavbeta5 antibody and heparin. Chloroquine and wortmannin were also able to inhibit degradation of both forms of vitronectin, indicating that both multimeric forms were following the same endocytic and degradative pathway. These results suggest that the organization of vitronectin into a multimeric form which will be recognized for endocytosis does not require disulfide bond stabilization. This study further suggests that recognition of vitronectin for endocytosis is dependent upon its conversion from a monomeric to a multivalent form (C.E. Wilkins-Port, P.J. McKeown-Longo, Mol. Biol. Cell 8:S:64A (1997).