Abstract
The activation peptide of blood coagulation factor XIII (AP-FXIII) has important functions
in stabilizing the FXIII-A2 dimer and regulating FXIII activation. Contributions of many of its 37 amino acids
to these functions have been described. However, the role of proline 36, which is
adjacent to the thrombin cleavage site at Arg37, has not yet been studied in detail.
We approached this question when we came across a patient with congenital FXIII deficiency
in whom we detected a novel Pro36Ser mutation. We expressed the mutant FXIII-A Pro36Ser
protein in Chinese hamster ovary cells and found that this mutation does not influence
FXIII-A expression but significantly inhibits proteolytic activation by thrombin.
The enzymatic transglutaminase activity is not affected as it can be induced in the
presence of high Ca2+ concentrations. We performed nuclear magnetic resonance analysis to investigate AP-FXIII–thrombin
interactions, which showed that the mutant Ser36 peptide binds less well to the thrombin
surface than the native Pro36 peptide. The Arg37 at the P1 position still makes strong interactions with the active site cleft but the P4–P2 residues (34VVS36) appear to be less well positioned to contact the neighbouring thrombin active site
region. In conclusion, we have characterized a novel mutation in AP-FXIII representing
only the fourth case of the rare FXIII-A type II deficiency. This case served as a
perfect in vivo model to shed light on the crucial role of Pro36 in the proteolytic
activation of FXIII-A. Our results contribute to the understanding of structure–function
relationship in FXIII.
Keywords
coagulation factor XIII - factor XIII activation peptide - congenital factor XIII
deficiency
