Shear flow has minimal role in unfolding of free von Willebrand factor

Von Willebrand factor (VWF) is an ultra-large polymeric blood protein which circulates in the blood and can act as a mechanical probe, responding to changes in flow to initiate platelet plug formation. Paradoxically, using single-molecule imaging our labs have previously observed the shear stress required to extend surface-tethered VWF is higher than the reported shear stress required to extend single-molecules of free, tumbling VWF. To resolve this inconsistency, we investigated the response of free VWF to shear flow using a single-molecule fluorescence imaging approach that we call PUlsed Laser Stroboscopic Imaging of Single-molecules (PULSIS).