Exploring the “minimal” structure of a functional ADAMTS13 by mutagenesis and small-angle X-ray scattering

Human ADAMTS13 is a multidomain protein with metalloprotease (M), disintegrin-like (D), thrombospondin-1 (T), Cys-rich (C) and spacer (S) domains, followed by 7 additional T and 2 CUB domains. ADAMTS13 inhibits the growth of von Willebrand factor (VWF)-platelet aggregates by cleaving the cryptic Tyr 1605 -Met 1606 bond in the VWF A2 domain. ADAMTS13 is regulated by substrate-induced allosteric activation: without shear stress, the distal T8-CUB domains markedly inhibits VWF cleavage, and binding of VWF domain D4 or selected monoclonal antibodies (MAbs) to distal ADAMTS13 domains relieves this autoinhibition. By small angle X-ray scattering (SAXS), ADAMTS13 adopts a hairpin-like conformation with distal T7-CUB domains close to the proximal MDTCS domains and a hinge point between T4 and T5. The hairpin projects like a handle away from the core MDTCS and T7-CUB complex and contains distal T domains that are dispensable for allosteric regulation. Truncated constructs that lack the T8-CUB domains are not autoinhibited and cannot be activated by VWF D4 but retain the hairpin fold. Allosteric activation by VWF D4 requires T7, T8 and the 58 amino acid residue linker between T8 and CUB1. Deletion of T3 to T6 produced the smallest construct examined (delT3-6) that could be activated by both MAbs and VWF D4. Columba livia (pigeon) ADAMTS13 (pADAMTS13) resembles human delT3-6, retains normal activation by VWF D4, and has a SAXS envelope consistent with amputation of the hairpin containing the dispensable T domains of human ADAMTS13. Our findings suggest that human delT3-6 and pigeon ADAMTS13 approach a “minimal” structure for allosterically-regulated ADAMTS13.