Intratesticular distribution of cyritestin, a protein involved in gamete interaction.

Cyritestin, a member of the ADAM family of proteins, has been shown to be involved in the interaction between sperm and egg during fertilization. The protein is a transmembrane protein associated with the sperm acrosome. In the present study, electron microscopy was used to trace the distribution of the cyritestin molecule in intratesticular germ cells, particularly in haploid round spermatids where the acrosomal structure is differentiating. Our results indicate that cyritestin is transported to the forming acrosomal vesicle through the Golgi apparatus to become part of the acrosomal membrane. Differential staining with antibodies recognizing either the metalloprotease-like domain or the cytoplasmic domain of cyritestin indicates that processing of the molecule leading to the loss of the pro- and metalloproteinase-like domains begins during germ cell stage 6 and is completed before stage 15.