Background: Factor XII (FXII) is proteolytically converted to the protease (FXIIa) by kallikrein or autoactivation on negatively charged surfaces. The FXII non-catalytic heavy chain (HC) contains six domains: (from the N-terminus) the fibronectin type II (FnII), epidermal growth factor-1 (EGF-1), fibronectin type I (FnI), EGF-2, and kringle domains, and the proline-rich region (PRR). FXII shares this arrangement with its homolog hepatocyte growth factor activator (HGFA). The FXII-HC mediates binding to surfaces during contact activation, and inhibits FXII activation by kallikrein in the absence of surface by blocking access to the activation cleavage site. The roles of the individual HC domains in these processes is incompletely understood, but replacing the FXII-HC with the HGFA-HC disrupts both functions.

Aims: Determine the importance of FXII-HC domains to FXII autoactivation and activation by kallikrein.

Methods: A panel of FXII variants with one HC domain replaced with the corresponding HGFA domain was prepared. FXII with the FnII replacement did not express, so FXII lacking FnII (DFnII) was used. Protein activation was assessed with chromogenic assays.

Results: The FXII/HGFA EGF-1 and kringle chimeras demonstrated defects in autoactivation in the presence of polyanionic polyphosphate. Interestingly EGF-1 autoactivated on a positively charged surface (polyethyleneimine). Other chimeras activated similarly to FXII. DFnII and the EGF-1, FnI, EGF-2 and kringle chimeras, but not the PRR chimera, showed significantly enhanced activation by kallikrein in the absence of polyphosphate compared to FXII.

Conclusions: The FXII EGF-1 and kringle domains appear to play important roles during autoactivation on polyphosphate. In the case of EGF1, this may include a charge interaction with the surface. The ability of FXII-HC to restrict activation by kallikrein in the absence of surface is sensitive to perturbations throughout the HC, suggesting an alteration in the precise positioning of the HC relative to the activation cleavage site.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/structure-function-relationships-in-factor-xii-studied-with-factor-xii-hgfa-chimeras/