A unique protein that only occurs in sperm could be the key to a new contraceptive for both men and women.

The protein, named CatSper, is an ion channel protein that controls the flow of calcium into sperm tails. Calcium is crucial in enabling sperm to swim, so a drug designed to block the channel would immobilise sperm and prevent fertilisation.

The discovery is reported in this week’s Nature by Dr David Clapham, from the Howard Hughes Medical Institute, and colleagues from Harvard Medical School.

“This is the only protein I’ve ever seen that localises to only one tissue,” said Dr Clapham.

“We looked carefully through at least fifty human tissues, and the only place it turned up was in the testes.”

“Futhermore, the only place it turned up within the testes was in sperm. And within sperm, it is found only in the principal piece of the tail.”

It is also specific to mature sperm and does not appear to be expressed in sperm precursor cells, or in immature sperm.

The very specific role of the protein means that a contraceptive designed to disrupt it would have far fewer side effects than a hormone-based contraceptive. Hormones are found in many different tissues.

Working with mice, the researchers discovered that the sperm of mice without the CatSper gene were unable to penetrate the outer covering of the egg, called the zona pellucida.

But when the researchers mixed the sperm with eggs that had the covering removed, they fertilised normally.

“It might be that this channel is involved in giving the sperm’s tail a kind of turbocharge at the last instant, when it needs more power to penetrate the zona pellucida,” Dr Chapman speculated.

A contraceptive designed to prevent fertilisation would not interfere with the gene but simply inhibit the ion channel protein, so that calcium could not pass through.

“If a drug could be designed to block this ion channel specifically, it could be taken by men or women,” said Dr Chapman.

“And, it would not have to be taken for a very long period to block fertilisation, perhaps only just before or after intercourse.”