Methods.

Detailed procedures used for the overexpression and purification of the intact mutant nTPx, and for the crystallization and data collection of tmTPx have been previously reported. 8, 12 The X-ray data collected at 100 K at the Pohang Light Source (Korea) were used for the structure determination. The structure was easily solved in space group P3 2 by molecular replacement using the computer program Beast 13 and a model based on the X-ray structures of human TPxB 14 (PDB code, 1qmv) and Streptococcus pneumoniae TPx (PDB code, 1psq). The model was built using the program O. 15 Refinement was carried out using CNS 16 with positional refinement, followed by simulated annealing to 4,000 K, and finally, individual B-factor refinement. Manual rebuilding was carried out between each run with σ A-weighted, 2Fo-Fc maps. Water molecules were picked up from the 2Fo-Fc and Fo-Fc difference maps at the bases of peak heights and distance criteria; however, these were discarded if the thermal parameter after refinement was above 60 Å 2. During the final stages of refinement, the maximum likelihood method implemented in the program Refmac 17 was used to refine atomic positions and isotropic B-factors. There were no electron densities for the N-terminal Ser57 and Ser58 residues and the C-terminal Glu215. A summary of data and refinement statistics is shown in Table I. Structural figures were generated using the program PyMOL (http://pymol. sourceforge. net/). The coordinates have been deposited in the PDB under accession code 2a4v.