Protein S binding in relation to the subunit composition of human C4b-binding protein.
The human regulatory complement component C4b-binding protein (C4BP) circulates in plasma either as a free protein or in a bimolecular complex with the vitamin K-dependent protein S. The major form of C4BP is composed of 7 identical, disulfide-linked 70 kDa subunits (alpha-chains), the arrangement of which gives the C4BP molecule a spider-like appearance. Recently, we identified a unique 45 kDa subunit (beta-chain) in C4BP. We have now isolated a subpopulation of C4BP, which does not bind protein S. This C4BP species, which had a molecular weight slightly lower than that of the predominant form, was found to lack the beta-chain. Another lower molecular weight form of C4BP was also purified. It contained the beta-chain and was efficient in binding protein S. Its subunit composition was judged to comprise six alpha-chains and one beta-chain. These results indicate C4BP in plasma to be heterogeneous at a molecular level vis-a-vis subunit composition and/or protein S binding ability and provide support for the concept that the beta-chain of C4BP contains the single protein S binding site.
Hillarp
A
A
Department of Clinical Chemistry, University of Lund, Malmö General Hospital, Sweden.
Hessing
M
M
Dahlbäck
B
B
D015153:Blotting, Western; D002352:Carrier Proteins / Q000378:metabolism* / Q000648:ultrastructure; D003169:Complement Inactivator Proteins; D003165:Complement System Proteins; D006023:Glycoproteins / Q000378:metabolism*; D006801:Humans; D046911:Macromolecular Substances; D008970:Molecular Weight; D011485:Protein Binding; D017293:Protein S
D002352:Carrier Proteins; D003169:Complement Inactivator Proteins; D006023:Glycoproteins; D046911:Macromolecular Substances; D017293:Protein S; D003165:Complement System Proteins