Truncated Semenogelin I Binds Zinc and Is Cleaved by Prostate-Specific Antigen

ABSTRACT: Semenogelins I and II are major coagulum-forming proteins in semen, and they are secreted mainly by the seminal vesicles. These proteins bind Zn²⁺ and act as substrates for prostate-specific antigen and transglutaminase. A variant semenogelin I lacking 60 amino acids has been described that occurs in different populations with an allele frequency of 1%–3%. To better understand the function of the semenogelins in vivo, our aim was to characterize the properties of the variant form and compare with the wild type. Recombinant proteins were synthesized in insect cells. Binding of Zn²⁺ was studied by titration of metal ions in the presence of a zinc (II) fluorophore chelator. SDS-PAGE was used to visualize the results of cleavage by prostate-specific antigen and cross-linking with transglutaminase. We found that the truncated and wild-type semenogelin molecules had similar Zn²⁺-binding properties (ie, a stoichiometry of at least 9–10 mol per mol of protein and an average dissociation constant of 5 μmol/L per site), and they showed also similar susceptibility for degradation by prostate-specific antigen. Furthermore, like the wild-type form, the truncated semenogelin I was able to serve as a substrate for transglutaminase. These findings imply that the studied characteristics do not depend on a well-defined tertiary structure, or that the deletion has no major effect on the structure responsible for these features.