Sperm defender has second role.

A protein that scientists suspect defends sperm and other cells from microbes also guides the maturation of sperm, a new study concludes.

Before a sperm cell can fertilize an egg, it must pass out of a testicle and through a coiled tube called the epididymis. It's during this passage that sperm gain their full capacity to swim and fertilize eggs, although what triggers this maturation remains murky.

Scientists studying the epididymis have found that it contains a number of proteins with antimicrobial properties. The proteins include several belonging to a molecular family called beta-defensins. Hsiao Chang Chan of the Chinese University of Hong Kong and her colleagues wondered whether a particular beta-defensin does more than ward off germs.

In the May Nature Cell Biology, the team reports that this defensin binds to the head of sperm cells. Moreover, immature, immotile sperm can swim forward only after they've been exposed to cells from outside the epididymis that had been engineered to make the defensin. To solidify the case for the defensin's sperm-boosting role, the investigators injected into rats' epididymides a compound that suppresses the production of the defensin. The sperm from those rodents were much more sluggish than typical sperm.--J.T.