Article: Protein S

Protein S is a vitamin K-dependent plasma glycoprotein synthesized in the liver. In the circulation, Protein S exists in two forms: a free form and a complex form bound to complement protein C4b.

Function

The best characterized function of Protein S is its role in the anti coagulation pathway, it functions as a cofactor to Protein C in the inactivation of Factors Va and VIIIa. Only the free form has cofactor activity.

Protein S can bind to negatively charged phospholipids via the carboxylated GLA domain. This property allows Protein S to function in the removal of cells which have turned into apoptosis. Apoptosis is a form of cell death that is used by the body to remove unwanted or damaged cells from tissues. Cells that have turned into apoptosis express negatively charged phospholipids on the cell surface. These negatively charged phospholipids are recognized by phagocytes such as macrophages. Protein S can bind to the negatively charged phospholipids and function as a bridging molecule between the apoptotic cell and the phagocyte. The bridging property of Protein S enhances the phagocytosis of the apoptotic cell.

The gene (PROS1) is located on the third chromosome (3p11.1-q11.2).

Protein S deficiency is a rare blood disorder which can lead to an increased risk of thrombosis.