Epididymal Binder of SPerm genes and proteins: what do we know a decade later?

Binder of SPerm (BSP) proteins from ungulates (more specifically bull, bison, buffalo, stallion, boar, goat, and ram) have been extensively studied in the past 30 years. These proteins secreted by seminal vesicles constitute between 1 and 60% of total seminal plasma proteins depending on the species. In addition to sharing many biochemical characteristics such as the ability to bind to gelatin, glycosaminoglycans, choline phospholipids, and lipoproteins, they also share a main function: promoting sperm capacitation. Over the last 10 years, new members of the BSP superfamily have been discovered. These proteins found in bulls, humans, mice, and rabbits are expressed in the epididymides rather than in seminal vesicles and constitute only a minute percentage of the seminal plasma proteins. However, they share many characteristics with BSPs expressed by accessory glands including their structure, their ability to bind to the aforementioned BSP ligands, as well as their ability to promote sperm capacitation. More investigations need to be done on epididymal BSP proteins, but studies described in this review constitute a solid foundation toward deciphering the significance of epididymal BSP proteins in male fertility.