Identification of ADAMTS13 Epitopes Required for Binding to von Willebrand Factor Using Lambda Phage Surface Display.

The metalloprotease ADAMTS13 cleaves multimeric von Willebrand factor (VWF) to regulate VWF-mediated thrombus formation. We planned to search core epitopes of ADAMTS13 that is required for its binding to VWF. We constructed a random cDNA fragment library expressing various peptides of ADAMTS13 on the surface of lambda phage and screened the library using immobilized VWF as a probe. After the first screening, the C-terminus of the spacer domain from Arg⁶⁷⁰ to Glu⁶⁸⁴ (termed as epitope-1) and the middle of the cysteine-rich domain from Arg⁴⁸⁴ to Arg⁵⁰⁷ (epitope-2) were determined as epitopes. When we added the synthetic epitope-1 peptide to the second screening, a new site, from Pro⁶¹⁸ to Glu⁶⁴¹ (epitope-3), was found in the middle of spacer domain. While the presence of synthetic epitope-2 peptide did not affect the subsequent screening, the presence of epitope-3 peptide enhanced the isolation of clones encoding epitope-1. These results suggest that ADAMTS13 epitopes-1, -2 and -3 may interact with each other for their binding to VWF. From screening in the presence of any combination or all of the three synthetic peptides, however, no new VWF binding site was uncovered. To examine the effect of divalent metal cations on the binding of ADAMTS13 epitopes to immobilized VWF, screening was carried out in the presence or absence of 5 mM of EDTA. No new epitope site was found. We next explored inhibitory effect of the synthetic epitope peptides on ADAMTS13 protease activity using recombinant ADAMTS13 and FRETS-VWF73 as a substrate. Synthetic epitopes-2 and -3 peptides markedly inhibited the cleavage of VWF by ADAMTS13, while the synthetic epitope-1 peptide did not as efficiently as epitopes-2 and -3. The stronger inhibitory effect of epitope-3 peptide than that of epitope-1 peptide was confirmed by SDS-agarose gel electrophoresis analysis of cleavage products of denatured multimeric VWF molecules by recombinant ADAMTS13. This was consistent with the dissociation constants for the three synthetic peptides with immobilized VWF determined by surface plasmon resonance, in which epitopes-2 and -3 have higher affinities for VWF than that of epitope-1. The results described above suggest that ADAMTS13 may initially bind to immobilized VWF through the sites of epitope-1 and epitope-2 with relatively weak affinity. The binding of epitope-1 to VWF may subsequently induce the conformational change of VWF, thereby exposing a binding site for epitope-3 for the efficient catalytic cleavage of VWF by ADAMTS13.