Seminal plasma proteins: functions and interaction with protective agents during semen preservation

Seminal plasma contains factors that are beneficial and/or detrimental to sperm function and/or storage. However, the nature and characteristics of these factors are not well understood. The major protein fraction (50-70%) of bovine seminal plasma is represented by a family of phospholipid-binding proteins collectively called BSP proteins. The BSP protein signature is characterised by two tandemly repeated fibronectin type 2 (Fn2) domains. It is now well established that BSP proteins and their relatives represent a new emerging superfamily of proteins in mammals. They bind to sperm membrane choline phospholipids at ejaculation. They also bind to capacitation factors, namely, high-density lipoproteins and glycosaminoglycans and promote sperm capacitation induced by these molecules, indicating their beneficial role in sperm function and fertility. In contrast, BSP proteins also induce changes in the sperm plasma membrane by stimulating cholesterol and phospholipid efflux. Thus, the continuous exposure of sperm to seminal plasma that contains BSP proteins is detrimental to the sperm membrane, which may render the membrane very sensitive to sperm storage in the liquid or frozen states. Interestingly, BSP proteins specifically bind low-density lipoproteins present in egg yolk, a compound commonly used in semen extenders. This interaction appears to abolish the detrimental effect of BSP proteins on the sperm membrane. Therefore, BSP proteins in seminal plasma act like a double-edged sword, being both beneficial and detrimental to sperm.