MINIREVIEW Nucleocytoplasmic shuttling of STAT transcription factors Thomas Meyer and Uwe Vinkemeier Abteilung Zellula ¨ re Signalverarbeitung, Leibniz-Forschungsinstitut fu ¨ r Molekulare Pharmakologie, Freie Universita ¨ t Berlin, Germany The s ignal transducer an d activator of transcription (STAT) proteins have initially been described as cytoplasmic proteins that enter the nucleus only after cytokine treatment of cells. Contrary to this assumption, it was demonstrated that STATs a re constantly shuttling between nucleus and cyto- plasm i rrespective o f c ytokine stimulation. This happens both via carrier-dependent as well as carrier-independent transportation. Moreover, it was also recognized that cyto- kine stimulation triggers nuclear retention of dimeric STATs, rather than affecting the rate of nuclear import. In summary, it is increasingly being appreciated that STAT nucleocytoplasmic cycling determines the quality of cytokine signaling and also constitutes an important area for micro- bial intervention. Introduction Multicellular organisms utilize a n integrated n etwork of cell–cell communications and humeral interactions to coordinate complex cellular processes such as proliferation, differentiation, and homeostasis. Cells recognize external stimuli and transfo rm the signals i nto a cellular response, which most often result in an alteration in the pattern of expressed genes. Many s ignal transducers that f unction as transcription factors have to traverse the barrier of the nuclear envelope in order t o gain a ccess to specific target genes within the nuclear compartment. The Janus kinase (JAK)-signal tr ansducer and activator of transcription (STAT) pathway is regarded as a paradigmatic model f or such a direct signal transduction, because it transmits information received from e xtracellular polypeptide signals without the interplay of second messengers directly to target promoters in the nucleus [1]. The STAT proteins comprise a family of evolutionarily conserved t ranscription factors and in mammalian cells seven known STAT proteins were identified, den oted STAT1, STAT2, STAT3, STAT4, STAT5a, S TAT5b, and STAT6, all of which are activated by a distinct set of cytokines a nd growth factors [1]. T hese proteins consist o f several conserved functional domains. The amino terminal N-domain is responsible for t etramerization of all STATs (with the probable exception of STAT2), and this domain also regulates receptor recognition and phosphatase recruitment for some STATs [2–5]. The N-domain is followed by a coiled-coil domain implicated in protein– protein interactions [6], a DNA binding domain [7], a linker domain that participates in DNA binding [8], an SRC homology 2 (SH2) domain that mediates dimeriza- tion and receptor binding [9], and a carboxy-terminal transactivation domain [10]. Best characterized is the role of STAT proteins in cytokine signaling. Upon binding of extracellular ligands such as interferons or interleukines to their cognate receptors, re ceptor-asso ciated Janus kinases, of w hich four have been described in mammalian cells (JAK1, JAK2, JAK3 and T YK2), undergo t yrosine a utophosphorylation and transphosphorylate tyrosine-containing motifs on the intracellular receptor chains, thus creating docking sites for the SH2 domain of STAT molecules [11]. Subsequently, the JAKs catalyze the phosphorylation o f a single tyrosine residue in the c arboxy t erminus of STAT proteins [10,12]. The tyrosine-phosphorylated STATs detach from the intracellular receptor tail and homo- or heterodimerize due to reciprocal phosphotyrosine-SH2 interaction ([1] and Fig. 1). Before exposure of cells to cytokines the STAT molecules are nontyrosine phosphorylated, but may assem- ble into dimeric and higher order complexes [13,14]. Structurally and functionally these aggregates remain sparsely characterized. Therefore, throughout this review we will use the term ÔdimerÕ as shorthand for Ôtyrosine- phosphorylated dimerÕ. A characteristic but until recently poorly understood phenomenon associated with cytokine stimulation o f cells is the inducible and transient accumulation of STAT proteins [10]. Once in the nucleus, STAT dimers can directly bind to nonameric DNA sequences known as gamma-activated sites (GAS) in the promoter region of cytokine-responsive genes resulting in gene transcription [7]. Several years ago, Yoneda and coworkers s howed that cytokine stimulation with concomitant dimerization of tyrosine-phosphorylated STATs induces their association with importin t ransport factors [15]. Next, we will describe what is presently known about the m olecular basis o f t his process. Correspondence to U. Vinkemeier, Abteilung Zellula ¨ re Signalverar- beitung, Leibniz-Forschungsinstitut f u ¨ r Molekulare Pharmakologie, Freie Universita ¨ t Berlin, Robert-Ro ¨ ssle-Str. 10, 13125 Berlin, Germany. Fax: +49 30 94793 179, Tel.: +49 30 94793 171, E-mail: vinkemeier@fmp-berlin.de Abbreviations: CRM1, chromosomal region maintenance 1; dsNLS, dimer-specific nuclear localization signal; GAS, gamma-activated sites; JAK, Janus kinase; NLS, nuclear localizati on signal; NPC, nuclear pore complex; S H2, SRC homology 2; STAT, signal transducer and activator of transcription. (Received 18 August 2004, accepted 7 October 2004) Eur. J. Biochem. 271, 4606–4612 (2004) Ó FEBS 2004 doi:10.1111/j.1432-1033.2004.04423.x Requirements for cytokine-induced nuclear import of STATs Macromolecules and ions alike have to traverse the nuclear membrane through specialized structures called nuclear pore complexes (NPCs) [16]. The NPCs constitute high- order octagonal c hannels that are an integral part of the nuclear envelope. They are composed of proteins called nucleoporins which are present in multiples, and s ome of them contain hydrophobic phenylalanine/glycine (FG)-rich repeat motifs [16]. Macromolecules exceeding a molec ular mass of  40 kDa are generally barred from freely crossing the nuclear membrane by random d iffusion [17]. Thus, the NPCs function as selectivity filters by r estricting the transport of some mac romolecules, while allowing the rapid translocation of o thers. Detailed mechanistic insight has been acquired into translocation mechanisms that rely on tra nsport receptors of the karyopherin superfamily of proteins [18]. Karyophe- rins mediate either import into or export from the nucleus and t hey are therefore also c alled i mportins or exportins, respectively. They recognize loosely conserved sequence motifs on the surface of their substrates (also called cargoes). These signals allow the association with cargo proteins and the subsequent passage of the complex through the nuclear pore. Importins and e xportins, although structurally rela- ted, differ in their sequence r equirements for cargo associ- ation, as nuclear localization signals (NLS) are usually rich in basic residues, while nuclear export signals are charac- terized by the presence of hydrophobic residues, usually leucines [19]. It is believed that the karyopherins act as chaperones during nucleocytoplasmic translocation. Pas- sage through the pore appears to require weak and transient binding to th e nucleoporin FG repeats, an inter action that by itself was shown to occur independently o f metabolic energy [20,21]. Energy consumption, however, confers directionality to this process, which therefore was also termed active transport. The driving force behind the active translocation is created by Ran-GTPase nucleotide exchange factors, which are distributed asymmetrically between cytosol and nucleus [22]. Nucleotide hydrolysis by RanGAP, the cytoplasmically localized RanGTPase- activating protein, r esults in high levels of RanGDP in the cytosol. In the presence o f RanGDP, i mportins are loaded with substrates and may translocate through the NPC into the nucleus, wh ile the export receptors are liberated from their cargo molecules in this environment. The reverse reactions take place in the nucleus. Here, a h igh RanGTP/ RanGDP ratio is maintained by the guanine nucleotide exchange factor RCC1, which catalyzes t he conversion of RanGDP to RanGTP. RanGTP was demonstrated to promote both the disassembly of importin/cargo complexes and the association of exportins such as chromosomal region maintenance 1 (CRM1) wit h their cargoes [19]. At present, the overwhelming majority of examples of protein nucleocytoplasmic shuttling belong to this active mode of translocation. STAT proteins have also been demonstrated to utilize components o f this Ran-dependent nuclear import m achinery [15,23]. The karyopherin i mpor- tin b (p97) has been identified as the c arrier that transports importin a complexed with STATs into the nuclear compartment ([15,23] and Fig. 2A). In interferon-stimula- ted cells dimerized STAT1 and STAT2 bind directly to importin a5 (NPI-1/hSrp1), a karyopherin t hat contains Fig. 2. STATs at the nuclear envelope. (A) Carrier-dependent import. Phosphorylated S TAT dimers expose a dimer-specific n uclear local- ization signal and associate w ith importin a. Through importin b-mediated interactions w ith the interio r of the nuclear po re (NPC) this complex migrates into the nucleus. The complex disassembles after the bindung of RanGTP. The exact stoich iometry and order of events have not b een established. (B) Carrier-dep endent export. Unphos- phorylated S TATs can bind to the exp ortin C RM1 via leucine-rich nuclear export signals and traverse the NPC. RanGTP enhances the interaction of CRM1 with cargo proteins. In the cytoplasm, the nuc- leotide hydrolysis of RanGTP leads to release of the cargo. (C) Carrier- independent nucleocytoplasmic translocation. For the STATs, the majority of translocation events occur via direct interactions with proteins of the nuclear pore. The resulting nucleocytoplasmic cycling proceeds independen tly of met abolic energy. Fig. 1. STATs at the c ell me mbrane . A schematic representation of the events lead ing to the tyro sine phosphorylation (activation) of S TATs. The activation of receptor-associat ed JAK kinases after cytokine sti- mulation results in tyrosine phosphorylation of the receptor. The STATs dock to these sites via their SH2 domains and become tyrosine phosphorylated conc omitan tly. The a ctivated STATs de tach and homo- or heterodim erize. Ó FEBS 2004 Nucleocytoplasmic shuttling of STAT transcription factors (Eur. J. Biochem. 271) 4607 10 armadillo repeats [15,24,25]. Only the very C-terminal armadillo repeats 8 and 9 bind to STAT1 homodimers and STAT1-STAT2 heterodimers, whereas classical NLS sequences interact wi th repeats 2–4, 7 and 8 [26]. The binding site for importin a5 o n the STAT1 dimer has been mapped to an unusual dimer-specific nuclear localiza- tion signal (dsNLS) within the DNA binding domain [24,25]. The homologous sequence in the DNA binding region of STAT3 was later r eported to a lso function as an NLS for the dimer [27]. It is interesting to note that binding of STATs to importin a5 does not appear to pose an obstacle to promoter binding and transcription, as STAT- target DNA can disrupt the i mportin a5 complex with STAT1 [25]. The dsNLS differs from conventional import signals in some respects (Fig. 3). First, it does not resemble the consensus sequence of classical mono- or bip artite NLSs, which consist of one or two arginine/lysine-rich clusters of basic a mino acids s eparated by a spacer region ranging from 10 to up to about 40 residues [28,29]. The STAT1 dsNLS, in contrast, contains only a few positively charged residues. Another distinguishing feature of the STAT dsNLS i s its nontransferability, because i t functions only i n t he context of t he STAT dimer, but not autonom- ously as is typical for conventional NLSs [28,29]. In addition, the STAT a mino termini a lso a ppear to p rovide signals for the c ytokine-inducible nuclear localization as judged from the inability of a mino terminal deletion mutants to a ccumulate in the nucleus [30]; and residues i n the coiled-coil domain seem to contribute to carrier- dependent nuclear import of some S TATs [27]. The c anonical m odel of the JAK-STAT pathway stated that unphosphorylated STATs a re cytoplasmic and do not participate in nucleocytoplasmic shuttling. However, this model has been challenged by the observation that some STAT family members undergo constitutive shuttling between the nuclear and cytosolic compartments even in the absence of cytokine stimulation. A growing body of evidence indicates that the nucleocytoplasmic cycling of STAT proteins is much more dynamic than initially thought. In the following we will describe and discuss the recent a dvances, which make necessary a fresh look at the principles of cytokine signaling. Continuous nucleocytoplasmic cycling of STATs Loss-of-function mutations of the STAT1 dsNLS block nuclear entry of tyrosine-phosphorylated STAT1 [29]. As anticipated, the dsNLS mutants failed t o activate interferon- inducible STAT target genes despite their unperturbed dimerization and DNA binding abilities. Moreover, the import defect was associated also with the loss of cytokine- induced nuclear accumulation. Despite that, ample amounts of unphosphorylated dsNLS m utants of STAT1 were found in the nucleus of unstimulated cells [29]. This w as taken a s the first indication that unphosphorylated STATs used nuclear import mechanism(s) that deviated from the importin-dependent translocation described for the phos- phorylated dimer. Further hints came from the observation of nuclear pools o f monomeric STAT1 and STAT3 in a variety o f unstimulated primary cells or established cell lines [31,32]. P oint mutations in either the SH2 domain or the tyrosine residue in position 701 that completely prevented the signal-dep endent dimerization had no effect on the intracellular STAT1 localization in resting cells [31,32]. The direct visualization of STAT1 nucleocytoplasmic shuttling in re sting cells was made possible by the intracellular microinjection of precipitating anti-STAT1 IgG [29]. Strik- ingly, upon the microinjection of a specific antibody, but not of an unspecific immunoglobulin, STAT1 was depleted from the noninjected compartment [29]. This assay was used to perform time-course experiments to assess the nucleo- cytoplasmic flux rates of e ndogenous STAT1 i n unstimu- lated cells [33]. It was found that the antibody-induced STAT1 clearance was rapid and complete in about 30 min, irrespective of w hether the a ntibody was i njected into the cytoplasm or the nucleus (Fig. 4A–C). Moreover, while energy-depletion of cells precluded nucleocytoplasmic trans- port of karyopherin-dependent cargo proteins, the unphos- phorylated STAT1 continued to exchange between nucleus and cytosol under this condition [33]. Thus, constitutive nucleocytoplasmic shuttling continued in the absence of metabolic energy and an intact RanGTP gradient. High exchange rates between the nuclear and cytoplasmic STAT pools were r eported also for STAT3 a nd STAT5 [ 34,35]. These findings were complemented by import assays with digitonin-permeabilized cells that retain an intact nuclear envelope, but which are devoid of cytoplasmic proteins such as importins [36]. These experiments revealed that exclu- sively unphosphorylated STAT1 c ould enter the nucleus in the absence of cytosolic proteins, whereas tyrosine-phos- phorylated STAT1 dimers required both metabolic energy and added cytosol for nuclear import. Identical observa- tions were also made for unphosphorylated STAT3 and STAT5 [ 33]. Moreover, it was found that the carrier-free transport i s s aturable and appears to o ccur t hrough d irect contacts between STAT proteins and FG repeat-containing nucleoporins [33]. Interestingly, in vitro alkylation with N-ethyl-maleimide of a single cysteine residue in the STAT1 linker domain precluded the translocation across the nuclear membr ane, s uggesting that the functionally poorly characterized linker domain plays a fundamental role in carrier-independent nucleocytoplasmic shuttling [33]. Although the structural details that determine the carrier- free passage of STATs through t he nuclear pore r emain to Fig. 3. The dimer-specific nuclear import signal (dsNLS) of STAT1. A short stretch from the DNA binding domain of STAT1 harbors overlapping e xport and import activities. Notably, the import activity is observed only in the native STAT dimer, whereas the exp ort activity is readily observable in the isolated peptide. Residues that were dem- onstrated to b e imp ortant for e xport (of isolated peptides) are depicted in a white box, residues that are required on ly for import (of th e dimer) are boxed in dark grey. Residues, mutation of which affected both import and export, are shown in a ligh t grey box. For comparison, the homologous sequence s of other STATs are listed: D, Drosophila; h, human. 4608 T. Meyer and U. Vinkemeier (Eur. J. Biochem. 271) Ó FEBS 2004 be established, it was s hown that truncated STAT mutants that lack the amino- and carboxy-termini entered the nucleus with identical kinetics as the full-length molecule. The nuclear export rate of these truncation mutants, on the other hand, was reduced [33], which indicated that the structural requirements are complex and possibly affect transport in a direction-specific manner. Taken together, STATs use two different import pathways: before cytokine stimulation, unphosphorylated STATs migrate via a car- rier-free mechanism that involves direct interactions with nucleoporins. Nuclear import of tyrosine-phosphorylated STAT dimers, on the other hand, is dependent on impor- tins, Ran, and metabolic energy. Both pathways operate simultaneously in cytokine-stimulated cells and it a ppears that phospho rylation-induced dimerization is the s witch from fac ilitated diffusion to carrier-mediated t ranslocation (Fig. 2). Notably, only one third of the STAT1 molecules are tyrosine phosphorylated at any moment during cytokine stimulation [37]. Work in our l aboratory identified a functional leucine- rich nuclear e xport signal i n S TAT1 and d emonstrated its role in vivo, t hus showing that nuclear export of STAT1 was occurring [38]. In the me antime, further putative leucine- rich nuclear export signals have been identified in varying locations in STAT1 [39], STAT3 [ 40], and STAT5 [35], a s well as in Dictyostelium STATa [41], and STATc [ 42]. Of note is the fact that characterization of the STAT export signals remains inc omplete, as export a ctivity in the full length molecule has not been demonstrated yet for some of them. Interestingly, a biphasic regulation was described for STATa in which extracellular cAMP initially directs nuclear import of tyrosine-phosphorylated STATa and phosphory- lation of amino terminal serine residues catalyzed by glycogen synthase kinase-3 promotes its subsequent export [41]. This raises the intriguing possibility of flux modulations via post-translational modifications also for mammalian STATs. However, the respective phosphorylation sites are not conserved. While the CRM1-mediated nuclear export was initially implicated only in the termination o f cytokine-induced nuclear accumulation of STATs, it is now clear that t his export pathway operates constitutively [33]. P reincubation of resting cells with the CRM1 inh ibitor leptomycin B did not cause the nuclear accumulation of STAT1, which by some was taken as an indication that STATs do not shuttle i n r esting cells [39]. I n addition, it was noted that leptomycin merely attenuated the cytoplasmic r elocation after cytokine-induced nuclear accumulation, but did not cause a complete block [38]. As described above, th is phenotype is explained by the existence of a carrier- independent an d h ence leptomycin-insensitive nuclear export mechanism [33]. STATs are predominantly cyto- plasmic in resting cells, although STAT- and cell type- specific differenc es were reported [32]. For STAT1, the underlying molecular mechanism was d etermined t o e ntail the cooperative action of both the carrier-free and the CRM1-dependent translocation mechanism (Fig. 2B,C). It was found that inactivation specifically of CRM1 or STAT1 Hoechst FITC-BSA A B C D Fig. 4. Nucleocytoplasmic s huttling o f STAT1 in resting and cytokine-stimulated cells. Anti- body microinjection assays with an un specific STAT3 antibody (A) or a specific STAT1 antibody (B–D). After antibo dy injection the cells were incubated for 30 min at 37 °C, before fixation and immunocytochemical detection of endogenou s STAT1. The site of injection was marked by the coinjection of fluorescine-conjugated bovine serum albumin. Arrows point at the injected cells. The control in (A) demonstrated that the ST AT1 distri- bution is not affected by microinjection of an unspecific ant ibod y. Th e injectio n of a STAT1-specific a ntibody reveale d the consti- tutive cycling of STAT1 in resting cells (B,C). Cytoplasmic injection of anti-STAT1 depleted endogenous STAT1 from the nucleus (B), whereas nuclear delivery of anti-STAT1 caused STAT1 accum ulation in the n ucleus (C). In (D) the cells were treated w ith inter- feron-c for 60 min to induce the nuclear accumulation of STAT1, before anti-S TAT1 was injected into the cytosol of th e indicated cell. After another 30 min, nuclear STAT1 was substantially diminished in the injected cell. Note the continued nuclear accumulation in the neighboring cells. Ó FEBS 2004 Nucleocytoplasmic shuttling of STAT transcription factors (Eur. J. Biochem. 271) 4609 generally of energy-consuming transport p athways caused a nuclear relocation, resulting in a pancellular STAT1 distribution [33]. Whether retention mechanisms such as the complexation w ith cytoplasmic anchoring f actors also contribute to the cytoplasmic a ccumulation in resting cells is currently unclear. As was mentioned already, c ytokine s timulation of cells triggers a dramatic translocation of STATs into the nucleus. This phenomenon, which depending on the stimulus and its intensity can last for several hours, was initially believed t o reflect an exclusively nuclear residence of STATs. However, nuclear accumulation was recognized to be a highly dynamic process, as the rapid nucleocytoplasmic cycling of STATs continues even during the accumulation phase. In the following we will outline how dimerization, the STAT/ DNA dissociation rate, and the activity of a nuclear phosphatase were identified as the crucial p layers that control retention and accumulation of STATs in the nucleus. The STAT/DNA dissociation rate is a central integrator of cytokine signaling Novel i nsight into the readily observable cytokine-stimula- ted nuclear accumulation of STATs has been gained in the recent past. It was long known t hat dimerization of phosphorylated STATs is an absolute r equirement for an observable accumulation in the nucleus [10]. However, it has become clear that t he concurrent switch to carrier-dep end- ent transport is not the cause of nuclear accumulation, as mutants were generated that were imported normally in response to cytokine stimulation, but that nevertheless were not capable of nuclear retention [ 43]. Based o n in vivo labeling experiments and subcellular fractionations, it was previously proposed that the duration of STAT nuclear accumulation was influenced by the activity of tyrosine phosphatases [37]. Several phosphatases, some of them nuclear, have been demonstrated to affect the rate o f STAT dephosphorylation in vivo [44]. Alternatively, ubiquitination followed by d egradation was p roposed to terminate S TAT signaling in the nucleus [45]. Recent work unambiguously demonstrated that tyro- sine-phosphorylated STAT1 is incapable of nuclear exit and has to be depho sphorylated in order to leave the nuclear compartment [4,43]. T his f act c onstitutes t he basis of the cytokine-induced nuclear accumulation of STATs. The i mportance o f r educed export for the induced nuclear accumulation was also shown for a STAT protein from Dictyostelium [42]. While the nuclear accumulation can last for s everal hours, the nuclear phosphatase activity results in almost instantaneous dephosphorylation. Therefore t he question arises as to the mechanisms that defer tyrosine dephosphorylation. Surprisingly, this mechanism was determined to be DNA binding. It was found that the sequence-specific off-rate from DNA was correlated with the half-life of the phosphorylated protein [43]. STAT dimers that were bound to high-affinity GAS sites resisted dephosphorylation better, as compared to STAT molecules bound to non-GAS sites (Fig. 5 ). Thus, contrary to the previous assum ption that dephosphorylation releases STATs from DNA, it was the other way around, and DNA binding protected STATs from the enzyme activity. This conclusion was supported by measurements of the intranuclear mobility of STAT1 in the presence and absence of phosphatase activity [4,43,46]. Even if the phosphatase activity was blocked, the mobility of S TAT1 remained close to the diffu sion limit. Normally, however, owing to t heir high D NA o ff-rate [2], t he protection from dephosphorylation conferred by DNA binding does not last for the entire time of nuclear accumulation. In vivo,the half-life of phosphorylated STAT1 and STAT3 was shown to not exceed 15–30 min even on a target promoter [37,47]. Thus, the apparently constant level of nuclear accumulated STAT molecules is maintained b y constant nuclear export and successive re-import [48,49]. The r esulting nucleocyto- plasmic cycling during nuclear accumulation was clearly demonstrated by cytoplasmic trapping of STAT1 after antibody microinjection ([43] and Fig. 4D). The central role of dimerization for nuclear retention of STATs was confirmed by a STAT1 mutant that had l ost its ability to recruit the inactivating phosphatase TC45 [43,50]. Ex- change of a single amino acid residue in the amino terminal domain could reverse the defective nuclear accumulation of a DNA binding mutant without rescuing the DNA binding phenotype [4]. These observations also contradicted a competing model for nuclear accumulation, which stated that DNA binding was a necessary prerequisite for nuclear accumulation [39]. Thus, the coupling of dephosphorylation and nuclear retention to t he sequence-specific DNA of f-rate constitutes a regulatory mechanism that integrates at least three important determinants of cytokine signaling. These are the half-life of the transcriptionally active STAT dimer, the duration o f p romoter o ccupancy, and finally the ability to link nuclear activity to the a ctivity of c ytok ine receptors i n the cell membrane. Fig. 5. STATs in the nucleus. STAT binding sites on DNA differ strongly in terms of their DNA off-rate, which is lowest at optimal tar- get sites (GAS). Enzymatic dephosphorylation of STATs is po ssible only when the molecule is off DNA. Thus, the activity of the STAT dimer is extended at promoters with op timal STAT binding site(s). 4610 T. Meyer and U. Vinkemeier (Eur. J. Biochem. 271) Ó FEBS 2004 STAT nucleocytoplasmic transport in disease It is increasingly becoming clear that nucleocytoplasmic cycling of signal transducers is an intricate process that affects signaling in many ways. It is therefore not surprising that several viral proteins, such as the V proteins from Nipah and Hendra viruses, both of which cause zoonotic diseases in animals and humans, have been shown to interfere with the nucleocytoplasmic translocation o f STAT proteins ([51–53]; reviewed in [54]). The interferon antag- onistic activity of these paramyxovirus V proteins included the cytoplasmic sequestration of STAT1 and STAT2 in high molecular mass c omplexes. It was shown that Nipah and Hendra V proteins alter the subcellular d istribution of STAT1 in resting cells and prevent nuclear import of both STAT1 and STAT2 in interferon-stimulated cells. Thus, inhibition of nucleocytoplasmic shuttling constitutes a viral strategy to evade the antiviral effects of interferons. In addition, impaired interleukine-12-dependent nuclear trans- location of STAT4 was reported in a patient with recurrent mycobacterial infection [55]. 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MINIREVIEW Nucleocytoplasmic shuttling of STAT transcription factors Thomas Meyer and Uwe Vinkemeier Abteilung Zellula ¨ re Signalverarbeitung, Leibniz-Forschungsinstitut. cells seven known STAT proteins were identified, den oted STAT1 , STAT2 , STAT3 , STAT4 , STAT5 a, S TAT5b, and STAT6 , all of which are activated by a distinct set of cytokines a nd growth factors [1] varying locations in STAT1 [39], STAT3 [ 40], and STAT5 [35], a s well as in Dictyostelium STATa [41], and STATc [ 42]. Of note is the fact that characterization of the STAT export signals remains