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C4b-binding protein (C4BP) beta-chain Short Consensus Repeat-2 specifically contributes to the interaction of C4BP
R H van de Poel1, J C Meijers, B Dahlbäck
1Thrombosis and Haemostasis Laboratory, Department of Haematology, University Medical Center Utrecht, 3508 GA Utrecht, The Netherlands.
Insights
C4b-binding protein (C4BP) regulates complement and protein S anticoagulant activity. Beta-chain SCR-2 specifically contributes to C4BP
Area of Science:
- Biochemistry
- Molecular Biology
- Immunology
Background:
- C4b-binding protein (C4BP) is a key regulator of the complement system and influences the anticoagulant activity of protein S.
- Protein S binding to C4BP reduces its cofactor activity for activated protein C, impacting anticoagulation.
- C4BP consists of alpha-chains and a beta-chain, each containing Short Consensus Repeats (SCRs).
Purpose of the Study:
- To clarify the role of C4BP's SCR-2 domain in binding to protein S.
- To resolve inconsistencies between previous studies regarding SCR-2's contribution to protein S interaction.
Main Methods:
- Construction and analysis of chimeric proteins involving C4BP beta-chain SCRs and alpha-chain SCRs fused to tissue-type plasminogen activator (tPA).
- Binding studies were performed using a novel chimera (Beta1alpha2) to assess the specific contribution of alpha-chain SCR-2.
Main Results:
- Previous studies suggested SCR-2 in both alpha and beta chains contributes to protein S binding, but with conflicting evidence.
- A chimera containing beta-chain SCR-1 and alpha-chain SCR-2 (Beta1alpha2) exhibited lower affinity for protein S compared to a chimera with SCR-1+2.
- This indicates that alpha-chain SCR-2 does not significantly contribute to protein S binding, and its effect may have been masked in earlier studies due to molecular structure.
Conclusions:
- The beta-chain SCR-2 domain specifically contributes to the interaction with protein S, in conjunction with SCR-1.
- Alpha-chain SCR-2 does not appear to play a significant role in protein S binding.
- These findings resolve discrepancies in prior research concerning the SCR-2 domain's function in C4BP-protein S interactions.
Abstract:
C4b-binding protein (C4BP) regulates the complement system and the anticoagulant activity of protein S. Protein S can bind to C4BP, resulting in a decreased cofactor activity of protein S for anticoagulant activated protein C. C4BP contains several identical a-chains and a single 3-chain. Each chain contains Short Consensus Repeats (SCRs). By making chimeras of 13-chain SCRs fused to tissue-type plasminogen activator (tPA chimeras), we found that 13-chain SCR-2 contributed to the interaction of 13-chain SCR-1 with protein S (van de Poel RHL, Meijers JCM, Bouma BN. J Biol Chem 274:15144-15150, 1999). Chimeras containing C4BP a-chains with SCR-1, SCR-l +2 or SCR-l +2+3 replaced by their 13-chain counterpart had affinities for protein S similar to C4BP (Hardig Y, Dahlb¿ck B. J Biol Chem 271:20861-20867, 1996). This was not in agreement with the finding that Beta-chain SCR-2 contributed to the interaction and could be explained by the possibility that alpha-chain SCR-2 in the alpha-chain chimeras contributed comparable with Beta-chain SCR-2 in the tPA chimeras. To investigate this we constructed a tPA chimera containing Beta-chain SCR-1 and alpha-chain SCR-2 (Beta1alpha2). Binding studies showed that Beta1alpha2 had a lower affinity compared with SCR-1 +2, indicating that alpha-chain SCR-2 did not contribute to the interaction. The difference with the alpha-chain chimeras may be explained by the fact that the alpha-chain chimeras were linked by their C-terminal cysteines, resulting in multiple binding sites in a single molecule. Thereby, the effect of a lower affinity of each alpha-chain chimera may have been masked. The studies performed here help to clarify the apparent inconsistencies in two previous reports about the contribution of the SCR-2 domain in C4BP to protein S binding. In conclusion, Beta-chain SCR-2 specifically contributes to the interaction of SCR-1 with protein S.