The proposed 2-site initial binding interaction mechanism between VWF and ADAMTS13. (A) A schematic diagram of ADAMTS13 shows its domains. VWF is represented in its globular conformation. The ADAMTS13 cleavage site in the VWF A2 domain is buried in the center of the molecule and not accessible to be cleaved by the metalloprotease. However, a binding site in the C-terminal region (D4CK) of VWF is constitutively exposed, allowing interaction with the ADAMTS13 distal domains (TSP5-8 and CUBs). How these domains are orientated and how precisely they interact with each other remains to be clarified: the model is indicative only in this respect. (B) Under condition of high shear, VWF unravels. The initial anchoring of the distal domains of ADAMTS13 to the C-terminal region of VWF may help the exposure of the VWF A2 domain binding site and favor the correct positioning of ADAMTS13 spacer domain. (C) Once the higher-affinity interaction between the spacer domain and the A2 domain is established, the ADAMTS13 protease domain can access and cleave the Y1605-M1606 bond in the A2 domain of VWF. Note that we do not suggest that direct cleavage of VWF by ADAMTS13, without the initial tethering step (Step 1), cannot take place. For example, murine investigations with MDTCS demonstrated cleavage of VWF in vivo.³⁹  Such cleavage can arise only when the VWF A2 domain is unfolded by shear and can then attract ADAMTS13. The longer time taken for association to take place, compared with when ADAMTS13 is already associated with VWF, however, may result in less efficient proteolysis. Proteolysis in vivo clearly depends upon several factors, including the location of unfolding, the local flow conditions, and the extent of tethering of VWF onto cellular surfaces.