Comparison of A1 tubule binding and GPIbα binding. In the upper left is a diagram of hydrogen bonding interactions (dashed lines; not to scale) made by the A1 edge strand with either Asn381 in the VWF tubule or with a β-switch strand in the GPIbα complex. The 2 hydrogen bonds made with the Asn381 side chain are replaced by interstrand hydrogen bonds in the GPIbα complex. In the structure images, A1 is shown looking down the central β-sheet. A semitransparent surface of E1 (blue) is shown to indicate where it contacts A1 (left); the complex between A1 and GPIbα (PDB code 1SQ0) (right).