Observed heparin-binding mode of PN1. (A) A single heparin disaccharide unit (yellow sticks) was built into electron density between 2 loosely associated monomers. This alternate asymmetric unit is “glued” together by heparin that is juxtaposed between the 2 highly basic heparin-binding sites (structural elements of both molecules are colored as in Figure 1, with remaining regions in gray or pink). (B) An extension of the disaccharide to a decasaccharide along the 2-fold axis nicely satisfies the charged groups along helix D (oriented as in Figure 1B).