Dynamics and assembly of vegetally localized RNP complexes in Xenopus laevis oocytes

Multiple eukaryotic cell types localize various cytoplasmic RNAs to facilitate regional protein expression, thereby creating asymmetry within a cell. Cell polarity is critical for functions ranging from cell motility and signaling in somatic cells to cell type specification during development. In Xenopus laevis oocytes, the localization of RNAs to the vegetal cortex is important for the generation of the primary germ layers during embryogenesis. Both Vg1 and VegT RNAs, which are involved in the specification of endoderm and mesoderm, localize to the vegetal cortex during mid-oogenesis. Cytoplasmic transport of these RNAs is dependent upon localization elements (LEs) within their 3' untranslated regions (3'UTRs), as well as several trans-acting factors. The work presented here demonstrates that Vg1 and VegT utilize a shared set of cis- and trans -acting factors for localization, thus providing evidence for a common vegetal localization machinery. Moreover, comparison of the cis-acting requirements for localization of these RNAs has revealed several common features, such that a consensus vegetal LE can now be defined. Analysis of Vg1 and VegT RNP complexes from both the nucleus and cytoplasm has uncovered novel steps in the vegetal localization pathway, demonstrating for the first time that cytoplasmic RNA localization begins with the formation of a core ribonucleoprotein (RNP) complex in the nucleus. This core complex is remodeled following export to the cytoplasm to include additional factors involved in subsequent steps of localization and anchoring. Future studies must therefore focus on understanding links between nuclear events, such as RNP complex formation, and cytoplasmic steps of RNA localization to further elucidate the mechanism of this conserved process.