Purification and preliminary X-ray crystallographic studies of beta-microseminoprotein from human seminal plasma

Three-dimensional structures of avian beta-microseminoproteins: insight from the chicken egg-specific beta-microseminoprotein 3 paralog

Beta‐microseminoproteins (MSMBs) are small disulfide‐rich proteins that are conserved among vertebrates. These proteins exhibit diverse biological activities and were mainly reported to play a role in male fertility, immunity, and embryogenesis. In this work, we focused on the chicken MSMB3 protein that was previously depicted as an egg antibacterial protein. We report that MSMB3 protein is exclusively expressed in the reproductive tissues of laying hens (in contrast to chicken MSMB1 and MSMB2 paralogs), to be incorporated in the egg white during the process of egg formation. We also showed that chicken MSMB3 possesses highly conserved orthologs in bird species, including Neognathae and Palaeognathae. Chicken MSMB3 was purified from egg white using heparin affinity chromatography and was analyzed by top‐down and bottom‐up proteomics. Several proteoforms could be characterized, and a homodimer was further evidenced by NMR spectroscopy. The X‐ray structure of chicken MSMB3 was solved for the first time, revealing that this protein adopts a novel dimeric arrangement. The highly cationic MSMB3 protein exhibits a distinct electrostatic distribution compared with chicken MSMB1 and MSMB2 structural models, and with published mammalian MSMB structures. The specific incorporation of MSMB3 paralog in the egg, and its phylogenetic conservation in birds together with its peculiar homodimer arrangement and physicochemical properties, suggests that the MSMB3 protein has evolved to play a critical role during the embryonic development of avian species. These new data are likely to stimulate research to elucidate the structure/function relationships of MSMB paralogs and orthologs in the animal kingdom.

Growth inhibition mediated by PSP94 or CRISP-3 is prostate cancer cell line specific

The prostate secretory protein of 94 amino acids (PSP94) has been shown to interact with cysteine-rich secretory protein 3 (CRISP-3) in human seminal plasma. Interestingly, PSP94 expression is reduced or lost in the majority of the prostate tumours, whereas CRISP-3 expression is upregulated in prostate cancer compared with normal prostate tissue. To obtain a better understanding of the individual roles these proteins have in prostate tumourigenesis and the functional relevance of their interaction, we ectopically expressed either PSP94 or CRISP-3 alone or PSP94 along with CRISP-3 in three prostate cell lines (PC3, WPE1-NB26 and LNCaP) and performed growth inhibition assays. Reverse transcription-polymerase chain reaction and Western blot analysis were used to screen prostate cell lines for PSP94 and CRISP-3 expression. Mammalian expression constructs for human PSP94 and CRISP-3 were also generated and the expression, localization and secretion of recombinant protein were assayed by transfection followed by Western blot analysis and immunofluorescence assay. The effect that ectopic expression of PSP94 or CRISP-3 had on cell growth was studied by clonogenic survival assay following transfection. To evaluate the effects of co-expression of the two proteins, stable clones of PC3 that expressed PSP94 were generated. They were subsequently transfected with a CRISP-3 expression construct and subjected to clonogenic survival assay. Our results showed that PSP94 and CRISP-3 could each induce growth inhibition in a cell line specific manner. Although the growth of CRISP-3-positive cell lines was inhibited by PSP94, growth inhibition mediated by CRISP-3 was not affected by the presence or absence of PSP94. This suggests that CRISP-3 may participate in PSP94-independent activities during prostate tumourigenesis.

Crystal structure of prostate secretory protein PSP94 shows an edge-to-edge association of two monomers to form a homodimer

Several recent genome-wide association studies have linked the human MSMB gene, encoding prostate secretory protein of 94 residues (PSP94), with prostate cancer susceptibility. PSP94 is one of the most abundant proteins from prostatic secretions and a primary constituent of human semen. PSP94 suppresses tumor growth and metastasis, and its expression gradually decreases during progression of the prostate cancer. It is a rapidly evolving protein with homologues present in several species with 10 conserved cysteine residues. PSP94 homologues show high-affinity binding with different proteins from the cysteine-rich secretory protein family, some of which have been shown to be ion channel blockers. Here, we report the crystal structure of human PSP94 at 2.3 A resolution. The structure shows that the amino and the carboxyl ends of the polypeptide chain are held in close proximity facing each other. A strong hydrogen bond between these ends, which are located respectively on the first and the last beta-strands, leads to formation of an almost straight edge in PSP94 structure. Crystal structure shows that these edges from two PSP94 monomers associate in antiparallel fashion, leading to formation of a dimer. Our studies further show that dimers dissociate into monomers at acidic pH, possibly through distortion of the straight edge. Further, based on several observations, we propose that PSP94 binds to cysteine-rich secretory proteins and immunoglobulin G through the same edge, which is involved in the formation of PSP94 dimeric interface.