The association of Xenopus nuclear factor 7 with subcellular structures is dependent upon phosphorylation and specific domains

The function of proteins is often regulated by their association with specific subcellular structures. Xenopus nuclear factor 7 (xnf7) is a putative transcription factor that is selectively retained in the cytoplasm from fertilization through the mid blastula transition (MBT). Cytoplasmic retention is dependent upon the presence of a 22-amino-acid cytoplasmic retention domain and the phosphorylation of two sites (site 1 and site 2) within the protein. We show that the N-terminal acidic domain of xnf7 transactivated a reporter gene in transfected cells, supporting its function as a transcription factor. During mitosis xnf7 was associated with the mitotic spindle and chromosomes, while during the short embryonic interphase it was associated with structures at the poles which were most likely centrosomes. The association with these structures was dependent upon the presence of protein domains and the phosphorylation of a specific phosphorylation site (site 2). In addition, we determined that association with the spindle or centrosomes was not necessary for cytoplasmic retention prior to the MBT. We suggest that the association of xnf7 with these structures is due to its interaction with other proteins that are colocalized.