Abstractβ -Microseminoprotein (MSP) and cysteine-rich secretory protein 3 (CRISP-3) are abundant constituents of human seminal plasma. Immunoprecipitation and gel filtration of seminal plasma proteins combined with examination of the proteins in their pure form showed that MSP and CRISP-3 form stable, non-covalent complexes. CRISP-3 binds MSP with very high affinity, as evidenced by surface plasmon resonance. Due to far higher abundance of MSP in prostatic fluid, it manifests large overcapacity for CRISP-3 binding. Structural similarity with an MSP-binding protein from blood plasma suggests that CRISP-3 binds MSP through its aminoterminal SCP-domain.
KeywordsCysteine-rich secretory protein; β-Microseminoprotein; PSP94; Seminal plasma; Prostate cancer; Protein complex; SCP-domain; Mass spectrometry; Surface plasmon resonance Human cysteine-rich secretory protein 3 (CRISP-3; also known as SGP28) was first isolated from neutrophilic granulocytes, but it is also a constituent of blood plasma and exocrine secretions, such as saliva and seminal plasma [1]. It belongs to a family of CRISPs found in mammals and reptiles, which is characterized by 16 highly conserved cysteine residues of a total of 220-230 amino acids and 35-85% identity in the primary structure [2][3][4]. Recently, the crystal structure of a snake venom CRISP was elucidated [5]. This confirmed the previously proposed two-domain structure of CRISPs with a large N-terminal SCP domain (after sperm coating protein, an alternative name for murine CRISP-1) and a smaller, compact C-terminal domain, separated by a hinge-region [3,5].A novel protein with an N-terminal SCP domain has been identified in human blood plasma and is called PSP94-binding protein (PSPBP) as it binds to β -microseminoprotein (MSP), also known as prostate secretory protein of 94 amino acids (PSP94) [6]. MSP is one of the three predominant proteins secreted by the prostate gland, but it is also present in other body fluids, e.g., tracheobronchial fluid. In general, MSP-synthesis is associated with tissue producing [7,8].It is yet unknown whether MSP binds to PSPBP through the SCP domain or via the large C-terminal part of the molecule [6]. One way to answer this question is to investigate whether MSP binds other proteins with an SCP domain, and the abundance of both MSP and CRISP-3 in human seminal plasma prompted us to test the hypothesis that these proteins form high-affinity complexes.
Materials and methods
MaterialsHuman MSP was purified from seminal plasma with the method described for the purification of pig MSP [9]. Human CRISP-3 was purified from granulocytes [10]. Specific antisera were raised in rabbits against recombinant C-terminally truncated human CRISP-3 and native human MSP [1,11]. Semen was collected from healthy volunteers and allowed to liquefy for 1 h at room temperature (RT), followed by centrifugation to remove spermatozoa and was subsequently stored at −20 °C until use.
Gel filtrationProteins were separated according to their molecular size by gel filtration on a Su...