Visualization of an N-terminal fragment of von Willebrand factor in complex with factor VIII.

Binding to the von Willebrand factor (VWF) D'D3 domains protects factor VIII (FVIII) from rapid clearance. We performed single-particle electron microscopy (EM) analysis of negatively stained specimens to examine the architecture of D'D3 alone and in complex with FVIII. The D'D3 dimer ([D'D3]2) comprises 2 antiparallel D3 monomers with flexibly attached protrusions of D'. FVIII-VWF association is primarily established between the FVIII C1 domain and the VWF D' domain, whereas weaker interactions appear to be mediated between both FVIII C domains and the VWF D3 core. Modeling the FVIII structure into the three-dimensional EM reconstructions of [D'D3]2-FVIII ternary and quaternary complexes indicates conformational rearrangements of the FVIII C domains compared with their disposition in the unbound state. These results illustrate the cooperative plasticity between VWF and FVIII that coordinate their high-affinity interaction.