A protein of the sea urchin cortical granules is targeted to the fertilization envelope and contains an LDL-receptor-like motif.

Cortical granules of eggs contain a population of heterogeneous proteins that are sequestered selectively within the cortical granule during oogenesis and that participate in the block to polyspermy during the fertilization reaction. To begin to understand the targeting mechanism of proteins to this organelle, we have identified cDNA clones that encode proteins of the cortical granule. Here we characterize the protein encoded by the cDNA, SFE 9, which is packaged specifically into the spiral lamellae of the cortical granules in the sea urchin Strongylocentrotus purpuratus. The SFE 9 protein showed a molecular mobility in PAGE of 150 kDa, close to the estimated 128 kDa predicted by cDNA sequence, and contained three regions of distinct repeating sequence. Each type of repeat sequence was of different length, frequency, and amino acid composition, but each type showed a high degree of conservation within a repeat type. A fourth region of sequence showed similarity to the low-density lipoprotein (LDL)-receptor motif. The N-terminal 2/3 of the protein contained an abundance of cysteine residues with regular and conserved spacings. Upon fertilization, SFE 9 was secreted and was rapidly incorporated into the fertilization envelope. The integration of SFE 9 into the fertilization envelope was complete within 5 min post-fertilization and was mediated by ovoperoxidase-catalyzed cross-linking. Incorporation of SFE 9 into the fertilization envelope was not uniform. Instead, it concentrated to the microvillar casts both in normal and in ovoperoxidase-inhibited fertilization envelopes. Thus, SFE 9 shows molecular heterogeneity within the fertilization envelope and the targeted integration is independent of ovoperoxidase activity. The strong repeat regions and the LDL-receptor motif of SFE 9 are postulated to be used in specific protein-protein interactions that occur either during traflicking to the cortical granules during biogenesis or to the fertilization envelope during the fertilization reaction.