The Molecular Mechanisms of Messenger RNA Nuclear Export.

Abstract

ABSTRACT. In eukaryotic cells, the nuclear membrane creates a barrier between the nucleus and the cytoplasm. Whereas RNA synthesis occurs in the nucleus, they mostly function in the cytoplasm; thus export of RNA molecules from the nucleus to the cytoplasm is indispensable for normal function of the cells. The molecular mechanisms involved in each kind of cellular RNA export is gradually understood. The focus of this review will be mRNA export. mRNAs are multiformed. In order to ensure that this variety of mRNA molecules are all exported, cells are probably equipped with multiple export pathways. A number of proteins is predicted to be involved in mRNA export. Ascertaining which proteins play crucial roles in the pathways is the key point in the study of mRNA export.Key words: nuclear transport/RNA export/WRNA/RNA-binding protein soluble nuclear export factors/RNA processing In eukaryotic cells, genomes, the templates for general transcription, are sequestered within the nucleus and this compartment is the only place in cells devoted to the synthesis of RNA molecules. After transcription, the RNA molecules undergo a variety of posttranscriptional processing steps. The majority of RNAs including transfer RNA, rinsbosomal RNA, small nuclear RNA and messenger RNA, are exported to the cytoplasm. Nuclear transport occurs through nuclear pore complexes (NPCs) located across the nuclear envelope. A nuclear pore complex is an assembly of up to one hundred different proteins termed nucleoporins (Nups) (Ohno, 1998;Yang, 1998;Stoffler, 1999). While relatively small molecules such as ions and proteins smaller than 40 kDa can diffuse through the nuclear pores, macromolecules including RNAs traverse the NPC by an active, temperature-dependent manner.Active transport both in and out of the nucleus requires soluble nuclear transport receptors, which first bind specific sequences (nuclear import or export signals) within the proteins to be transported (cargos) and then transport them in concert with Nups (for reviews see Izaurralde and Adam, 1998;Mattaj and Englmeier, 1998;Nakielny and Dreyfuss, 1999). Directionality of nuclear transport is provided by a small GTPase Ran (Dahlberg and Lund, 1998). In the nucleus Ran exists mostly as the GTP-bound form and causes dissociation of cargos from the nuclear import factors upon translocation across the nuclear membrane (Görlich et al., 1996;Rexach and Blobel, 1995). In the case of nuclear export, however, RanGTP is required in efficient binding of nuclear export factors to the molecules to be exported (Arts et al., 1998a;Fornerod et al., 1997;Kutay et al., 1997). Cytoplasmically localized RanGAP and RanBP1 facilitate hydrolysis of Ran-bound GTP to GDP. The exchange of RanGTP to RanGDP causes dissociation of the exported cargos from the nuclear export factors upon translocation to the cytoplasm (Görlich, 1998;Kehlenbach et al., 1999;Kutay et al., 1997). Whether hydrolysis of RanGTP is the energy source for the nuclear transport is still questionable (Englmeier et al., 1999;Izaurralde et...

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