A cell integrates mitogenic signals received at the plasma membrane with intracellular biochemical changes to direct the events of cell division. Oocytes from Xenopus laevis offer a system that allows molecular dissection of pathways controlling cell growth and division in response to extracellular cues. Xenopus oocytes, physiologically arrested in a G2 like state, respond to the hormone progesterone to reinitiate meiosis and mature into a fertilizable egg. Signals received at the oocyte membrane induce translation of dormant maternal mRNAs that not only drive meiotic entry but also maintain the cell cycle arrest in an egg. A major pathway controlling the translation of these mRNAs is cytoplasmic polyadenylation, facilitated by the Cytoplasmic Polyadenylation Element Binding protein (CPEB) through cis-acting elements in their 3'untranslated regions (3'UTRs). Cytoplasmic polyadenylation requires the phosphorylation of serine174 on CPEB by Aurora-A as well as the translation of a hitherto unknown mRNA. The transcript of the RINGO/Spy gene is a putative candidate for this unknown upstream regulator of CPEB function. RINGO/Spy mRNA is translationally repressed in immature oocytes by a ribonucleoprotein (RNP) complex consisting of the repressor Pumilio-2, the putative activator Deleted in Azoospermia-like (DAZl) and embryonic poly A binding protein (ePAB). Progesterone signaling leads to the dissociation of Pumilio-2 from the mRNP and the ensuing RINGO/Spy protein synthesis, in turn, promotes cytoplasmic polyadenylation and oocyte maturation.

Pumilio and its associated proteins, such as Drosophila Brain tumor (Brat) and DAZl, in addition to their cytoplasmic roles have ill-defined functions within the nucleus. We detected DAZl within the nucleoli of telomerase-immortalized human retinal pigment epithelial (RPE) cells in interphase and on acrocentric chromosomes during mitosis. DAZl colocalizes with the RNA polymerase I associated Upstream Binding transcription Factor (UBF), most likely through pre-ribosomal RNA and is a likely component of the Nucleolar Organization Region (NOR). Stably knocking down DAZl in RPEs using short hairpin RNAs results in loss of nucleolar segregation, the physiological outcome of which is under investigation. These preliminary findings indicate an additional role for DAZl within the nucleolus, one likely to be independent from cytoplasmic translational control.