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SP-40,40 is a two-chain serum protein which acts in vitro as a potent inhibitor of the assembly of the membrane attack complex of human complement. It contains 10 cysteine residues, the numbers and locations of which are conserved in several mammalian species. Evidence is presented that all the cysteine residues are involved in interchain (alpha-beta) disulphide bonds. There are no free cysteine residues. The disulphide bond motif established in this study for SP-40,40 is unique and bears no obvious homology to those complement components whose disulphide bonds have been assigned, nor is there any homology apparent between SP-40,40 and other multi-chain proteins containing disulphide bonds.

The CD8 Ag is a cell surface heterodimer which demarcates predominantly cytotoxic T cells which are restricted by class I MHC Ag. The disulfide bonds within the murine structure were assigned in this study and the alpha-beta-interchain bond involves one or more cysteine residues located in each chain proximal to the plasma membrane or included within it. The location of the intrachain disulfide loop within the CD8 beta-chain confirms its proposed structural homology to an IgV domain but no corresponding disulfide loop is present within the alpha-chain. The invariant IgV disulfide loop has been replaced by a unique, short loop involving an unusual cysteine which is conserved in the CD8 alpha-chains of man, mouse, and rat. Despite its lack of precedent in other Ig-related structures, this unusual disulfide loop can be parsimoniously accommodated into a modified domain which has retained the major features of the Ig structural motif.