Trans homophilic contacts between adjacent IgD1/D2 and IgD2/D2 domains. (A-B) Interchain IgD1/D2 interfaces present in the crystal lattice formed by the 2 PECAM-1 IgD1-D2 molecules in the asymmetric unit. (C-D) Interchain IgD2/D2 interfaces in the crystal lattice formed by the 2 PECAM-1 IgD1-D2 molecules asymmetric unit. (E) The intrachain interface between IgD1 and IgD2. Interfacial residues are shown as sticks with red oxygens and blue nitrogens. Carbohydrates are shown as sticks with green carbons. Hydrogen bonds are dashed in green. Residues that are important for homophilic binding, as identified by previous mutagenesis studies, are boxed. The equivalent residues that differ in mouse PECAM-1 are shown in parentheses. The interfacial residues are also indicated in Figure 2.