EXOGENOUS TAN RNA IS LOCALIZED IN OOCYTES - POSSIBLE EVIDENCE FOR EVOLUTIONARY CONSERVATION OF LOCALIZATION MECHANISMS

The multistep pathway leading to intracellular RNA localization is kno wn to involve cis-acting signals in targeted mRNAs, which are presumab ly recognized by specific RNA-binding proteins and interact with a fun ctional cytoskeleton. Tau RNA is localized to the proximal hillock of rat axons, and this movement requires intact microtubules. Because Xen opus oocytes demonstrate a clear polarity involving microtubule-mediat ed RNA localization, we have studied the distribution of tau RNA injec ted into oocytes. We find that a fragment from the 3'-untranslated reg ion of tau RNA is localized to the vegetal cortex of stage III/IV oocy tes in a distribution indistinguishable from Vg1 RNA, a vegetally loca lized oocyte mRNA. A fragment from the tau RNA coding region, however, is homogeneously distributed in oocytes. Tau RNA contains a functiona l binding site for Vg1 RBP, a Xenopus microtubule-associated protein t hat binds vegetally localized oocyte RNAs with high affinity, and this binding correlates with vegetal localization ability. The present stu dies demonstrate, for the first time, localization of heterologous RNA in oocytes. Given the role of Vg1 RBP as a mediator of specific RNA-m icrotubule interactions, these results are strong evidence that Vg1 RB P is involved in the vegetal localization of RNAs in oocytes and raise the intriguing possibility of the existence of proteins with similar function in neurons. (C) 1996 Academic Press, Inc.