Gamete interaction and fusion is a complex process involving multiple proteins on the surface of both the sperm and the egg. The sperm protein IZUMO1 has been shown to be essential for gamete fusion, as sperm from IZUMO1 knockout mice fail to fuse with the egg.  Interestingly, IZUMO1 does not appear to be directly responsible for binding ligands on the egg plasma membrane.  Rather, it has been shown that the transmembrane and/or cytoplasmic domain of IZUMO1 facilitates its interaction with other sperm proteins thus raising the possibility that it serves as a scaffold to mediate the organisation or stabilisation of multi-protein complexes essential for membrane fusion.  However, the identity of its interacting partners has not yet been defined. Through the use of blue native electrophoresis we have confirmed that IZUMO1 forms a number of high molecular weight protein complexes in mouse spermatozoa. Mass spectrometry analysis identified a prominent interacting protein as the glioma pathogenesis-related 1–like 1 (GLIPR1L1).  This protein is of interest as it is a member of the CAP superfamily of proteins that have been implicated in various aspects of cell to cell adhesion and male fertility. Our previous work on mouse GLIPR1L1 has shown that is has a testis-enriched expression profile and becomes localised to the sperm head during post-testicular maturation. Sperm-egg interaction assays also indicate that GLIPR1L1 has a role in binding and fusion of the sperm to the egg. In the present study we have acquired several lines of evidence to support the interaction between IZUMO1 1 and GLIPR1L1 and have demonstrated that this interaction is profoundly influenced by the maturation status of the spermatozoa.  Collectively, these data strengthen the hypothesis that IZUMO1 and GLIPR1L1 work together to support the formation of multiprotein complexes essential for sperm-egg fusion.