title: Characterization of the interaction between human protein S and C4b-binding protein (C4bp).
abstract: C4b-binding protein, C4bp, is a regulatory factor of the complement system and is also known to be a binding protein of vitamin K-dependent coagulation factor, protein S. Whereas the C4b-binding site is known to be located in the middle part of the subunit chains of C4bp, the location and properties of protein S-binding site are uncertain. Therefore, we have examined the characteristics of the interaction between human protein S and C4bp. Proteolysis of C4bp-protein S complex with chymotrypsin yielded N-terminal-derived 48-kDa fragments of C4bp subunit chains and a C-terminal-derived 160-kDa core fragment of C4bp, to which protein S was still bound. This result suggested that the protein S-binding site is located in the core domain of C4bp. Gel filtration of guanidine-treated C4bp-protein S complex in the absence of guanidine resulted in the separation of C4bp and protein S. Binding assay with 125I-labeled protein S showed that the guanidine-treated C4bp lacked the protein S-binding activity. This result suggests that the protein S-binding site in C4bp is denatured irreversibly by guanidine treatment and therefore seems to be dependent on a specific conformation of C4bp. The C4bp-binding site of protein S was lost upon thrombin treatment, suggesting that the N-terminal thrombin-sensitive region of protein S may be related to the C4bp-binding site. Although free protein S was susceptible to chymotrypsin, leukocyte elastase, and cathepsin G, C4bp-bound protein S was found to be resistant to these proteases.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D002352:Carrier Proteins / Q000378:metabolism*; D002850:Chromatography, Gel; D003169:Complement Inactivator Proteins; D006023:Glycoproteins / Q000378:metabolism*; D006146:Guanidines / Q000494:pharmacology; D006801:Humans; D006868:Hydrolysis; D005779:Immunodiffusion; D007457:Iodine Radioisotopes; D010447:Peptide Hydrolases; D017293:Protein S; D013917:Thrombin / Q000494:pharmacology
chemical_list: D002352:Carrier Proteins; D003169:Complement Inactivator Proteins; D006023:Glycoproteins; D006146:Guanidines; D007457:Iodine Radioisotopes; D017293:Protein S; D010447:Peptide Hydrolases; D013917:Thrombin