Analysis of the process of localization of fertilin to the sperm posterior head plasma membrane domain during sperm maturation in the epididymis

Fertilin is a heterodimeric (subunits alpha and beta) sperm plasma membrane protein. Both subunits belong to the ADAM protein family of surface proteins that contain a disintegrin and a metalloprotease domain. Fertilin functions in sperm-egg fusion by binding the sperm to the egg plasma membrane via a binding site in the disintegrin domain of fertilin beta. On testicular sperm of guinea pig, fertilin is distributed on the plasma membrane over the entire sperm head, but is found only on the posterior head once sperm have passed through the epididymis. This redistribution of fertilin to the posterior head can be partially mimicked in vitro if testicular sperm are briefly treated with trypsin. In this study we used immunofluorescence and digital image analysis to analyze how fertilin becomes restricted to the posterior head. We found that fertilin became restricted to the posterior head by migration of anterior head fertilin molecules into the posterior head domain. Comparison of immunofluorescence patterns and immunoblots of fertilin from seven regions of the epididymis showed a temporal correlation between the beginning of fertilin's migration to the posterior head and the proteolytic processing of the full-length fertilin beta precursor (the 85-kDa pro-beta form) to a 75-kDa intermediate, pro-beta*. Completion of the migration coincided with the further cleavage of pro-beta* to the 25- to 28-kDa mature form. Our data suggest that the cleavage of fertilin pro-beta to pro-beta* may initiate fertilin's migration into the posterior head domain and, after localization to that membrane domain, pro-beta* is cleaved to mature beta. We also report evidence that a common mechanism may be used to change the localization pattern of other sperm surface molecules. Other surface proteins were shown to become localized to either the posterior or the anterior head membrane domains on sperm at the same time fertilin became localized to the posterior head. These restrictions of surface protein localizations were also shown to immediately precede the development of the sperm's ability to swim and undergo the acrosome reaction, and thus redistribution of surface proteins may be necessary before sperm become functional.