| The subcellular localization of mRNAs is a widespread and important form of gene regulation that establishes polarity in eukaryotic cells. In the African clawed frog Xenopus laevis, many mRNAs are localized to the vegetal cortex of the full-grown oocyte. These RNAs typically utilize either an early or a late pathway during oogenesis for their localization. Here I show that early and late pathway RNAs compete for common localization machinery in vivo. In addition, while previous work demonstrated that the molecular motor kinesin II is required for localization of the late pathway mRNA Vg1 to the vegetal cortex, I discovered that localizing RNAs recruit kinesin II to the vegetal cortex, suggesting a direct role for this motor in the localization process. This finding also revealed a novel ability of cargo RNAs to differentially recruit the molecular motor that localizes them.;A distinguishing feature of the early pathway is the pre-localization of RNAs to an evolutionary conserved structure called the Balbiani body in stage I oocytes. Early pathway RNAs have been proposed to localize solely by a diffusion and entrapment mechanism, however, I demonstrate that their localization to the Balbiani body is highly dependent on temperature as well as the intracellular concentration of ATP. These findings implicate, for the first time, enzymatically-driven processes in the early RNA localization pathway. Moreover, RNA localization in stage I oocytes is reduced by expression of a kinesin II dominant-negative peptide fragment. Thus, kinesin II not only mediates the localization of RNAs to the vegetal cortex of late-staged oocytes, but also to the Balbiani body in earlier-staged oocytes. Together, these studies show that vegetally localized RNAs utilize a common kinesin II-dependent pathway throughout oogenesis in Xenopus and, more importantly, suggest a mechanism by which differential recruitment of RNA localization factors may be used to specify a wide temporal and spatial array of localized mRNAs in the vertebrate oocyte. |
