The SpEGF III gene encodes a member of the fibropellins: EGF repeat-containing proteins that form the apical lamina of the sea urchin embryo.

We have identified a gene in the sea urchin Strongylocentrotus purpuratus that encodes a protein with multiple epidermal growth factor(EGF)-like motifs. The SpEGF III cDNA sequence predicts a 570 amino acid protein with a complex domain structure similar to that of the fibropellins--the protein products of the SpEGF I gene. A putative hydrophobic leader sequence is followed by an EGF-like motif, a domain similar to complement Cls, seven tandem EGF-like repeats, and a carboxy terminal domain similar to avidin. The 2.9-kb SpEGF III mRNA is expressed from a single copy gene. SpEGF III mRNA is present at low levels in unfertilized eggs and during early cleavage, then rapidly increases in abundance between late morula and mesenchyme blastula stages to maximal levels maintained through subsequent stages. Polyclonal antisera from SpEGF III fusion proteins reveal that the protein is a 100-kDa fibropellin which is present in unfertilized eggs but does not accumulate substantially until the mesenchyme blastula stage. The protein is localized to the apical lamina, a structurally complex component of the extracellular matrix that is made up of three major proteins, two of which are the differentially spliced products of SpEGF I. The size and localization of the SpEGF III protein, and the results of immunoprecipitation assays which reveal that it is tightly associated with the products of SpEGF I, indicate that it is the third major protein component of the apical lamina. The timing of SpEGF III protein accumulation is coincident with an increase in the structural complexity of the apical lamina, and with the developmental period when the apical lamina plays an important role in gastrulation.