ULTRASTRUCTURE OF THE PROTEOLIAISIN-OVOPEROXIDASE COMPLEX AND ITS SPATIAL-ORGANIZATION WITHIN THE STRONGYLOCENTROTUS-PURPURATUS FERTILIZATION ENVELOPE

Ovoperoxidase is a cortical granule-derived enzyme that hardens the se a urchin fertilization envelope by catalyzing the formation of dityros ine residues. Ovoperoxidase works in concert with a second protein, pr oteoliaisin, which anchors ovoperoxidase to the nascent fertilization envelope in a divalent cation-dependent manner. In this study, we exam ined the Ca2+-dependent interaction of proteoliaisin with ovoperoxidas e in rotary-shadowed Pt replicas. Ovoperoxidase, a uniformly sized glo bular molecule, binds to a distal portion of rod-shaped proteoliaisin when low concentrations of Ca2+ are present. Higher Ca2+ concentration s lead to the formation of extended proteoliaisin strands that are dec orated along their lengths with ovoperoxidase. Using immunogold labeli ng, we also examined the assimilation of these two proteins into the f ertilization envelope in quick-frozen, deeply etched samples. Both pro teins are abundant in the fertilization envelope as early as one minut e after fertilization. Coincident with paracrystalline coating of the envelope, the labeling density is markedly reduced, suggesting that an tigenic sites may be masked by the paracrystalline coat. This suggests that the ovoperoxidase-proteoliaisin complex resides within the centr al portion of the fertilization envelope, rather than in the paracryst alline coat.