Hagfish VWF A1 and A2 domain structures. (A) Comparative model of hagfish A1 (backbone ribbon representation on surface volume renderings) in comparison with the experimentally determined human A1 structure (PDB ID 1AUQ). The structures are highly superimposable with the exception of α4, where hagfish is truncated by 4 residues, similar to known bird, amphibian, and fish sequences, impacting the length of the helix and β4α4 loop conformation. (B) Superimposition of the hagfish A1 onto experimentally determined human A1 bound to GPIbα (green ribbon) (PDB ID 3SQ0). The many amino acids shown to contribute to the GPIbα binding interface that are conserved in hagfish A1 are highlighted (gray sticks) along with those few not conserved in hagfish A1 (yellow sticks). In addition, charge is conserved between hagfish and human among 5 of the 7 electrostatically charged residues, as is the hydrophobic character of 3 of 4 residues. (C) Comparative model structure of hagfish A2 in comparison with the experimentally determined human A2 structure (PDB ID 3ZQK). Like human, hagfish A2 is distinguished from other VWA domains by a relatively long unstructured loop at helix 4 (α4). (D) Modeling of the α4-less loop environment in hagfish A2. The α3- and α5-helix dipole moments are symbolized. The strictly conserved buried hydrophobic I1622 side chain atoms, and D1620 N-cap and R1624 C-cap side chain atoms are shown (yellow sticks). Although the hagfish α4-less region contains 5 additional residues compared with human, the α4-less loop environment is well conserved.