Functional processing of fertilin: evidence for a critical role of proteolysis in sperm maturation and activation.

Fertilin is a sperm surface protein with an essential role in fertilization. It is required for the migration of spermatozoa through the oviduct, for binding to the zona pellucida, and for efficient binding to the egg plasma membrane. Fertilin consists of two subunits, fertilin alpha and beta, both of which belong to the metalloprotease-disintegrin protein family (ADAMs). Fertilin alpha and beta are made as larger precursors that are processed proteolytically at different stages of sperm maturation in the testis and epididymis. Fertilin alpha is processed first, most likely by a pro-protein convertase in the secretory pathway of testicular cells. Fertilin beta is processed later, while spermatozoa are in transit through the epididymis. The processing of fertilin beta in the epididymis correlates with the acquisition of fertilization competence in spermatozoa, exposes an epitope that has a role in sperm-egg interactions, and triggers the relocalization of fertilin from the whole sperm head to the posterior head. These findings indicate that the proteolytic processing of fertilin and perhaps also other sperm proteins plays an important role in sperm maturation and activation in the epididymis. Further evaluation of the functional significance of proteolysis for sperm maturation should lead to new and exciting insights into the mechanism of sperm maturation, and may also uncover the cause of certain types of male infertility. The identification of the responsible proteases could provide novel targets for contraceptive drugs.