The initial crude homology models were built using MOD-ELLER and refined using modules for loop refinement and CHARMM minimization in DS2.5. Thus, the final structures were of high quality. The Profiles-3D scores of all amino acid residues in the four structures were greater than zero (Fig. 2) , while an evaluation of the stereochemical quality of the models using PROCHECK showed that no residues were in the disallowed regions of the Ramachandran plots (Fig. 3) . This indicated that the backbone dihedral angles, phi and psi, of the four structure models were reasonably accurate.