Identification of Novel Isoforms of Vitellogenin Expressed in Ascidian Eggs

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Vitellogenin is a precursor of yolk protein that is necessary for embryonic development. This protein is a large multi-domain protein consisting of a signal peptide, a heavy-chain lipovitellin, a phosvitin, a light-chain lipovitellin, a von Willebrand factor type D domain (vWF-D), and a C-terminal coding region (CT), which are processed to respective domains after uptake into oocytes. It is currently believed that only lipovitellin and phosvitin domains are necessary for nutrient supply to oocytes. Thus, molecular species of vitellogenin lacking these domains are not known. Here, we show that two novel isoforms of vitellogenin, both of which possess vWF-D and CT domains but not a lipovitellin or phosvitin domain, are expressed in the gonad of the ascidian Halocynthia roretzi. In situ hybridization revealed that mRNAs of these proteins are specifically expressed in oocytes and test cells, accessory cells in the perivitelline space of ascidian eggs. Immunohistochemistry showed that these proteins are localized around the surface of test cells in immature oocytes. Immunoelectron microscopy revealed that vitellogenin associates with vesicles located beneath the vitelline coat (VC) before fertilization but that it dissociates from the VC after fertilization. These results, together with our previous results showing that vWF-D and CT domains are capable of binding to the two sperm proteases HrProacrosin and HrSpermosin, led us to propose that novel isoforms of vitellogenin, which are expressed in oocytes and test cells and released to the perivitelline space during oocyte maturation, may participate in gamete interaction upon fertilization.