P58-A and P58-B: novel proteins that mediate skeletogenesis in the sea urchin embryo.

During sea urchin embryogenesis, the skeleton is produced by primary mesenchyme cells (PMCs). PMCs undergo a sequence of morphogenetic behaviors that includes ingression, directed migration, and cell-cell fusion. Ultimately, PMCs deposit the calcite-containing biomineral that forms the endoskeleton of the late embryo and early larva. The endoskeleton has a stereotypical structure and is the major determinant of the distinctive, angular shape of the larva. Although many candidate biomineralization proteins have been identified, functional data concerning these proteins are scant. Here, we identify and characterize two new biomineralization genes, p58-a and p58-b. We show that these two genes are highly conserved in Strongylocentrotus purpuratus and Lytechinus variegatus, two sea urchin species whose ancestors diverged approximately 100 mya. The p58-a and p58-b genes lie in tandem on the chromosome, suggesting that one of the two genes arose via a gene duplication event. The two genes encode closely related, type I transmembrane proteins. We have established by whole mount in situ hybridization that p58-a and p58-b are expressed specifically in the PMCs in both species. Knockdown of either gene by morpholino antisense oligonucleotides leads to profound defects in skeletogenesis, although skeletal elements are not completely eliminated. The P58-A and P58-B proteins do not appear to play a role in the specification, directed migration or differentiation of the PMCs, but most likely are directly involved in biomineralization during sea urchin embryonic development.