The non-glycosylated N-terminal domain of human thrombopoietin is a molten globule under native conditions.

Human thrombopoietin (hTPO) is a primary hematopoietic growth factor that regulates megakaryocytopoiesis and platelet production. The non-glycosylated form of 1-163 residues of hTPO (hTPO ) including the N-terminal active site domain (1-153 residues) is a candidate for treating thrombocytopenia. However, the autoantigenicity level of hTPO is higher than that of the full-length glycosylated hTPO (ghTPO ). In order to clarify the structural and physicochemical properties of hTPO , circular dichroism (CD) and differential scanning calorimetry (DSC) analyses were performed. CD analysis indicated that hTPO undergoes an induced-fit conformational change (+19.0% for helix and -16.7% for β-strand) upon binding to the neutralizing antibody TN1 in a manner similar to the coupled folding and binding mechanism. Moreover, DSC analysis showed that the thermal transition process of hTPO is a multistate transition; hTPO is thermally stabilized upon receptor (c-Mpl) binding, as indicated with raising the midpoint (T ) temperature of the transition by at least +9.5 K. The conformational variability and stability of hTPO indicate that hTPO exists as a molten globule under native conditions, which may enable the induced-fit conformational change according to the type of ligands (antibodies and receptor). Additionally, CD and computational analyses indicated that the C-terminal domain (154-332 residues) and glycosylation assists the folding of the N-terminal domain. These observations suggest that the antibody affinity and autoantigenicity of hTPO might be reduced, if the conformational variability of hTPO is restricted by mutation and/or by the addition of C-terminal domain with glycosylation to keep its conformation suitable for the c-Mpl recognition.