Proteolysis of VWF by ADAMTS13. VWF circulates in plasma as a multimeric molecule (A) that adopts a quiescent globular conformation. Each multimer is composed of disulphide linked VWF monomers (B). In its globular conformation, the A3 domain collagen binding site is exposed. ADAMTS13 can bind to this globular VWF via its TSP (5-8) and CUB domains (C), step 1. This enables VWF and ADAMTS13 complexes to form and circulate in plasma. Under elevated shear forces (which can occur on secretion, collagen binding, or passage through the microvasculature), VWF can unravel to expose A1 domain binding site for GPIbα. These shear forces also remove molecular plug formed by the vicinal disulphide bond in the A2 domain, which causes A2 domain unfolding (D), step 2 (see Figure 1). This unfolding reveals cryptic exosites that enable residues in the ADAMTS13 spacer domain to bind to the unfolded A2 domain (E), step 3 (see Figures 1D, 2B). Thereafter, a critical low-affinity interaction between D1614 and the Dis domain helps approximate and position the cleavage site (F), step 4 (see Figure 3). This enables further interactions between the MP domain to occur, including an essential interaction via an S3 subsite with L1603 in VWF (G), step 5 (see Figure 3). Together, these interactions allow the MP to engage via S1 and S1′ subsites with the cleavage site (YM; H), step 6, after which proteolysis can occur, step 7.