The Fibronectin Type II Domain of Factor XII Ensures Zymogen Quiescence

Author:

Clark Chantal C.1,Hofman Zonne L. M.12,Sanrattana Wariya1,den Braven Lyanne1,de Maat Steven1,Maas Coen1

Affiliation:

1. Department of Clinical Chemistry and Haematology, University Medical Center Utrecht, Utrecht University, Utrecht, The Netherlands

2. The Laboratory for Translational Immunology, University Medical Center Utrecht, Utrecht University, Utrecht, The Netherlands

Abstract

AbstractFactor XII (FXII) zymogen activation requires cleavage after arginine 353 located in the activation loop. This cleavage can be executed by activated FXII (autoactivation), plasma kallikrein (PKa), or plasmin. Previous studies proposed that the activation loop of FXII is shielded to regulate FXII activation and subsequent contact activation. In this study, we aimed to elucidate this mechanism by expressing and characterizing seven consecutive N-terminally truncated FXII variants as well as full-length wild-type (WT) FXII. As soon as the fibronectin type II domain is lacking (FXII Δ1–71), FXII cleavage products appear on Western blot. These fragments display spontaneous amidolytic activity, indicating that FXII without the fibronectin type II domain is susceptible to autoactivation. Additionally, truncated FXII Δ1–71 is more easily activated by PKa or plasmin than full-length WT FXII. To exclude a contribution of autoactivation, we expressed active-site incapacitated FXII truncation variants (S544A). FXII S544A Δ1–71 is highly susceptible to cleavage by PKa, indicating exposure of the activation loop. In surface binding experiments, we found that the fibronectin type II domain is non-essential for binding to kaolin or polyphosphate, whereas the following epidermal growth factor-like domain is indispensable. Binding of full-length FXII S544A to kaolin or polyphosphate increases its susceptibility to cleavage by PKa. Moreover, the activation of full-length WT FXII by PKa increases approximately threefold in the presence of kaolin. Deletion of the fibronectin type II domain eliminates this effect. Combined, these findings suggest that the fibronectin type II domain shields the activation loop of FXII, ensuring zymogen quiescence.

Funder

Netherlands Thrombosis Foundation and Landsteiner Foundation for Blood Transfusion Research

Utrecht University Medical Center

Royal Thai Government

Publisher

Georg Thieme Verlag KG

Subject

Hematology

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