In addition to 24 kDa/E-2, several other p-defensin-like molecules appear to be specific to the epididymis. These include Bin-1b in the rat and ESC42, hE2, and EP2 in primates. Of these, HE2 and Bin-1b have been shown to possess antimicrobial action in vitro. Interestingly, although ESC42 and ESP13.2 possess an N-ter-minal defensin-like domain, they share with the 24 kDa/E-2 a C-terminal domain that is absent from any true p-de-fensin, suggesting that they may form a small subfamily.
The role of this C-terminal domain is unclear, although it is noteworthy that ESC42 has been shown to bind to the surface of maturing sperm, whereas neither Bin-1B nor ESP13.2 has been reliably detected on the sperm surface. Thus, it is possible that the C-terminal domain has a role in the interaction of the protein with the sperm plasma membrane. inhaler proventil
It remains to be determined why those epididymal de-fensin-like molecules that bind to the sperm surface do not appear to have antimicrobial activity and vice versa. An intriguing speculation is that the latter group protects the vas deferens and epididymis from bacterial infections, whereas the former group may have a role during fertilization (e.g., initiating cortical granule release) as a result of activation through intermixing with components on the ool-ema.