Protein-protein interactions between the FVIIa protease domain and the K1 domain of XK1. (A) Cryo-EM density map (shown as a mesh) and model of the TF/FVIIa/XK1 complex, with density for the 10H10 Fab, nanodisc, and MBP subtracted for clarity. Color scheme: FVIIa, orange; TF, purple; XK1, light sea green. Residues from 31 to 41 of the XK1 K1 domain, which form a hairpin-like conformation, are shown in dark red. The cryo-EM density map is shown at contour level 0.1, whereas the density for EGF2 is visible at higher contour levels. (B) View of the interaction between the protease domain of FVIIa and the K1 domain of XK1 with the density map at contour level 0.1. Boxes indicate areas of the structure magnified (C-D). Panels C-D show magnified views of specific regions of the FVIIa-XK1 interface boxed in panel B. Critical residues are shown as sticks and labeled, with carbon atoms colored according to the respective protein, oxygens in red, and nitrogen atoms in blue. Selected atomic distances are indicated with gray dotted lines. The backbones of G365 and G375 are shown in gray. The cryo-EM density in panels C-D is shown at contour level 0.13.