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Fig. 5. Model illustrating the dependence of the fate of localized endogenous and exogenous RNAs in Xenopus oogenesis on their nuclear/cytoplasmic history. Endogenous (A) and exogenous (B) early- and late-pathway RNAs. (1) In stage I oocytes, the early-pathway RNAs, such as Xcat2 mRNA, bind nuclear proteins (possibly Sm proteins) that facilitate transport (via nuclear pores) to the mitochondrial cloud (yellow) and germinal granules (red spheres). The late-pathway RNAs, such as Vg1 mRNA, bind to Vg1RPB/Vera and hnRNP, which form a core complex facilitating export from the nucleus, and diffuse uniformly within the oocyte cytoplasm. (2) Later in oogenesis (starting from late stage II or early stage III), Staufen and Prrp proteins are added to the core complex assembled on late-pathway RNAs. These bind to a molecular motor, such as kinesin I and/or II, that transports RNAs on the MT tracks that form a wedge around the remnants of the mitochondrial cloud. (3) Early- and late-pathway RNAs injected into the nuclei of stage I/early stage II oocytes that bind the appropriate nuclear factors and mimic the localization pattern of their endogenous counterparts. There is no information on the fate of early- or late-pathway RNAs injected into the cytoplasm of stage I oocytes. (4) Early-pathway RNAs injected into the nucleus or cytoplasm of stage III, or older, oocytes behave like late-pathway RNAs migrating on the MTs towards the vegetal cortex. This indicates that the early-pathway binding factors present in the nuclei of stage I/early stage II oocytes are either absent or unavailable in older oocytes. However, the early-pathway RNAs can bind some of the cytoplasmic factors of the late-pathway machinery, and they either mimic the movement of late-pathway RNAs or piggyback on late-pathway RNAs. Late-pathway RNAs injected into the nuclei or cytoplasm of older oocytes bind the appropriate factors and after export into the cytoplasm behave like their endogenous counterparts - either assembling their own transport complexes or piggybacking on endogenous RNAs.
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