Helical arrangement of VWF in tubular striations of WPBs. (A) The orderly twisting of the tubules within WPBs is illustrated by the surface rendered tubules on a tomographic slice. Three tubules stop halfway into the WPB (arrowheads) and 2 tubules display kinks (arrows). (B) Reconstruction of VWF tubules assembled in vitro at pH 6.2 from purified dimeric D′D3 and D1D2 domains. Left, whole tubule demonstrating an outside diameter of 25 nm. Middle, cutaway view shows the inner diameter of the tubule of 12 nm. Right, arrangement of the different domains within the helix. The D′D3 domain in the center is flanked by 2 D1D2 domains (because the orientation of the propeptide is not yet known, a and b represent either D1 or D2). The arrows indicate 4 distinct noncovalent domain-domain interactions. Panel B was adapted from Huang et al.⁴⁸