Abstract Number: OC 09.2
Meeting: ISTH 2021 Congress
Background: The first ever approved nanobody therapy – Caplacizumab targets the discontinuous autoinhibitory module (AIM) of Von Willebrand Factor (VWF) which consists of flanking regions of the A1 domain that unfold under shear force and activate VWF. A mechanism for this has been suggested but no structural evidence exists.
Aims: To capture VWF-A1 domain in a novel conformation using the monomeric form of Caplacizumab – VHH81 and solve the associated structure.
Results: In the structure, many residues, including residues 1262-1267 and some in the A1 domain, are in direct contact with all three complementarity-determining regions (CDR) loops of VHH81. For instance, VWF residue R1274 forms a salt bridge to the side chain of E105 in CDR3, and residues 1262-1267 pack around Y32 in CDR1. Also pertinent to this study is the hydrogen bond between side chains of E1264 and R1341, which has not been observed in any previous structures of A1. Moreover, comparison of the AIM-A1/VHH81 complex structure to previously reported A1 structures, for example when bound to GPIbα (PDB:4C2B) reveals that the largest difference in VWF conformation lies in the α1β2 loop, as well as N and C-terminal AIM residues, such that these residues appear to move away from the α1β2 loop upon binding of GPIbα. In the AIM-A1/VHH81 complex, with residues 1463-1466 in position close to the α1β2 loop, the unresolved residues beyond 1466 (i.e. residues 1467-1481) would clearly interfere with GPIBα binding to A1.
Conclusions: We provide the first structural evidence of the tensile conformation changes that activate VWF-A1 as well as demonstrating how VHH81 exploits this to non-competitively inhibit the binding of VWFA1 to GPIbα.
To cite this abstract in AMA style:Brown A, Emsley J, Arce N, Li R. Crystal Structure of the von Willebrand Factor A1 Domain – Caplacizumab Complex [abstract]. Res Pract Thromb Haemost. 2021; 5 (Suppl 2). https://abstracts.isth.org/abstract/crystal-structure-of-the-von-willebrand-factor-a1-domain-caplacizumab-complex/. Accessed November 29, 2023.
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