We are aware that fertilisation occurs when an egg and a sperm recognise each other and fuse together to form an embryo. What we do not know is that what triggers this 'life-initiating' handshake.
The Izumo protein on the sperm that recognises the egg was identified in 2005 by Japanese researchers who named it Izumo after a Japanese marriage shrine.
But its mate on the egg has remained a mystery.
Now, researchers at the Wellcome Trust Sanger Institute in Britain have discovered interacting protein on the surface of egg essential to begin life.
"We identified a single protein that paired with Izumo and is necessary for fertilisation. The protein is named 'Juno' after the Roman Goddess of fertility and marriage," explained Gavin Wright, senior author from the Sanger Institute.
"We have solved a long-standing mystery in biology by identifying the molecules displayed on all sperm and egg that must bind each other at the moment we were conceived," he informed.
Without this essential interaction, fertilisation just cannot happen. We may be able to use this discovery to improve fertility treatments and develop new contraceptives, Wright added.
These proteins offer new paths towards improved fertility treatments and the development of new contraceptives.
The scientists created an artificial version of the Izumo protein and used this to identify binding partners on the surface of the egg taken from mice.
Using this approach, they discovered that Izumo on the sperm interacted with Juno on the surface of the egg to initiate fertilisation.
"The Izumo-Juno pairing is the first known essential interaction for sperm-egg recognition in any organism," added Enrica Bianchi, first author from the Wellcome Trust Sanger Institute.
The binding of the two proteins is very weak, which probably explains why this has remained a mystery until now.
The team is now screening infertile women to understand whether defects in the Juno receptor are a cause of infertility.