Subcellular trafficking of the nuclear receptor COUP-TF in the early embryonic cell cycle.
The nuclear receptor SpCOUP-TF is the highly conserved sea urchin homologue of the COUP family of transcription factors. Previous results from our laboratory demonstrated that SpCOUP-TF transcripts are localized in the egg and asymmetrically distributed in the early embryonic blastomeres (A. Vlahou et al., 1996, Development 122, 521-526). To examine the subcellular localization of SpCOUP-TF protein, polyclonal antibodies were separately raised against the divergent N-terminus as well as the conserved DNA-binding and ligand-binding domains. Immunohistochemical analyses suggest that SpCOUP-TF is a maternal protein residing in the cytoplasm of the unfertilized egg. After fertilization, and as soon as the two-cell-stage embryo, most of the receptor translocates from the cytoplasm to the cell nuclei. During the rapid embryonic cell division, SpCOUP-TF was found to shuttle from the interphase nuclear periphery to the condensed chromosomes in mitosis, in a cell-cycle-dependent manner. In an attempt to confirm these observations, the subcellular localization of myc-tagged human COUP-TF I introduced into the sea urchin embryo by RNA injection of fertilized eggs was examined. The pattern of human COUP-TF I subcellular localization, detected with a monoclonal myc antibody, recapitulated the essential features described for the endogenous SpCOUP-TF trafficking. Replacement of the N-terminus of the human receptor with the unique sea urchin N-terminus enhanced its localization to the nuclear rim during interphase. Deletion of the DNA-binding domain of human COUP-TF I resulted in loss of all aspects of nuclear periphery and chromosomal localization. Taken together these data suggest that SpCOUP-TF transcriptional activity is keyed on a cell-cycle-dependent mechanism that regulates chromosomal protein traffic.
Vlahou
A
A
Department of Cell Biology, Baylor College of Medicine, 1 Baylor Plaza, Houston, Texas 77030, USA.
Flytzanis
C N
CN
D000818:Animals; D001692:Biological Transport; D002453:Cell Cycle; D002467:Cell Nucleus / Q000378:metabolism; D002875:Chromosomes; D004625:Embryo, Nonmammalian / Q000166:cytology; D005306:Fertilization; D006801:Humans; D008938:Mitosis; D018160:Receptors, Cytoplasmic and Nuclear / Q000276:immunology / Q000378:metabolism*; D012617:Sea Urchins / Q000196:embryology*; D013347:Subcellular Fractions / Q000378:metabolism; D014157:Transcription Factors / Q000276:immunology / Q000378:metabolism*
D018160:Receptors, Cytoplasmic and Nuclear; D014157:Transcription Factors