Protein C

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Introduction

The protein C system controls blood coagulation by regulating factor VIIIa (FVIIIa) and factor Va (FVa), which are cofactors for activating factor X and prothrombin, respectively. Vitamin K-dependent protein C, a key component of this system, circulates in the blood as a zymogen for an anticoagulant serine protease and is efficiently activated by thrombin on the endothelial cell surface. Protein C is further stimulated and activated by the endothelin C receptor (EPCR). Activated protein C (APC) and its cofactor protein S inhibit coagulation by degrading FVIIIa and FVa on the surface of phospholipid membranes. Protein S and intact FV are necessary for APC to degrade FVIIIa. APCs have anti-inflammatory and anti-apoptotic functions in addition to their excellent anticoagulant properties, which play an important role in APCs binding to EPCR and cleaving protease-activated receptor 1 (PAR-1). The protein C system is physiologically essential, and genetic defects affecting the system are the most common risk factors for venous thrombosis. The proteins of the protein C system are composed of multiple domains, and structural information for several proteins has been obtained. The mysteries of the protein C system are gradually being solved, giving us new insights into this complex molecule at the atomic level.

Activation of Protein C on the Surface of Endothelial Cells

Protein C consists of four domains, a γ-carboxyglutamic acid residue (Gla)-rich domain, two epidermal growth factor (EGF)-like domains, a short activation peptide, and a serine protease domain (SP) (Figure 1).

Figure 1. Schematic representation of blood coagulation and the protein C anticoagulant system (Dahlbäck, B.; Villoutreix, B.O. 2005)

The binding of Gla residues to calcium is important for proper folding of this domain. The Gla domain of protein C/APC binds and interacts with phospholipid membranes and EPCR, both of which are essential for the physiological function of protein C (Figures 1 and 2). All vascular endothelium contains TM, especially in the capillaries. High concentrations of TM in the capillaries ensure that thrombin binds to TM, and the procoagulant properties of thrombin are lost on binding to TM because TM occupies the exosite I important for thrombin, thereby blocking its interactions with other thrombin-binding proteins. TM is a type I membrane protein comprising an N-terminal type C lectin domain followed by 6 EGF-like domains, a Ser/Thr rich region containing a chondroitin sulfate side chain, a transmembrane section, and a short cytoplasmic tail. Thrombin bound to TM is potently inhibited by antithrombin (AT) and protein C inhibitors. Thus, TM is an activator that converts thrombin to protein C and accelerates thrombin inhibition.

Figure 2. Overall view of the EPCR-protein C-T-TM complex (Dahlbäck, B.; Villoutreix, B.O. 2005)

Protein S Affects the Complement System and the Phagocytosis of Apoptotic Cells

Protein S is a vitamin K-dependent protein that contains an N-terminal phospholipid-binding Gla domain, a thrombin-sensitive region, 4 EGF-like domains, and 2 laminin G-type (LamG) domains in its three-dimensional structure (Figure 3). Protein S has high affinity for phospholipid membranes and forms a membrane-bound complex with APC. The domains of protein S are important for the interaction with APC. It has been suggested that the interaction of APC and protein S can decrease the distance between the active site of APC and the phospholipid membrane, which may be important for the proper localization of the cleavage site.

Figure 3. Protein S and the protein S-C4BP complex (Dahlbäck, B.; Villoutreix, B.O. 2005)

Antiinflammatory and Antiapoptotic Effects of the Protein C Pathway

In addition to acting as anticoagulants, components in the protein C system have other biological effects. Studies have shown that protein C and APC can directly inhibit the adhesion of neutrophils to the endothelial cell surface and the transmigration of neutrophils. The lectin domain of TM has direct anti-inflammatory properties, and studies of mice having a selective deficiency of the TM-lectin domain have shown that this domain reduces leukocyte adhesion and extravasation. Both protein S and protein S-C4BP complex have excellent antiinflammatory properties. C4BP acts as a potent regulator of the complement system, and the protein S-C4BP complex is thought to localize on phospholipid membranes, which, for example, on apoptotic cells. APCs also have direct anti-inflammatory and anti-apoptotic properties on a variety of cell types in vivo and in vitro. These properties are both influenced by the presence of EPCR and protease-activated receptor 1 (PAR-1) in the membrane, as well as proteolytic cleavage of PAR-1 (Figure 4).

Figure 4. APC bound to EPCR activates PAR-1 (Dahlbäck, B.; Villoutreix, B.O. 2005)