CRYSTAL STRUCTURE OF EXTRACELLULAR DOMAIN OF TISSUE FACTOR COMPLEXES WITH AN INHIBITORY FAB. Mingdong Huang, Rashid J. Syed, Enrico A. Stura, Wolfram Ruf, Thomas S. Edgington, and Ian A. Wilson. Department of Molecular Biology The Scripps Research Institute, La Jolla, CA 92037.

The macromolecular assembly of Tissue factor (TF) with factor VIIa plays the central role in the cellular activation of the blood coagulation cascades. An Fab (5G9) was found to be an effective immediate anticoagulant in plasma, which binds strongly to TF and displaces VIIa from the preformed TF-VIIa complex. We report here the crystal structure of the complex between Fab 5G9 and the extracellular domain of TF. The interdomain angle of TF in the complex is basically the same as for the free TF, demonstrating the strong interdomain interaction in TF. The current model of the complex is consistent with the TF mutagenesis data. For example, K169 and N171 interact with L1, L2, L3, and H3 CDR loops of Fab 5G9 with large contact area (138Ų), which explains when these residues are mutated to alanines both the binding of TF to Fab 5G9 and TF's function are greatly reduced. The TF-Fab complex is the first protein structure in P2 space group (based on current pdb release). The L shape of the complex molecule may explain the adoption of this rare space group for protein and the weak diffraction of the crystal. The crystal used for data collection diffracted to 3Šwith an overall I/s(I) of 7.5 and an Rsymm of 13%. The structure is currently refined to an R value of 24.0% and an R-free of 28.5% with strict NCS applied to the two NCS-related molecules in the asymmetric unit.