An alternative hypothesis is that the 2D6 carbohydrate epitope is generic to several different glycoproteins that are secreted in different regions of the epididymis and that change their reactivity to the McAb during epididymal maturation. Thus, initially a —250-kDa component is secreted in the distal caput epididymidis that binds to spermatozoa in the lumen of the duct. In the corpus and cauda epidi-dymidis, however, this component is removed or the epitope modified, and instead 2D6 McAb now recognizes several unrelated proteins secreted in the corpus and cauda regions, only one of which (the —48-kDa component) interacts with the sperm plasma membrane.
Implicit in this interpretation is that although 2D6 McAb recognizes several different antigens, they all bind to the same area of the sperm plasma membrane (i.e., the tail). Several precedents are known of endoproteolytic cleavage and ectodomain shedding of peptides from surface membrane antigens in the epididymis, most notably fertilin and CE9. The fate of the peptides released is not known, nor is it known if cleavage is important for activating the portion of the protein remaining attached to the membrane (as found with protease-activated receptors for the G-protein-coupled family ).