In addition to its known activity in hemostasis, the A1 domain of the von Willebrand factor (VWF) glycoprotein has a direct role in VWF tubule assembly for storage in Weibel-Palade bodies, according to a recent study published in Blood.
“It was previously observed that the presence of the A1 domain in recombinant VWF amino-terminal fragments led to more robust tubule formation in vitro and that the A1 domain was required for tubule formation in vivo,” the researchers explained in their report.
The team aimed to determine the structural basis for the discrepancy between the geometries of in vitro vs in vivo VWF tubules.
They constructed VWF tubules containing different VWF protein domains in vitro, including the D1D2 propeptide, D’D3 segment, dimeric D’-A1 segment, D’-A3 segment, or D’-C6 segment. They also reconstituted the D1-A1 segment containing the R763G mutation, which results in the propeptide remaining covalently attached.
The researchers analyzed the tubules using cryogenic electron microscopy and compared those assessed in previous studies, composed of the D1D2 and D’D3 segments, with the other tubules.
With 3-dimensional reconstructions following cryo-EM, the team found the formation of tubules with the helical geometry observed for VWF in intracellular Weibel-Palade bodies requires the A1 domain but not the A2 or A3 domains.
They reported that the tubules with and without the A1 domain appeared similar but discovered the A1 domain is inserted between helical turns of the D-assembly tubule, replacing interhelical contacts, altering helical parameters, and leading to increased robustness of tubule.
“In summary, we show that the VWF A1 domain is directly involved in VWF tubule assembly, and likely disassembly, complementing the well-known role of the A1 domain in hemostasis following secretion,” concluded the researchers.
Javitt G, Yeshaya N, Khmelnitsky L, Fass D. Assembly of von Willebrand factor tubules with in vivo helical parameters requires A1 domain insertion. Blood. 2022;140(26):2835-2843. doi:10.1182/blood.2022017153