Sperm-specific post-acrosomal WW-domain binding protein (PAWP) does not cause Ca2+ release in mouse oocytes

Mature mammalian oocytes undergo a prolonged series of cytoplasmic calcium (Ca2+) oscillations at fertilization that are the cause of oocyte activation. The Ca2+ oscillations in mammalian oocytes are driven via inositol 1,4,5-trisphosphate (IP3) generation. Microinjection of the sperm-derived phospholipase C-zeta (PLCζ), which generates IP3, causes the same pattern of Ca2+ oscillations as observed at mammalian fertilization and it is thought to be the physiological agent that triggers oocyte activation. However, another sperm-specific protein, ‘post-acrosomal WW-domain binding protein’ (PAWP), has also been reported to elicit activation when injected into mammalian oocytes, and to produce a Ca2+ increase in frog oocytes. Here we have investigated whether PAWP can induce fertilization-like Ca2+ oscillations in mouse oocytes. Recombinant mouse PAWP protein was found to be unable to hydrolyse phosphatidylinositol 4,5-bisphosphate in vitro and did not cause any detectable Ca2+ release when microinjected into mouse oocytes. Microinjection with cRNA encoding either the untagged PAWP, or yellow fluorescent protein (YFP)-PAWP, or luciferase-PAWP fusion proteins all failed to trigger Ca2+ increases in mouse oocytes. The lack of response in mouse oocytes was despite PAWP being robustly expressed at similar or higher concentrations than PLCζ, which successfully initiated Ca2+ oscillations in every parallel control experiment. These data suggest that sperm-derived PAWP is not involved in triggering Ca2+ oscillations at fertilization in mammalian oocytes.