Choosing the Right Path RNA molecules are synthesized in the cell nucleus, yet many have to be moved to the cytoplasm to be processed and/or to effect their function. Different classes of RNA are transported from the nucleus by different transport systems. Messenger RNAs (mRNAs) and uridine-rich small nuclear RNAs (U snRNAs) are transcribed by RNA polymerase II and are capped and bound by the cap-binding machinery in the nucleus but are exported by different protein complexes. The feature that distinguishes the two classes of RNA is their length: U snRNAs are short and mRNAs are long. Using an in vitro system and human tissue culture cells, McCloskey et al. (p. 1643 ) show that the length of the RNAs is measured by the heterogeneous nuclear ribonicleoprotein (hnRNP) C tetrameric protein complex. The hnRNP C cannot bind to the short U snRNAs, allowing the U snRNA-specific export adaptor protein, PHAX, to bind and mediate export. Longer mRNAs are bound by hnRNP C, which prevents the binding of PHAX, thus identifying these RNAs for export from the nucleus via the mRNA pathway.

Eukaryotic cells are separated by the nuclear envelope into two major compartments, the nucleus and the cytoplasm. This compartmentalization requires transport through the nuclear pore complexes, the channels for material exchange across the nuclear envelope barrier (reviewed in reference 40). The vast majority of RNA species, following their synthesis and processing in the nucleus, are exported to the cytoplasm. Different RNA species, such as tRNAs, U snRNAs, mRNAs, and rRNAs, utilize distinct export pathways, i.e., distinct sets of export factors (reviewed in reference 5). Accumulating evidence shows that the pathway of RNA export can influence the fate of a given RNA in the cytoplasm (5), indicating the biological importance of the choice of RNA export pathway. However, how export factors of each pathway are loaded onto the corresponding RNA is largely unknown.The major exporter for mRNAs is the TAP-p15 heterodimer (10,16,33). TAP is one of the few non-importin␤ family transport receptors known to date. Although TAP itself can bind RNA, its recruitment to mRNAs requires adaptor proteins. RNA binding adaptor proteins, including Aly/REF and shuttling SR proteins, should first be recruited to mRNA, and these adaptor proteins in turn recruit mRNA exporter TAPp15 heterodimer onto the RNA through protein-protein interactions (12,13,31,37,39,41).In yeast (Saccharomyces cerevisiae), the recruitment of adaptor proteins is coupled to transcription. Aly/REF (Yra1 in yeast) together with the DEXD-box RNA helicase UAP56 (Sub2 in yeast) interacts with the transcription elongation complex (THO complex), forming a larger protein complex (1, 38).Owing to this complex formation, Aly/REF and UAP56 are recruited onto the nascent mRNA transcript during transcription elongation, and hence this larger protein complex is termed TREX (transcription/export) complex (1, 38). In vertebrates, however, splicing rather than transcription plays an important role in loading these adaptor proteins onto RNA. Aly/REF and UAP56 are loaded onto mRNAs in a splicingdependent manner as components of exon junction complex and TREX complex (3,17,20,25). Interaction between Aly/ REF and UAP56 plays an important role in this recruitment (23,36).However, splicing is not essential for recruiting export factors onto mRNAs. mRNAs produced from genes without introns (intronless mRNAs) are also exported efficiently. A certain class of intronless mRNAs contains specific RNA elements, e.g., the intronless transport element in histone H2A mRNA (14). Such RNA elements function as high-affinity binding sites for specific RNA binding adaptor proteins (12,13). Another class of intronless mRNAs, which apparently does not contain such specific elements, still can recruit adaptor proteins such as Aly/REF. It is already well known that Aly/REF interacts with mRNAs in a splicing-independent manner, since Aly/REF plays a role in the export of intronless mRNAs without specific elements (6,18,26,29,31). We previously showed that a stretch of unstructured RNA region of a cer...

Different RNA species are exported from the nucleus by distinct mechanisms. Among the different RNAs, mRNAs and major spliceosomal U snRNAs share several structural similarities, yet they are exported by distinct factors. We previously showed that U1 snRNAs behaved like an mRNA in nuclear export if various ∼300-nucleotide fragments were inserted in a central position. Here we show that this export switch is dependent on the length of the insertion but independent of its position, indicating unequivocally that this switch is indeed the result of RNA length. We also show that intronless mRNAs can be progressively converted to use the U snRNA export pathway if the mRNAs are progressively shortened by deletion. In addition, immunoprecipitation experiments show that the protein composition of export RNPs is influenced by RNA length. These findings indicate that RNA length is one of the key determinants of the choice of RNA export pathway. Based on these results and previous observations, a unified model of how an RNA is committed to a specific export pathway is proposed.[Keywords: RNA export; mRNA; U snRNA; RNA length] Supplemental material is available at http://www.genesdev.org.

Intron-containing pre-mRNAs are retained in the nucleus until they are spliced. This mechanism is essential for proper gene expression. Although the formation of splicing complexes on pre-mRNAs is thought to be responsible for this nuclear retention activity, the details are poorly understood. In mammalian cells, in particular, very little information is available regarding the retention factors. Using a model reporter gene, we show here that U1 snRNP and U2AF but not U2 snRNP are essential for the nuclear retention of pre-mRNAs in mammalian cells, showing that E complex is the major entity responsible for the nuclear retention of pre-mRNAs in mammalian cells. By focusing on factors that bind to the 3¢-splice site region, we found that the 65-kD subunit of U2AF (U2AF 65 ) is important for nuclear retention and that its multiple domains have nuclear retention activity per se. We also provide evidence that UAP56, a DExD-box RNA helicase involved in both RNA splicing and export, cooperates with U2AF 65 in exerting nuclear retention activity. Our findings provide new information regarding the pre-mRNA nuclear retention factors in mammalian cells.