This is the C-terminal oligomerization domain found in C4b-binding protein (C4BP), which contains 14 cysteines that form 7 intermolecular disulfide bridges. C4BP is a plasma glycoprotein complex of 570 kDa, which is mainly produced in the liver. C4BP is the major inhibitor of complement activation, the major isoform consists of 7alpha and one beta-chain where each alpha-chain comprises eight complement control domain proteins (CCPs) and a C-terminal oligomerization domain. This domain carries a stretch of 42-45 amino acids that has been shown to be required for ring formation [1].

1: Arranged sevenfold: structural insights into the C-terminal oligomerization domain of human C4b-binding protein. Hofmeyer T, Schmelz S, Degiacomi MT, Dal Peraro M, Daneschdar M, Scrima A, van den Heuvel J, Heinz DW, Kolmar H; J Mol Biol. 2013;425:1302-1317. PMID:23274142