title: Transient appearance of Strongylocentrotus purpuratus Otx in micromere nuclei: cytoplasmic retention of SpOtx possibly mediated through an alpha-actinin interaction.
abstract: At the 16-cell stage, the sea urchin embryo is partitioned along the animal-vegetal axis into eight mesomeres, four macromeres, and four micromeres. The micromeres, unlike the other blastomeres, are autonomously specified to produce skeletogenic mesenchymal cells and are also required to induce the vegetal-plate territory. A long-held belief is that micromeres inherit localized maternal determinants that endow them with their cell autonomous behavior and inducing capabilities. Here, we present evidence that an orthodenticle-related protein, SpOtx appears transiently in nuclei of micromeres but not in nuclei of mesomeres and macromeres. At later stages of development, SpOtx was translocated into nuclei of all cells. To address the possibility that SpOtx was retained in the cytoplasm at early developmental stages, we searched for cytoplasmic proteins that interact with SpOtx. A proline-rich region of SpOtx resembling an SH3-binding domain was used to screen an embryo cDNA expression library, and a cDNA clone was isolated and shown to be alpha-actinin. A yeast two-hybrid analysis showed a specific interaction between the proline-rich region of SpOtx and a putative SH3 domain of the sea urchin alpha-actinin. Because micromeres lack an actin-based cytoskeleton, the results suggested that, at the vegetal pole of the 16-cell stage embryo, SpOtx was translocated into micromere nuclei, whereas in other blastomeres SpOtx was actively retained in the cytoplasm by binding to alpha-actinin. The transient appearance of SpOtx in micromere nuclei may be associated with the specification of micromere cell fate.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Texas M.D. Anderson Cancer Center, Houston 77030, USA.
authors.forename: C K
authors.initials: CK
authors.lastname: Chuang
authors.forename: A H
authors.initials: AH
authors.lastname: Wikramanayake
authors.forename: C A
authors.initials: CA
authors.lastname: Mao
authors.forename: X
authors.initials: X
authors.lastname: Li
authors.forename: W H
authors.initials: WH
authors.lastname: Klein
mesh_terms: D000185:Actinin / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D003593:Cytoplasm / Q000378:metabolism; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D009693:Nucleic Acid Hybridization; D011392:Proline; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013320:Strongyloidea / Q000196:embryology / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D015003:Yeasts
chemical_list: D018398:Homeodomain Proteins; D009419:Nerve Tissue Proteins; D011993:Recombinant Fusion Proteins; C092316:SpOtx protein, Strongylocentrotus purpuratus; D014157:Transcription Factors; D000185:Actinin; D011392:Proline