The role of stallion seminal proteins in fertilisation

Seminal plasma proteins are secretory proteins originating mainly from the epididymis and the accessory sex glands. They are involved in the remodelling of the sperm surface which occurs during sperm transit through the male genital tract and continues later at ejaculation. During this process, collectively called post-testicular sperm maturation, the spermatozoa acquire the ability to fertilise an egg. Seminal plasma proteins have been shown to contribute to early and central steps of the fertilisation sequence, e.g. the establishment of the oviductal sperm reservoir, modulation of capacitation and gamete interaction. The major equine seminal plasma proteins belong to three protein classes, which contain widely occurring protein modules. Fn-2 type proteins are characterised by two or four tandemly arranged Fn-2 modules and have been implicated in the modulation of sperm capacitation. Multiple members of the cysteine-rich secretory proteins (CRISP) have been identified in the male genital tract of a number of species. CRISP proteins have been shown to be involved in various functions related to sperm–oocyte fusion, innate host defense function and ion channel blockage. Spermadhesins occur only in ungulate species. Their carbohydrate- and zona pellucida-binding properties would suggest a role of these proteins in gamete recognition. The major proteins of equine seminal plasma have been isolated and characterised regarding their expression along the male genital tract, protein structure and their functions.