CELL STRUCTURE AND FUNCTION 25: 227–235 (2000)© 2000 by Japan Society for Cell Biology

REVIEW

The Molecular Mechanisms of Messenger RNA Nuclear Export
Mikiko C. Siomi
Institute for Genome Research, University of Tokushima, 3-18-15 Kuramoto, Tokushima 770-8503, Japan

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 areprobably 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

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 (Artset al., 1998a; Fornerod et al., 1997; Kutay et al., 1997).
Cytoplasmically localized RanGAP and RanBP1 facilitatehydrolysis 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 theenergy source for the nuclear transport is still questionable(Englmeier et al., 1999; Izaurralde et al., 1997b; Kose et al.,
1997; Nakielny and Dreyfuss, 1998; Ribbeck et al., 1999;
Richards et al., 1997; Schwoeble et al., 1998) and needs to
be investigated further.
Among all the nuclear import factors so far identified, thebest known is importin > (Görlich et al., 1995). This protein
associates with the cargos through a 60 kDa protein termed
importin =, which directly recognizes the nuclear import signals within the cargos. The majority of the nuclear transport
factors show some but significant similarities to importin >and they are known as members of the importin > family(Nakielny and Dreyfuss, 1999 and references therein). For
the active nuclear import, a large number of factors havebeen identified and well characterized and it has also been
revealed that some import factors do not belong to the importin > family (Ribbeck et al., 1998; Smith et al., 1998). In
contrast to the nuclear import, studies in the nuclear export,
especially of RNA molecules, seem to be somewhat behind.
Even so, the molecular mechanisms of U snRNA and tRNA
export have been becoming clear as results of the recent in-

In eukaryotic cells, genomes, the templates for generaltranscription, are sequestered within the nucleus and thiscompartment is the only place in cells devoted to thesynthesis 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) locatedacross 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 andproteins 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 specificsequences (nuclear import or export signals) within the proteins to be transported (cargos) and then transport them inconcert with Nups (for reviews see Izaurralde and Adam,
1998; Mattaj and Englmeier, 1998; Nakielny and Dreyfuss,1999). Directionality of nuclear transport is provided by asmall 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 upontranslocation across the nuclear membrane (Görlich et al.,Institute for Genome Research, University of Tokushima, 3-18-15
Kuramoto, Tokushima 770-8503, Japan.
Tel: +81-88-633-9456, Fax: +81-88-633-9451
E-mail: siomim@genome.tokushima-u.ac.jp

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M.C. Siomi

is essential for the process (Eckner et al., 1991; Hamm andMattaj, 1990; Jarmolowski et al., 1994; Sun et al., 1992). In
the case of U snRNA, however, the same 5’ cap structure is
known to be necessary for its export. As briefly describedabove, the cap-binding complex (CBC) comprising CBP80
and CBP20 first binds to the m7G cap and then a novel 55
kDa protein termed PHAX interacts to the complex, which
in turn is recognized by exportin 1 and finally the export
complex comes into existence (Ohno et al., 2000). It is very
interesting that the requirement of an identical structure (i.e.
5’ cap) differs in the case of mRNA and U snRNA export(Hamm and Mattaj, 1990; Izaurralde et al., 1995; Jarmolowski et al., 1994). It has been shown under immunoelectron microscopy that CBP20 at least accompanies
mRNAs known as Balbiani ring in Chironomus tentans
salivary glands while they are exported through the nuclear
pores to the cytoplasm (Visa et al., 1996b). Therefore, it is
not difficult to speculate that the contribution of the 5’ cap/
CBC complex to the mRNA export could be greater than weknow at this date.

tensive investigation (for reviews see Cullen, 2000; Strasser
and Hurt, 2000). The factors identified for U snRNA andtRNA export are exportin 1/CRM1 (Fornerod et al., 1997)
and exportin-t (Arts et al., 1998a; Kutay et al., 1998; Sarkar
and Hopper, 1998), respectively. Although both belong to
the importin > family, their binding styles to the cargos differ from each other. While exportin-t can bind tRNA directly (Arts et al., 1998b; Lipowsky et al., 1999; Lund andDahlberg, 1998), exportin 1 associates with U snRNA only
through particular proteins, cap-binding complex (CBC)and a novel protein termed PHAX (Izaurralde et al., 1995;
Ohno et al., 2000).
Despite intensive investigation, the molecular mechanisms of mRNA export have not become completely clear
yet. However, recent studies on mRNA export revealed anumber of RNA-binding proteins which fall under the category of trans-acting mRNA export factors. Interestingly
some proteins among them act as importin > does; namely
showing the ability to bind both the cargos and NPCs and todirectly mediate mRNA export, although they certainly do
not belong to the importin > family. In this review the soluble factors which play critical roles in mRNA export, mainly in the process occurring within the nucleus of vertebratecells, will be focused and described.

hnRNP proteins
A subset of specific proteins which bind nascent premRNAs upon transcription are known as heterogeneous ribonucleoprotein (hnRNP proteins) (Dreyfuss et al., 1993).
The hnRNP proteins comprise about 20 major proteins andform higher order structures on pre-mRNAs that are refer toas hnRNP complexes (Krecic and Swanson, 1999). It is
widely believed that the hnRNPs are the structural entities
that serve as substrates for the processing reactions that generate mature mRNAs. Although the subcellular localization
shown by immunostaining is predominantly nuclear, morethan several hnRNP proteins such as hnRNP A1 continuously shuttle between the nucleus and the cytoplasm (Krecic and Swanson 1999; Pinol-Roma and Dreyfuss, 1991).
Other hnRNP proteins are retained in the nucleus once
they are imported to the nucleus after translation in thecytoplasm (Nakielny and Dreyfuss, 1996).
The shuttling hnRNP proteins, especially hnRNP A1, aremost likely to play key roles in the mRNA export (Fig. 1).
The reasons are following; first, hnRNP A1 is a nuclear
RNA-binding protein that shuttles between the nucleus andthe cytoplasm. Second, hnRNP A1 is shown to bindmRNAs not only in the nucleus but also in the cytoplasm(Pinol-Roma and Dreyfuss, 1992) and at last an A1-likeprotein in C. tentans has been shown under immunoelectron
microscopy to translocate through the nuclear pores in association with Balbiani ring (Daneholt, 1999; Visa et al.,1996a). It could be thought that hnRNP A1 was just carried(piggy-backed) to the cytoplasm by binding to mRNA molecules, but it became very unlikely since a nuclear export
signal (NES) with no detectable RNA-binding activity was
identified within the sequence of hnRNP A1 (Michael et al.,1995). The NES of hnRNP A1 corresponds to the same re-RNA processing and mRNA export
In eukaryotic cells, RNA synthesis is mostly carried out
by RNA polymerase I, II, or III. RNA molecules transcribedby RNA polymerase II can be divided into two groups; snRNA (small nuclear RNA) and the major products of the enzyme, hnRNA (heterogeneous nuclear RNA). The precursors of actually translatable mRNAs, pre-mRNAs, belong to
the latter group. Upon transcription, various kinds of RNAbinding proteins interact with pre-mRNAs and a variety of
posttranscriptional processing events take places. Three major processing events are 5’ capping, removal of introns,
and polyadenylation to the 3’ end. After the events, premRNAs become to be matured mRNAs and then are exported from the nucleus to the cytoplasm for the expression ofgenetic information.
Important links between splicing and export have recently
emerged (Kataoka et al., 2000; Zhou et al., 2000). Apparently, mRNPs that have undergone intron-removal event have a
different structure and at least one protein composition thanmRNPs containing identical RNA that was not produced bysplicing. Some of the proteins associated specifically withspliced mRNPs are mediators of mRNA export and are discussed in more detail below. Suffice to say here, the processof splicing appears to result in the loading of mRNA exporfactors. This provides one way to couple splicing and export
such that RNA molecules that have not passed through the
splicing machinery do not exit the nucleus.
Both the 5’ cap structure and the 3’ poly (A) tail of
mRNAs can influence the rate of mRNA export but neither
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Fig. 1. Various trans-acting proteins for mRNA export in cells. pre-mRNAs transcribed by RNA polymerase II are bound with a number of RNA-binding
proteins including hnRNP proteins and hnRNP complexes are formed on the nascent transcripts. After a variety of RNA processings occur, pre-mRNAs become matured mRNAs and some proteins identified recently as trans-acting factors such as TAP and Aly are recruited to the mRNP complexes. Nonshuttling hnRNP proteins which likely function in retaining intron-containing pre-mRNAs in the nucleus are released and as the result the export of mRNP complexes takes place. During translocation through the NPCs, Dbp5p may work on the RNA molecules in the mRNP complexes to unwind them and /or torelease from the complexes in the cytoplasm. Whether proteins belonging to importin b family are involved in mRNA export is not known yet. CBC may
also be involved in mRNA export as trans-acting factors since CBP20 has been shown to accompany to Balbiani ring transcripts in C. tentance.

by binding of RanGTP to TRN in the nucleus upon translocation (Izaurralde et al., 1997a; Siomi et al., 1997). TRN in
the nucleus then seems to remain binding to RanGTP untill
TRN itself returns to the cytoplasm, meaning that although
M9 acts as both the NLS and the NES of A1, TRN is unlikely to be also the export factor of A1. Despite further investigation, the export factor for hnRNP A1 has not been identi-

gion identified as the nuclear import signal (NLS), a 38amino acid domain termed M9 (Siomi and Dreyfuss, 1995;
Weighardt et al., 1995). An importin > family member,transportin (TRN), mediates the nuclear import of hnRNP
A1 while it binds the M9 region specifically (Pollard et al.,1996). As is the case for the import pathway mediated with
importin >, hnRNP A1/TRN complexes become dissociated229

M.C. Siomi

though, the inhibition of the mRNA export was not observed. This inhibition was most likely triggered by M9 titrating the endogeneous factors needed for the mRNA export, supporting the idea that M9 is one of the export signals
crucial for the mRNA export in living cells. For references,fied yet.
The other data which supports that hnRNP A1 is one of
the mRNA export factors is that nuclear microinjections of
A1 in Xenopus oocytes inhibit mRNA export (Izaurralde etal., 1997a). When A1 lacking M9 was injected instead,Fig. 2. (A) Schematic drawing of the structures of hnRNP A1 and Aly. The RBDs and the other regions rich in glycine in both proteins are shown by blackand blue boxes, respectively. The RGG box are highlighted in yellow. M9 identified as the NLS and NES of hnRNP A1, to which TRN specifically interacts
and mediates the nuclear import of hnRNP A1, is shown by a green box. hnRNP A1 is known to bind RNAs through the RBDs and hence the RBD in Aly islikely responsible for the RNA-binding activity of the protein. (B) The amino acid sequence alignment of Aly, REF2, Yra1p and Y14. The region
corresponding to an RBD in Aly is shown with a gray line on the top of the sequence. Amino acid residues in REF2, Yra1p, and Y14 identical or very
similar to those in Aly (including residues in Aly) are highlighted in gray.

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the yeast homolog of hnRNP A1 is Npl3p. NPL3 was foundin a genetic screen for poly(A)+RNA export and hence it issuggested that Npl3p plays an critical role in mRNA export
pathway (Strasser and Hurt, 1999). These data also strongly
support the hnRNP A1 function in mRNA export.
Among nonshuttling proteins, the best-characterized is
hnRNP C1. A nuclear retention signal (NRS) has been identified within the protein (Nakielny and Dreyfuss, 1996). It
has been demonstrated that the NRS can override an NES
identified within shuttling hnRNP proteins like M9. The
data led a scenario that pre-mRNA associated with both
NES-containing and NRS-containing proteins is retained in
the nucleus until the NRS-containing proteins are released
from the complex. As a matter of fact, hnRNP C1 is known
to preferably bind intron-like sequences (for a review see
Dreyfuss et al., 1996) and some insect hnRNP proteins,
likely the functional homologs of nonshuttling human
hnRNP proteins, are released from mRNP complexes justbefore mRNA export (Sun et al., 1998). This may be one of
the mechanism that ensures only fully matured mRNA molecules are exported.

member.
Mex67p is the yeast ortholog of TAP. Mex67p was identified in a genetic screen as a protein essential for the exportof poly(A)+RNA in yeast (Segref et al., 1997). Like TAP,
Mex67p also binds poly(A) +RNA and interacts with NPCs.
Involvement of a protein termed Mtr2p seems to be criticalfor both interactions (Santos-Rosa et al., 1998). Although
the sequence at the amino acid level is not homologous, p15
was identified in vertebrates as a protein functionally equivalent to Mtr2p (Katahira et al., 1999). The combination of
TAP and p15 could rescue the viability of yeast cells lacking the function of Mex67p and Mtr2p, suggesting that Tap
plays a critical role in cellular mRNA export (Katahira
et al., 1999). It has also been demonstrated that in Xenopus
oocytes exogeneously injected TAP could overcome theinhibition of nonviral mRNA export caused by excess
amounts of CTE, which in addition supports the idea of
TAP involvement to the cellular mRNA export (Fig.
1)(Gruter et al., 1998; Pasquinelli et al., 1997). In vertebratecells, however, p15 is demonstrated to be not essential for
NPC binding and shuttling of TAP and more curiously for
the CTE-containing RNA export mediated by TAP (Bachiet al., 2000; Kang and Cullen, 1999). At least, p15 can forma ternary complex with TAP and CTE, suggesting that p15
plays an important but unknown role in the mRNA export.
Studies on the structural properties of TAP have revealed
that a domain near the carboxyl terminus is the NPCdocking domain and functions as both the NLS and NES of
TAP. It is also reported that TAP contains an additional
NLS which interacts with TRN, the nuclear import factor
originally identified for hnRNP A1 (see above)(Bachi et al.,
2000; Bear et al., 1999; Truant et al., 1999). Tap may
choose one of the signals to suit the occasion depending on,
for example, the concentration of TAP demanded in the
nucleus.

TAP
Nuclear export of intron-containing mRNA is indispensable for retroviral gene expression and replication. In studies of the molecular mechanisms in type D retroviruses suchas the simian retrovirus type1 (SRV-1), the constitutive
transport element (CTE) was identified in the viral RNA genome (Bray et al., 1994). CTE is a structured RNA elementthat induces unspliced viral mRNA export in infected cells.
Functionally, CTE is very similar to the Rev response element (RRE) identified in the mRNA of human immunodeficiency virus-1 (HIV-1) (Pollard and Malim, 1998). In thecase of RRE, the molecular mechanism of how the unspliced RRE-containing mRNA is exported in infected cells
has been well characterized and it is clear that a virally encoded RNA-binding protein, Rev, promotes the export
while it binds RRE directly (Pollard and Malim, 1998). Rev
is known to contain a leu-rich NES, with which the export
factor, exportin 1, interacts subsequently and mediates theexport of the complex. It has been demonstrated that the
RRE in the HIV mRNA can be functionally replaced by theCTE export signal from SRV-1 (Bray et al., 1994; Zolotukhin et al., 1994). This strongly suggests that a cellular
protein binds the CTE specifically and plays a similar roleto the viral Rev protein. TAP was identified as such a cellular protein (Braun et al., 1999; Gruter et al., 1998; Kang andCullen, 1999). Upon further investigation, TAP is now
known to continuously shuttle between the nucleus and thecytoplasm and to directly interact with some nucleoporins(Bachi et al., 2000; Kang and Cullen, 1999; Katahira et al.,1999). These characteristics of TAP raised an interesting
possibility that in contrast to Rev, TAP-mediated unspliced
mRNA export does not depend on any importin > family

Aly and Y14
In order to identify Mex67p-interacting proteins otherthan Mtr2p, synthetic lethal screening was carried out and aprotein termed Yra1p was discovered as Mex67p-binding
protein (Strasser and Hurt, 2000). Yra1p was originally reported as a novel nuclear protein with RNA annealing activity in yeast (Portman et al., 1997). It has been reported thatin mouse there are at least two Yra1p orthologs termed
REF1 and REF2 (RNA and export factor binding proteins)(Stutz et al., 2000). REF1 is also known as Aly, which
was originally reported as a novel context-dependent transcriptional coactivator of two proteins (LEF-1 and AML-1)
in the T-cell receptor alpha enhancer complex (Bruhn et al.,1997). Recent studies on mRNA export revealed that Aly is
recruited to mRNP complexes generated by splicing but
does not associate with artificially produced intron-lessmRNP (Zhou et al., 2000). Aly seems to be recruited during
splicing assembly and consistent with this, the protein co231

M.C. Siomi

quired for the mRNA export (Tseng, 1998; Schmitt, 1999;
Snay-Hodge, 1998). Dbp5p contains a DAED box andtherefore is likely with RNA helicase activity. Dbp5p is
predominantly localized at the cytoplasmic side of the
NPCs, although it is known to shuttle between the nucleus
and the cytoplasm. In fact, Dbp5p was reported to interact
with Yra1p (Schmitt et al., 1999), the Aly yeast ortholog,
which is known to be localized in the nucleus in yeast cells(Portman et al., 1997). Because of the probable RNA
helicase activity of Dbp5p, the predicted function of thisprotein is unwinding of RNA molecules during and/or
shortly after the translocation through the nuclear pores orremoving proteins from mRNP complexes in the cytoplasm.

localizes with splicing factors in the nucleus. Aly also meetsanother criterion to be a trans-acting protein participating inmRNA export; namely it shuttles between the nucleus andthe cytoplasm and more importantly, it specifically stimulates mRNA export in Xenopus oocytes. In conclusion, Aly
possibly functions as a regulator to ensure that only spliced
mRNAs are efficiently exported and as an intermediary between the two events, splicing and mRNA export (Fig. 1).
Aly (REF1) was originally described as an hnRNP-likeprotein (Stutz et al., 2000), because it is a nuclear RNAbinding protein containing an RNA-binding domain (RBD)
often observed in hnRNP proteins (Fig. 2A). However, it
was clearly shown that Aly is not likely so, since it is not an
H-complex (an assembled complex on pre-mRNA molecules upon transcription) component while all hnRNP proteins are found in the complex (Zhou et al., 2000). Structurally, though, hnRNP A1 and Aly show significant
homology; i.e. they both contain not only RBD(s) but alsoan RGG box, the remaining regions of the proteins are richin glycine, and even the sizes of the proteins are similar toeach other. Recently and coincidentally, novel proteinsquite homologous to Aly (Fig. 2B), have been reported, oneof which is Y14 (Kataoka et al., 2000), a protein identified
as an interacting protein to RanBP5, an importin >-like import factor (Deane et al., 1997; Jakel and Görlich 1998;
Rout et al., 1997). Y14 shows interesting similarities to Aly;
namely Y14 is also a nuclear RNA-binding protein with one
RBD, shuttles between the nucleus and the cytoplasm, is localized within nuclear speckles, and finally is associatedpreferentially with spliced mRNAs. Nevertheless the cellular function of Y14 seems not to completely overlap withthat of Aly. Whether all the proteins similar to Aly can bethought of as members of one family or what determines
among those proteins to function as mRNA export factor
are the questions needed to be addressed in this area. Considering that Aly is only recruited to mRNP complexes generated by splicing but not to artificially produced intron-lessmRNP, the protein is likely associated with export machinery through protein-protein interactions. Which region in
Aly is responsible for its specific recruitment to the machinery is one of the interesting questions to be answered.

Perspective
None of the proteins described in this review as trans-acting mRNA export factors belong to the importin > family of
nuclear transport factors. Is any one of the importin > family
members in fact necessary for mRNA export (Fig. 1)? As
described above, tRNA export is mediated by exportin-tbelonging to the importin > family. It has been reported that
under the condition where RanGTP was depleted from thenucleus, tRNA export was completely blocked and theblock was relieved by supplement of RanGTP (Izaurralde
et al., 1997b). The results were quite reasonable since
exportin-t interacts with tRNA molecules only in thepresence of RanGTP. Under the same conditions mRNA
molecules behaved similarly, except that the inhibition
caused by depletion of RanGTP from the nucleus was partial and a portion of the mRNA molecules were still
exported to the cytoplasm. All things considered, in the caseof mRNA export it is possible that molecule(s) belonging to
the importin > family are also involved and play certain
roles to some extent. Compared to tRNA and U snRNA,
mRNA represents a much more complex group of cargomolecules. Therefore, it is easy to imagine that mRNAs areexported by multiple mechanisms. The intricacy certainly is
the main hindrance to the study of mRNA export. Needlessto say, identifying and understanding the factors and mechanisms underlying mRNA export is of great consequence.
Emergence of novel RNA export factors is clearly to be
expected and this will surely reveal unanticipated aspects ofcontrol of gene expression.

Other potential factors
Several other proteins have been reported that might play
important roles in the mRNA export. One example is Rae1
(Brown, 1995; Bharathi, 1997). In S. pombe, Rae1 is essential for poly(A)+RNA export and this deficiency can becomplemented by the human Rae1. Human Rae1 meets thecriteria to be a mRNA export factor; Rae1 shuttles between
the nucleus and the cytoplasm and predominantly associateswith NPCs (Kraemer and Blobel, 1997; Pritchard, 1999). It
has also been demonstrated to associate with poly(A)+ RNA
in cells. However, the role played by Rae1 in the mRNA export is not clear yet. Dbp5p is also known as a protein re-

Acknowledgments. I thank Dr.Sara Nakielny, Dr. Paul Eder, Dr. Tetsuya
Taura, and Dr. Haruhiko Siomi for discussion and comments on the manuscript. I would like to apologize for omitting many important citations dueto the limited space. I am supported by grants from FRAXA Research
Foundation and Mitsubishi Foundation.
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(Received for publication, October 24, 2000
and accepted, October 24, 2000)

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