Localized RNA

The past decade exploded in the identification of numerous localized RNAs, and in the studies of RNA localization mechanisms. Prior to 1985 only two localized RNAs were known, and between 1985 and 1997 over 40 additional RNAs were discovered. RNA localization is the universal phenomenon occurring both in somatic and germ cells of different animal species, and plays a role in the establishment of cell polarity. In the case of coding RNAs (mRNAs) the RNA localization targets specific proteins of different subcellular compartments. There are also few examples of localized non-coding RNAs which may play a role in the anchoring of other types of localized RNAs to the cytoskeletal elements. The transport of localized RNAs to their destined location depends on microtubule and/or microfilaments, and involves special zipcodes or tags usually localized in 3′ end of RNA molecule. RNA localization during invertebrate oogenesis is critical for the establishment of initial polarity in the radial symmetrical egg, and for the proper axial pattering of the future embryo. Our studies of oogenesis in Xenopus have shown that there are two pathways of RNA localization to the vegetal cortex of growing Xenopus oocytes. First of these two pathways which we have called METRO (MEssenger TRransport Organizer) operates in stage 1-3 oocytes, involves Balbiani body (mitochondrial cloud, nudge), and localizes Xlsirts, Xcat2 and Xwnt11 RNAs to the vegetal cortex of the oocyte. Second pathway localizes RNAs such as Vg1 and functions in later stages of oogenesis (stages 2-4). We have found that although the late pathway machinery is dependent upon the prior functioning of the METRO pathway. These results demonstrate that the localization of RNAs within the same cell type is an extremely complex process which involves several different mechanism operating in a well-defined temporal and spatial order.