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

Maturemammalianoocytes undergoa prolongedseries of cytoplasmic calcium(Ca²⁺) oscillations at fertilizationthat are thecauseof 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⁺ releasewhenmicroinjected intomouse 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 inmouse oocytes.The lack of response inmouse oocytes was despite PAWPbeing 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.