The protein, on the sperm's surface, is synthesised by a gene that the scientists have dubbed Izumo, after a Japanese shrine dedicated to marriage, they report in a study published on Thursday.
Fusion problems have long been recognised as one of the big hurdles in pregnancy.
Recent research has discovered that a protein called CD9, on the egg membrane, is essential for amalgamating with sperm, and the discovery of Izumo is the first time that an equivalent has been found for the male gamete.
Masaru Okabe and colleague at Osaka University's Genome Information Research Centre started by looking at a monoclonal antibody that was already known to block egg-sperm fusion.
They then searched for the protein on sperm to which this antibody adheres.
The next step was to test the theory by producing mice that were genetically-engineered to lack the new-found Izumo gene.
The knockout mice were all healthy, but males turned out to be sterile.
A close look under a microscope found that their sperm, while appearing quite normal, were unable to penetrate beyond the egg's thick outer membrane, the zona pellucida.
The finding "promises benefits in the clinical treatment of fertillity and the potential development of new contraceptive strategies", Okabe's team write in the British weekly science journal Nature.
