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Studies on the interaction between vitamin K-dependent protein S and complement regulator C4b-binding protein:
1Department of Laboratory Medicine, Division of Clinical Chemistry, Lund University, University Hospital, Malmö, Sweden. Joanna.Webb@klkemi.mas.lu.se
Insights
Protein S directs C4b-binding protein (C4BP) to apoptotic cells via phosphatidylserine, retaining C4BP
Area of Science:
- Immunology
- Molecular Biology
Background:
- The complement system is crucial for innate immunity but requires strict regulation to prevent host cell damage.
- C4b-binding protein (C4BP) is a key regulator, composed of alpha and beta chains, binding C4b and protein S.
- Protein S, a vitamin K-dependent protein, circulates bound to C4BP, with free protein S acting as an anticoagulant cofactor.
Purpose of the Study:
- To elucidate the functional significance of the C4BP-protein S complex formation.
- To investigate the role of protein S in localizing C4BP to apoptotic cells.
- To identify the binding interaction between C4BP and protein S.
Main Methods:
- Utilized recombinant proteins with introduced mutations based on a 3D-homology model of the C4BP beta-chain.
- Performed binding studies to characterize the interaction between C4BP and protein S.
- Investigated the localization of C4BP on apoptotic cells mediated by protein S.
Main Results:
- Demonstrated that protein S localizes C4BP to the surface of apoptotic cells by binding to exposed phosphatidylserine.
- Confirmed that C4BP bound to apoptotic cells via protein S retains its ability to bind C4b.
- Identified a specific hydrophobic binding site for protein S on C4BP.
Conclusions:
- Protein S acts as a bridge, targeting the complement regulator C4BP to apoptotic cells.
- This localization mechanism ensures complement regulation at apoptotic cell sites, preventing inflammation.
- The study reveals the molecular basis of the C4BP-protein S interaction.
Abstract:
Complement is a cascade-like system that is part of the innate immune defence. It is an explosive system, potentially harmful also for the host cells, and needs to be strictly regulated. An important down-regulator of complement is C4b-binding protein (C4BP). C4BP contains two different types of subunits, seven identical alpha-chains and one unique beta-chain. The alpha-chains bind to C4b, C4BP's target in the complement system. The beta-chain binds to vitamin K-dependent protein S. Approximately 70% of all protein S in plasma circulates in a high affinity complex with C4BP. Free protein S, the remaining 30%, functions as an important cofactor in the anticoagulant system. The reason for the complex formation between C4BP and protein S has remained an intriguing enigma. Protein S has a very high affinity to negatively charged phospholipids for protein S. One area where such phospholipids are present is the surface of the apoptotic cell, where the exposure of phosphatidylserine is an early event. Physiological apoptosis is characterized by a lack of inflammatory response in surrounding tissues, indicating that cells are rapidly cleared before leaking cytoplasmic components into the extracellular space. A number of studies demonstrate that early complement proteins are important for the removal of apoptotic cells, but that subsequent assembly of later complement components and anaphylatoxin release must be prohibited in order not to provoke an inflammatory response. We demonstrate that protein S localizes C4BP to the surface of apoptotic cells via binding to the exposed phosphatidylserine. The C4BP attached to the apoptotic cell through protein S was still able to bind C4b, suggesting that C4BP retains its physiological function also when localized to the apoptotic cell surface. In addition, we have also pinpointed a hydrophobic binding site for protein S on C4BP. The binding studies between C4BP and protein S were performed on recombinant proteins where mutations had been introduced. Mutations were chosen based on a 3D-homology model of the C4BP beta-chain.