2 DISTINCT PATHWAYS FOR THE LOCALIZATION OF RNAS AT THE VEGETAL CORTEX IN XENOPUS OOCYTES

We found that there are two major pathways by which RNAs are localized at the vegetal cortex during oogenesis of Xenopus laevis. One of thes e, through which Xlsirts, Xcat2 and Xwnt11 are localized, involves tra nsport during stages 1 and 2 of oogenesis via a region of the mitochon drial cloud that we call the message transport organizer or METRO. Thi s pathway involved three steps, transport of RNA from the GV to the mi tochondrial cloud, sorting of the RNAs to specific regions of the METR O, and translocation to and anchoring at the vegetal cortex. These thr ee RNAs exhibit a distinct pattern of spatial localization within the METRO when they approach the vegetal cortex. The other pathway is used by Vg1. We detected Vg1 throughout the oocyte cytoplasm during stages 1 and 2. During stage 3 it was translocated to the vegetal cortex and associated with the cortex overlapping the region at which the Xlsirt , Xcat2, and Xwnt11 RNAs are anchored. Our results also showed that an choring of these RNAs was dependent in part on actin microfilaments, b ut was independent of microtubules. These results demonstrate a novel mechanism of translocation and RNA sorting used by RNAs several of whi ch may be involved in the establishment of the embryonic body axis.