Semenogelin I and semenogelin II, the major gel‐forming proteins in human semen, are substrates for transglutaminase

The major seminal vesicle secreted proteins in human semen, semenogelin I and semenogelin II, interact non‐covalently and via disulphide bridges to instantly form a coagulum upon ejaculation. The coagulum is liquefied after a few minutes due to the action of a prostatic serine protease, prostate‐specific antigen (PSA). In contrast to rat semen, which forms an insoluble plug within minutes of expulsion, no transglutaminase‐mediated cross‐linking has been demonstrated in ejaculated human semen. However, we here show that semenogelin I and semenogelin II, both in seminal vesicle fluid and purified from semen, are substrates for factor XIIIa, the fibrin cross‐linking transglutaminase. The cross‐linking of the semenogelins, which was conformation‐dependent, and the incorporation of a fluorescence‐labelled amine, were visualised by SDS/PAGE and Western blot. Purified semenogelin I and semenogelin II could be cross‐linked separately into complexes. Moreover, digestion of semenogelin with PSA produced fragments, some of which were cross‐linked into complexes by factor XIIIa. We also found that PSA was unable to release any semenogelin fragments during exposure of the high molecular‐mass complexes of cross‐linked semenogelin to active PSA.