Sending RNAs into the future: RNA localization and germ cell fate

RNA localization is a cellular mechanism used to localize proteins to subcellular domains and to control protein synthesis regionally. In oocytes, RNA localization has profound implications for development, setting up local concentrations of regulatory proteins that will establish regional fates in the future embryo. One such fate is that of the germ cell lineage. In a diverse number of organisms, including Drosophila and Xenopus, the germ cell lineage is determined by the inheritance of germ plasm assembled during oogenesis. This process requires the recruitment of specific RNAs, many now identified, to the germ plasm. Complex signals located in the 3' UTR direct RNAs to their destinations. These signals are sites for protein binding and assembly into particles competent to localize. Three different mechanisms have been described that operate during oogenesis or embryogenesis to localize RNAs in the germ plasm: motor driven transport, differential stability, and entrapment. Each of these localization mechanisms must be coordinated with translation and anchoring mechanisms to achieve functional germ plasms. Here we review what is known about these processes in germ cells, but the cellular mechanisms that select and transport RNAs are likely to be conserved among somatic cells as well.