Molecular cloning and characterization of the novel, human complement-associated protein, SP-40,40: a link between the complement and reproductive systems.

Open Access
Publisher Website
Google Scholar
Abstract
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.