Structural and thermodynamic change of the Fab upon binding of human thrombopoietin

Arai, Shigeki; Tamada, Taro; Maeda, Yoshitake*; Kuroki, Ryota

The mouse antibody TN1 recognizes the human thrombopoietin (hTPO) that primarily stimulates megakaryocytopoiesis and platelet production. The TN1 neutralize the TPO activity preventing from the homo-dimerization of its receptor. In order to investigate the structural change of the TN1 upon antigen binding, the crystal structure of TN-1 Fab was determined to 2.1 resolution and was compared with that of TN1-Fab / hTPO complex (1V7M). It was found that each domain comprising TN1-Fab was strikingly similar (rmsd 0.6) between the antigen bound and unbound forms of TN1-Fab including three complementarity determining regions. The relative locations of the variable- and constant-regions of the TPO unbound form of TN1-Fab was slightly shifted from those of TN1-Fab / hTPO complex, which may be caused by difference of their crystal packing. These results indicate that the TN1-Fab need not accompany the conformational changes upon antigen binding.