Mammalian fertilization depends upon successful binding and fusion between the membranes of the spermatozoon and the oocyte. These processes are thought to be mediated by a series of protein-protein interactions in which sperm antigens known as fertilins are thought to play a key role. Using a recently developed fluorescence technique, the interactions of the oligopeptide sequence corresponding to the fusogenic domain of mouse fertilin-alpha (MF alpha P) and phospholipid vesicles have been investigated. Following stopped-flow mixing, MF alpha P bound rapidly to phospholipid membranes in a co-operative manner with a Hill coefficient of 2.4 and binding rate constants in excess of 1000 s-1. The co-operative nature of the binding process is suggested to represent evidence of a structural mechanism to prevent egg fertilization by immature spermatozoa. The subsequent membrane insertion was found to take place over a longer time period (with rate constants of up to 6.3 s-1), and was linear with respect to peptide concentration. Comparison of these processes with similar time-resolved circular dichroism measurements revealed that changes in peptide secondary structure were very rapid. Fourier transform infrared spectroscopy measurements confirmed changes in the secondary structure of MF alpha P during interaction with PC phospholipid membranes, indicating that the peptide is mainly present in a beta-structure with a small proportion of alpha-helix. These results are consistent with the hypothesis that fertilin-alpha is the fusogenic species with an important role in fertilization.
