Characterization and Localized Expression of the Laminin Binding Protein/p40 (LBP/p40) Gene during Sea Urchin Development

Abstract

We have isolated and characterized the expression of a cDNA clone from the sea urchin Strongylocentrotus purpuratus that encodes a protein very similar to LBP/p40, originally identified as a nonintegrin, 67-kDa laminin binding protein. The deduced amino acid sequence of the protein, which we call spLBP/p40, shows significant similarity with the LBP/p40 from other sources, although significant divergence does occur at the carboxyl end. The S. purpuratus mRNA is present as a maternal transcript and its level remains constant until activation of zygotic transcription at the hatching blastula stage, whereupon the total spLBP/p40 increases through the pluteus larval stage. Adult tissues also contain the spLBP/p40 mRNA. Both maternal and zygotic transcripts are translated as determined by their presence in polysomes. Immunoblot analysis using an antibody raised against a recombinant fusion protein indicates that the concentration of the spLBP/p40 protein remains constant during development despite the postblastula increase in mRNA concentration. However, the spatial distribution of the protein changes from a uniform, intracellular distribution in all cells of cleavage and blastula stages to localized, elevated levels in cells of the gut, primary mesenchyme, and oral epithelium of prism larvae. The distribution of spLBP/p40 mRNA at different developmental stages, analyzed by in situ hybridization, reflects that of the protein. Our results argue against a laminin binding function for this protein; instead they place the spLBP/p40 gene in a class of previously described sea urchin genes involved in growth and proliferation.