LIG_FXI_DFP_1

The contact activation system (CAS) is central to the crosstalk between coagulation and inflammation and contributes to diverse disorders affecting the cardiovascular system. There are two branches of the CAS: 1) the inflammatory branch activates contact factors FXII and prekallikrein (PK) on the surface of endothelial cells, resulting in the release of the peptide bradykinin (BK), while 2) the plasma coagulation branch activates FXII and FXI on the surface of platelets. CAS factors seem to serve a supportive role in normal hemostasis, rather than a major role in initiating clot formation as evidenced by patients with FXII or FXI deficiency exhibiting no severe bleeding tendencies (Pathak,2018).
Factor XI (FXI) is the zymogen of a plasma protease, factor XIa (FXIa) that contributes to thrombin generation during blood coagulation by proteolytic activation of several coagulation factors, and most notably factor IX (FIX). FXI is structurally distinct from the vitamin K-dependent proteases that form the core of the vertebrate coagulation mechanism. Rather, it is a close homolog of prekallikrein (PK), a component of the plasma kallikrein-kinin system. Since the duplication of the ancestral gene FXI has undergone important structural changes that are required for its procoagulant activities (Mohammed,2018; Pathak,2018; Wong,2016). Uniquely among coagulation factors, FXI circulates as a disulfide-linked dimer. Each subunit has four apple domains (A1-4) (also named PAN domains PFAM: PF00024) and a C-terminal trypsin-like catalytic domain. Each apple domain consists of seven β-strands supporting a single α-helix, and the four of them assemble into a saucer-shaped disc. While A4 domains provide the dimerization interface, the two A2 domains are separated furthest from each other in the FXI dimer. PK has the same domain structure but is a monomeric protein. No other proteins with a similar domain arrangement have been described (Wong,2016).
Almost all FXI and most of PK circulate in complex with high molecular weight kininogen (HK), a highly glycosylated 120-kDa plasma protein with 6 domains. The physiologic importance of the PK-HK interaction seems clear as HK cleavage by α-kallikrein (activated PK) liberates the vasoactive peptide bradykinin from HK, and HK enhances PK activation by FXIIa. The importance of the FXI-HK interaction is less clear (Mohammed,2018).
Early studies identified that a region within the C-terminal D6 domain of HK, with at least 58 amino acids (HK residues 556-613) is required for normal FXI binding, and 31 amino acids for PK binding (Tait,1987), is responsible for the binding of FXI and PK A2 domains (Renne,2002; Renne,2002).
A dedicated random peptide-binding screen with FXI identified the DFP core motif (precisely a [FYWHIL].DF [PD] motif pattern) as an optimal binding partner for FXI (Wong,2016). This DFP motif was then detected not only within the C-terminal D6 domain of HK, but also in other extracellular proteins that could potentially interact with FXI and PK, e.g. laminin and collagen V. X-ray structures of the complexes between FXI and the corresponding motif of HK (5I25), laminin A1 (5EOD), and P39 peptide from the random screen (5EOK) show that the motifs bind to the same site on FXI. The Phe residue of the DFP occupies a major hydrophobic pocket, while the hydrophobic/aromatic residue preceding the DFP core motif inserts into a more minor pocket on the 2nd apple domain of FXI. The dimeric FXI orients 2 bound DFP peptides onto the same surface (Wong,2016). Surface plasmon resonance (SPR) studies also confirmed the binding of full-length heterotrimeric complexes, laminin-111, -411 and -511 to FXI, as well as the binding of a second motif instance within HK and one in collagen V (Wong,2016).
Efficient coagulation factor cleavage reactions typically take place only on a designated surface. While vitamin K–dependent coagulation factors have an N-terminal Gla domain that is capable of binding directly to negatively charged phospholipids present in activated platelets, FXI does not have a dedicated domain for this purpose. Laminins play important roles in plasma coagulation and thrombus formation, and the interaction between platelet integrins and laminins has been well characterized. Also, laminin has been previously shown to provide a surface that can promote plasma coagulation and thrombus formation in the absence of platelets (Schaff,2013; White-Adams,2010). Finally, HK and coagulation FXII (which activates FXI) have both previously been reported to bind laminin (Schousboe,2006; Schousboe,2009). Therefore the interactions observed for FXI with laminin and collagen V are most probably important for its colocalization with other intrinsic pathway coagulation factors to sites in the extracellular matrix (Wong,2016).
In sum, the HK cofactor and components of the extracellular matrix all seem to employ DFP motifs to interact with FXI and PK (Wong,2016).