RNA Export

RNA export from the nucleus can be divided into two general phases. The first involves ribonucleoprotein (RNP) movement from the site of RNA transcription and RNP assembly to the nuclear pore complex (NPC). Proposed mechanisms for this step include diffusion and motor-driven transport along a nuclear skeleton or matrix (reviewed by Rosbash and Singer, 1993), but definitive support for either hypothesis is currently lacking. Intranuclear movement is followed by translocation of the RNP through the NPC. In favorable cases (when the RNP is very large), this can be visualized in the electron microscope (see Figure t). The study of RNA export is still at an early stage, and our understanding of the process is fragmentary. This review will therefore mainly summarize the data implicating various factors in the export process. Some nucleoporins (NPC proteins) have roles in RNA export, and the small GTPase Ran, together with its cofactors RCC1 and RNA1, is involved. In addition, a few RNA-binding proteins that may mediate RNP transport have been identified. However , nothing is known about the detailed mode of action of any of these factors in export, nor about how they interact with each other. It is highly likely that translocation through the NPC is associated with alterations in RNP composition, but details of these changes or of their functional significance for export have not been elucidated. Diverse approaches have been used to study RNA export, and we will not attempt to portray the field as an integrated whole. Instead, we will discuss how different aspects of the problem have been tackled and where they may converge in the future. Experimental Approaches To date, two radically different strategies have provided much of our insight into RNA export. These are microinjec-tion into Xenopus oocytes and examination of yeast mutants. In the former, export of RNA can be monitored after nuclear injection either of purified or in vitro transcribed RNA or of DNA templates that are transcribed in the oo-cyte. Similar results are obtained following DNA or RNA injection, with accurate discrimination between substrates for nuclear retention versus export. In addition, similar ki-netics of RNA export and susceptibility of export to mutations in the RNA are seen using both approaches RNA packaging and export can be uncoupled from transcription and processing events. The first basic observations in the field, for example, that RNA export is energy dependent and saturable and thus a carrier-mediated process …