Characterization of the Snail gene superfamily in sea urchins

In a wide variety of animals, the Snail family of zinc-finger (ZF) transcription factors (TFs) plays a central role in embryonic development, being involved in cell movement and morphogenesis (1). We have identified two members of the Snail family in sea urchins: Snail1/Snail and Snail2/Snai2/Slug, while a third gene (Snail3/Snai3), previously named SMUC, is absent from the echinoderm clade. A single member of the Scratch family is present in the genomes of sea urchins. Common structural features of the Snail superfamily are the SNAG trans-repression domain at the most N- terminal ends, and four to six C2H2-type ZF DNA binding motifs at the C-terminal of all proteins. Family-related consensus sequences of each ZF (five in sea urchins) are highly conserved among urchins and vertebrates, allowing us to ascribe each sea urchin member to the vertebrate counterpart. A highly conserved scratch domain signature (69% identities and 75% positives) is unambiguously detected in sea urchins' Scratch predicted proteins, compared to human and Drosophila Scratch members. By contrast, we recognize only a slug-like area in the sea urchin Snai2, displaying 29% identity/ 53% positivity with the human canonical slug domain. However, apart from this difference, individual ZFs of the sea urchin Snai2 contain unique residues, typical of the Snail2/Snai2 TFs. qPCR data show that Snai2 expression starts before and at higher levels than that of Snail, both reaching the maximum at prism stage, while Scratch is poorly expressed during Paracentrotus lividus embryogenesis.