Sperm trypsin-like proteases are known to play important roles in fertilization, but their detailed functions are still unknown. We previously explored the binding partners of sperm trypsin-like proteases, HrProacrosin and HrSpermosin, in the ascidian Halocynthia roretzi, and we isolated several candidate proteins on the vitelline coat. We found that some of these proteins are identical to the C-terminal coding region (CT) and von Willebrand factor type D (vWF-D) domain of vitellogenin. We also found that CT on the vitelline coat disappears after fertilization. Vitellogenin is a large lipid transfer protein that is enzymatically processed during vitellogenesis. Although the processed domains including phosvitin and lipovitellin are known to function as yolk nutrient proteins, the roles of the CT and vWF-D domain remain elusive. Our results showed that the CT and vWF-D domain of vitellogenin are processed and attached to the vitelline coat, which in turn participate in fertilization as the binding partners of sperm proteases. (C) 2010 Elsevier Inc. All rights reserved.