OC 15.1 - Crystal structure of the von Willebrand factor A1 domain in complex with a novel inhibitory nanobody

Background: Activation of von Willebrand factor (VWF) is a tightly controlled process governed primarily by local elements around its A1 domain. The O-glycosylated sequences flanking the A1 domain constitute a discontinuous and force-sensitive autoinhibitory module (AIM) . Anti-AIM nanobodies, are inhibitory to VWF platelet binding.

Aims: We aim to investigate a novel inhibitory nanobody that targets the VWFA1 domain and modulates its function. The high-resolution complex structureswill provide us with molecular basis of how these nanobodies inhibit and their potential as novel anti-thrombotic agents.

Methods: We will use X-ray crystallography to elucidate atomic details of a novel nanobody-VWFA1 complex in order to describe the mode of inhibition as competitive or allosteric. We will assess the overlap of the nanobody binding sites with other VWFA1 domain ligands and inhibitors including platelet glycoprotein (GP)Ibα using molecular graphics.

Results: A crystal structure of AIM-A1 complex with a novel nanobody was determined to 3.0Å resolution. In the nanobody complex structure, it interacts with the proximal portion of the CAIM and residues from the A1 domain. Additional structure in the proline rich region of the CAIM is observed together with the O-glycosylated residue T1468. The conformation of the NAIM is different to those observed in other VWFA1 crystal structures likely due to O-glycosylated residue .

Conclusion(s): Overall, this novel structure shows the nanobody binds to the AIM and stabilizes its conformation to assist VWF autoinhibition. This structure will facilitate future studies of VWF functions and modes of inhibition to target VWF in pathological settings.