Ovoperoxidase, the enzyme that hardens the sea urchin fertilization envelope, is inserted into the assembling extracellular matrix through the action of an intermediary protein, proteoliaisin (PLN). The domain structure of PLN, a large, rod-shaped protein that binds to ovoperoxidase and the vitelline layer, was examined by limited proteolytic cleavage. Purified proteolytic fragments of PLN were tested for their ability to bind ovoperoxidase, inhibit the binding of 125I-PLN to the vitelline layer, or act as substrates for the hardening reaction. Based on these results, the vitelline layer-binding domain can be placed near the amino terminus, followed by the binding site for ovoperoxidase; the distal two-thirds of the protein contain sites for ovoperoxidase-catalyzed dityrosine formation. The pentapeptide GRGDS (but not RGD) inhibited PLN-vitelline layer binding half-maximally at 0.2 mM. Moreover, PLN promoted adhesion of bovine aortic endothelial cells to plastic dishes, a process inhibited by GRGDS. Thus PLN is a new member of the adhesive protein family, the function of which is to coordinate the morphogenesis of a specific, rapidly assembled extracellular matrix.