In 2005, Japanese researchers identified a sperm protein important for this process, named Izumo after a Japanese marriage shrine. Until recently, the binding partner present on the egg had proved more difficult to identify.
However, by creating an artificial version of the Izumo protein, researchers were able to find the protein that binds to it to initiate fertilisation.
They have named this new protein Juno, after the Roman goddess of fertility and marriage.
'We have solved a long-standing mystery in biology by identifying the molecules displayed on the sperm and egg which must bind each other at the moment we were conceived', says Dr Gavin Wright, senior author from the Wellcome Trust Sanger Institute. 'Without this essential interaction, fertilisation just cannot happen'.
The study reports that eggs from mice lacking Juno cannot fuse with sperm, making them infertile. Male mice lacking Izumo are also infertile, highlighting the importance of these proteins for fertility.
Lastly, the researchers also discovered that the Juno protein is quickly lost from the egg's surface around 40 minutes after fertilisation.
This may explain how the egg prevents additional sperm fusing with it once it has already been fertilised. This is important to ensure the embryo has the correct number of chromosomes to survive.
As a result, Dr Wright has speculated that 'we may be able to use this discovery to improve fertility treatments and develop new contraceptives'.
The team is now screening infertile women to determine whether problems with Juno are responsible for cases of currently unexplained infertility.
Dr Allan Pacey, a leading fertility expert not involved in this study, has said: 'We know that fertilisation failure in IVF is quite rare, and so I suspect the lack or dysfunction of this protein is probably not a major cause of infertility in couples. However, it would be useful to know how many women have eggs that lack this protein so we can properly assess this'.
In such cases, regular IVF treatment would fail. Therefore, doctors could instead suggest using intracytoplasmic sperm injection (ICSI), where the sperm is directly injected into the egg. This would bypass the need for Izumo and Juno to bind, and could reduce the expense and stress often involved in assisted fertility treatments.
The research was published in the journal Nature.