Molecular, cellular, and developmental characterization of sea urchin seawi: A microtubule/mRNA binding protein

Molecular cloning experiments indicate that the sea urchin 100 kDa dynamin-like tubulin dimer binding protein is the sea urchin ortholog of Drosophila piwi and thus named seawi. Seawi is a member of the piwi subgroup of the piwi/argonaute protein family that is involved in stem cell establishment/maintenance and developmental axis formation. In sea urchin eggs and embryos, seawi is a major component of microtubule ribonucleoprotein (MT-RNP) complexes containing silenced mRNAs. The N-terminal region of seawi's piwi domain is 55% identical and 69% similar to the equivalent domain from mouse miwi that has been shown to be necessary and sufficient for RNA binding. Seawi binding mRNA was confirmed when seawi was run on SDS-PAGE gels, transferred to nitrocellulose and probed with bep4 message. Indirect immunolocalization studies identified an enrichment of seawi within the small micromeres, which interestingly have been shown to have non-detectable levels of bep4 protein, during the cleavage stage. Consequently, seawi's association with bep messages as part of MT-RNP silencing complexes may regulate the expression of bep transcripts in the micromeres resulting in the regulation of beta-catenin localization to either cell-cell junctions or nuclei, thereby specifying the animal-vegetal axis. An additional developmental role for seawi comes from immunolocalization studies of later staged sea urchin embryos. Seawi remains enriched in the small micromeres, located at the base of the vegetal plate in mesenchyme blastula and the tip of the invaginating archenteron in gastrula, and in their descendants, the cells of the coelomic pouches. The small micromeres are a group of cells set-aside (stem cells) early in development that will ultimately serve as a source of non-committed pluripotent cells used to generate a number of adult tissue types and structures. Seawi's ability to bind mRNAs and previously obtained data on its microtubule binding support a model whereby seawi may act as a molecular tether between localized mRNAs and the cytoskeleton.