The process of localizing a maternal messenger RNA in Xenopus oocytes

The maternal mRNA Vg1 is localized to the vegetal pole during oogenesis in Xenopus. We have cultured oocytes in vitro to begin to understand how this localization occurs. Endogenous Vg1 mRNA undergoes localization when oocytes are cultured in vitro, and synthetic Vg1 mRNA injected into such oocytes is localized in the same fashion. Vg1 mRNA is associated with a detergent-insoluble fraction from homogenized oocytes, suggesting a possible cytoskeletal association. The use of cytoskeletal inhibitors reveals a two-step process for localizing Vg1 mRNA. Microtubule inhibitors such as nocodazole and colchicine inhibit the localization of Vg1 mRNA in late stage III/early stage IV oocytes, but have no effect on Vg1 mRNA once it is localized. The microfilament inhibitor cytochalasin B, however, has little effect on the translocation of Vg1 mRNA in middle-stage oocytes but causes a release of the message in late-stage oocytes. We propose a model for the localization of Vg1 mRNA in which translocation of the message to the vegetal cortex is achieved via cytoplasmic microtubules and the anchoring of the message at the cortex involves cortical microfilaments.