The cDNA sequence encoding the human complement‐associated protein, SP‐40,40, is reported. The two chains of SP‐40,40 are coded in a single open reading frame on the same mRNA molecule, indicating the existence of a biosynthetic precursor protein which matures post‐synthetically by the proteolysis of at least one peptide bond. The precursor is preceded by a signal sequence for vectorial export and contains six N‐linked glycosylation sites distributed equally between the two chains of the structure. The sequence of the SP‐40,40 precursor bears a 77% identity to a rat sulphated glycoprotein‐2 (SGP‐2) which is the major secreted product of Sertoli cells. The presence of SP‐40,40 within human seminal plasma at levels comparable to those in serum was demonstrated, indicating that SP‐40,40 and SGP‐2 are serum and seminal forms of the same protein. A sequence of 23 amino acids within the beta‐chain of SP‐40,40 exhibited significant homology to corresponding segments located within complement components C7, C8 and C9. The short cysteine‐containing motif represented the only evidence of a possible vestigial relationship between SP‐40,40 and other complement components. The precise role of SP‐40,40 is not known in either blood or semen but the present findings document an intriguing link between the immune and the reproductive systems.

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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 The invariant IgV disulfide loop has been replaced by a unique, short loop involving an unusual cysteine which is conserved in the CD8 alpha-chains of man, mouse, and rat.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.