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 lab oratory demonstrated that SpCOUP-TF transcripts are localized in the egg an d asymmetrically distributed in the early embryonic blastomeres (A. Vlahou ct al., 1996, Development 122, 521-526). To examine the subcellular localiz ation of SpCOUP-TF protein, polyclonal antibodies were separately raised ag ainst the divergent N-terminus as well as the conserved DNA-binding and lig and-binding domains. Immunohistochemical analyses suggest that SpCOUP-TF is a maternal protein residing in the cytoplasm of the unfertilized egg. Afte r fertilization, and as soon as the two-cell-stage embryo, most of the rece ptor translocates from the cytoplasm to the cell nuclei. During the rapid e mbryonic 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 subcellu lar localization of myc-tagged human COUP-TF I introduced into the sea urch in embryo by RNA injection of fertilized eggs was examined. The pattern of human COUP-TF I subcellular localization, detected with a monoclonal myc an tibody, recapitulated the essential features described for the endogenous S pCOUP-TF trafficking. Replacement of the N-terminus of the human receptor w ith the unique sea urchin N-terminus enhanced its localization to the nucle ar 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 l ocalization. Taken together these data suggest that SpCOUP-TF transcription al activity is keyed on a cell-cycle-dependent mechanism that regulates chr omosomal protein traffic. (C) 2000 Academic Press.