Murine Binder of SPerm Homolog 2 (BSPH2): The Black Sheep of the BSP Superfamily1

Abstract

Proteins of the Binder of SPerm superfamily are known to bind choline phospholipids on sperm membrane and promote sperm capacitation. The current study focuses on the biochemical and functional characterization of the murine Binder of SPerm homolog 2 (BSPH2). A recombinant protein (rec-BSPH2) was expressed in Escherichia coli Rosetta-gami B (DE3)pLysS cells using pET32a vector. It was purified by immobilized metal ion affinity chromatography and refolded on column using a decreasing urea gradient. Rec-BSPH2 was found to share some binding characteristics with other BSP proteins, such as binding to gelatin, heparin, and epididymal sperm. Rec-BSPH2 as well as murine recombinant BSPH1 were found to have different immunofluorescence patterns when bound to uncapacitated versus capacitated sperm, indicating a rearrangement of these proteins on sperm surface during or following capacitation. Surprisingly, rec-BSPH2 was unable to bind phosphorylcholine liposomes or promote sperm capacitation. It is the first time that such results are reported for proteins of the BSP family. The results indicate that murine BSPH1 and BSPH2 might not have redundant functions, as is the case with bovine BSPs. This study could lead to a better understanding of the role of BSP proteins in sperm functions and the existence of redundant BSP proteins in the reproductive tract.

Gelatin Binding Proteins in Reproductive Physiology

“…Further, HDL and glycosaminoglycans (GAGs) such as heparin have been displayed to induce capacitation in the female reproductive tract and GAGs interact with BSP proteins perhaps regulating capacitation [11,29], which is essential for fertilization. Recombinant human BSPH1 has been shown to induce capacitation in humans in a similar way to the murine rec-BSPH1 in murine species [9,29]. The functions of murine BSPH2 in capacitation is not known, although a high sequence similarity with murine BSPH1 has been observed.…”

“…In addition, BSP homologous DNA sequences were also characterized in the genomes of other mammals including humans, mice, rabbits, dogs and chimpanzees [8]. In the mouse and in humans, BSP homologs were characterized as mouse BSP homolog 1-3 (mBSPH1, mBSPH2 and mBSPH3) and human BSP homolog 1 (hBSPH1) [7,9]. The significant difference between the BSP proteins found in other species and those in mice and humans is the site of their expression in accessory sex glands and their concentration found in seminal plasma.…”

“…The significant difference between the BSP proteins found in other species and those in mice and humans is the site of their expression in accessory sex glands and their concentration found in seminal plasma. BSP homologs are expressed in the epididymis in mice and humans, whilst the site of expression is commonly the seminal vesicles in other mammals [9]. Approximately 60 % of seminal plasma proteins are made up of BSP proteins in most mammalian species which is in stark contrast to the 0.01 % found in the mouse and human [8,9].…”

“…BSP homologs are expressed in the epididymis in mice and humans, whilst the site of expression is commonly the seminal vesicles in other mammals [9]. Approximately 60 % of seminal plasma proteins are made up of BSP proteins in most mammalian species which is in stark contrast to the 0.01 % found in the mouse and human [8,9]. This variation in the site of expression amongst species may be because of differences in species specificity in terms of the maturation of spermatozoa, in contrast to the bull, the semen of mice and humans coagulates just after ejaculation and the coating of spermatozoa by seminal plasma proteins may not occur extensively.…”

“…The molecular weight of proMMPs is usually 10 kDa less than the active form. Human seminal plasma possesses latent as well as active types of MMP 2 and MMP 9 with latent MMPs more [7,9,11] Mouse Epididymis m BSPH1 and mBSPH2 [7,9,11] Porcine Seminal plasma pB1 [7] Buffalo Seminal plasma BSP homologs [11,41] Indian J Microbiol (Oct-Dec 2016) 56(4):383-393 385 predominant than the active form. In addition, the expression of proMMP 2 and MMP 9 increases sharply up to maximum levels 2 h after capacitation, which implies a role in triggering the hyperactivation of spermatozoa [21].…”