Steve Duff of Precision BioLogic forwarded a reference to a mechanical and enzymatic mechanism that controls the length of von Willebrand factor (VWF) multimers. According to Zhang X, Halvorsen K, Zhang CZ, Wong WP, Springer TA: Mechanoenzymatic cleavage of the ultralarge vascular protein von Willebrand factor. Science 2009;324:1330-4, shear force exerted on the VWF molecule “unrolls” the molecule and exposes a portion of the A2 domain to the WVF-cleaving protease ADAMTS13. Ultra-large VWF multimers (ULVWF), tethered to the endothelial cell from which they originated, are more easily unrolled and cleaved than small multimers. Their length and ability to be cleaved make VWF molecules part of hemostasis control. The experimental model described involves some intriguing microtechnology termed “laser tweezers.”
Here is the process explained in an accompanying comment in : Christof J, Gebhardt M, Rief M. Biochemistry. Force signaling in biology. Science 2009; 324:1278-80.
VWF is initially secreted in a highly active ultralong form (ULVWF). Because hydrodynamic forces act along the whole contour of a molecule, the long ULVWF chains are more easily stretched in shear flow. However, this
increases the risk of causing thrombosis through premature aggregation of platelets. The body must therefore counteract and regulate uncontrolled activation of ULVWF. This regulation is also mediated by shear flow forces. If stretched, ULVWF can be cleaved at its domain A2 by the metalloprotease ADAMTS13. Tsai has speculated that domain A2 in the VWF monomer unfolds upon shear force–induced stretching of ULVWF, exposing an otherwise buried cleavage site for ADAMTS13. ADAMTS13 can then cleave ULVWF.
Zhang et al. now provide direct evidence for force-induced unfolding and cleavage of A2. Using optical tweezers, they directly tested the activation mechanism of ADAMTS13-mediated cleavage of A2 on the single-molecule level. They found that an individual A2 domain unfolds at forces of ~11 pN when stretched mechanically. When relaxed to lower forces, A2 remains unfolded for about 2 s before it refolds. In their assay, cleavage of A2 by ADAMTS13 occurs only in the unfolded configuration; A2 is not cleaved if the applied force remains below the unfolding threshold. Mechanical force thus acts as a cofactor for ADAMTS13.