An Integrative Structural Biology Analysis of Von Willebrand Factor Binding and Processing by ADAMTS-13 in Solution

Metalloprotease domain latency protects ADAMTS13 against broad-spectrum inhibitors of metalloproteases while maintaining activity toward VWF

BACKGROUND

ADAMTS13 is a circulating metalloprotease that cleaves von Willebrand factor (VWF) in a shear-dependent manner. ADAMTS13 is secreted as an active protease but has a long half-life, suggesting that it is resistant to circulating protease inhibitors. These zymogen-like properties indicate that ADAMTS13 exists as a latent protease that is activated by its substrate.

OBJECTIVES

To investigate the mechanism of ADAMTS13 latency and resistance to metalloprotease inhibitors.

METHODS

Probe the active site of ADAMTS13 and variants using alpha-2 macroglobulin (A2M), tissue inhibitors of metalloproteases (TIMPs), and Marimastat.

RESULTS

ADAMTS13 and C-terminal deletion mutants are not inhibited by A2M, TIMPs, or Marimastat, but cleave FRETS-VWF73, suggesting that the metalloprotease domain is latent in the absence of substrate. Within the metalloprotease domain, mutating the gatekeeper triad (R193, D217, D252) or substituting the calcium-binding (R180-R193) or the variable (G236-S263) loops with corresponding features from ADAMTS5 did not sensitize MDTCS to inhibition. However, substituting the calcium-binding loop and an extended variable loop (G236-S263) corresponding to the S1-S1' pockets with those from ADAMTS5, resulted in MDTCS-GVC5 inhibition by Marimastat, but not by A2M or TIMP3. Substituting the MD domains of ADAMTS5 into full-length ADAMTS13 resulted in a 50-fold reduction in activity compared with the substitution into MDTCS. However, both chimeras were susceptible to inhibition, suggesting that the closed conformation does not contribute to the latency of the metalloprotease domain.

CONCLUSION

The metalloprotease domain protects ADAMTS13 from inhibitors and exists in a latent state that is partially maintained by loops flanking the S1 and S1' specificity pockets.

ADAMTS13 conformations and mechanism of inhibition in immune thrombotic thrombocytopenic purpura

ADAMTS13, a plasma metalloprotease that cleaves von Willebrand factor, is crucial for normal hemostasis. Acquired autoantibody-mediated deficiency of plasma ADAMTS13 results in a potentially fatal blood disorder, immune thrombotic thrombocytopenic purpura (iTTP). Plasma ADAMTS13 protease appears to exist in multiple conformations. Under physiological conditions, plasma ADAMTS13 exists predominantly in its "closed" conformation (or latent form), which may be activated by lowering pH, ligand binding, and binding of an antibody against the distal domains of ADAMTS13. In patients with iTTP, polyclonal antibodies target at various domains of ADAMTS13. However, nearly all inhibitory antibodies bind the spacer domain, whereas antibodies that bind the distal C-terminal domains may activate ADAMTS13 through removing its allosteric inhibition. Additionally, the anti-C-terminal antibodies may alter the potency of inhibitory antibodies towards ADAMTS13 activity. This review summarizes some of the most recent knowledge about the ADAMTS13 conformation and its mechanism of inhibition by its autoantibodies.