NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.

Madame Curie Bioscience Database [Internet]. Austin (TX): Landes Bioscience; 2000-2013.

Cover of Madame Curie Bioscience Database

Madame Curie Bioscience Database [Internet].

Show details

Calreticulin-Mediated Nuclear Protein Export

and .

The role of calreticulin (CRT) as a molecular chaperone that functions in the endoplasmic reticulum (ER) is well established. This involves transient binding of CRT to hydrophobic residues and carbohydrate chains in polypeptides undergoing folding reactions in the lumen of the ER. The issue of CRT distribution and function outside of the ER, though controversial for several years, has now been addressed by rigorous biochemical fractionation and cell biological analysis. Cytosolic CRT, which refers to the non-ER form of the protein that shuttles between the cytoplasm and nucleus, can function as a receptor that mediates nuclear export of the glucocorticoid receptor (GR). The signal recognized by CRT is contained within the DNA binding domain (DBD) of GR. In this chapter, we introduce the topic of nuclear export and summarize the characterization of cytosolic CRT as an export receptor. We also review the evidence that the DBD functions as a signal for export of GR. The DBD is likely to function as the export signal for other members of the nuclear receptor (NR) superfamily, which is the largest family of transcription factors in higher eukaryotes. Our working model is that the non-ER form of CRT contributes to the regulation of multiple cellular pathways through a nuclear export-based mechanism.

Nucleocytoplasmic Transport Pathways

Nuclear import and export pathways generally use cis-acting signals to direct cargoes to the nucleus and cytoplasm.1,2 These signals are recognized and bound by specific receptors that facilitate translocation through large channels in the nuclear envelope, termed nuclear pore complexes (NPCs).35 The leucine-rich or hydrophobic nuclear export signal (NES), which is the most common signal for export, was first identified in the HIV-1 Rev protein and protein kinase inhibitor (PKI).6,7 The leucine-rich NES in Rev (LPPLERLTL) and PKI (LALKLAGLDIN) is recognized by the export receptor Crm1, a member of the importin β (also called karyopherin β) family of nuclear transport receptors.8,9 Crm1 binds the NES and mediates export to the cytoplasm.10,11 This export pathway is regulated by the GTPase Ran, which, in its GTP-bound form, assembles into a stoichiometric complex with Crm1 and the NES cargo.12 Following translocation, the export complex is disassembled in the cytoplasm through the action of several factors including the Ran GTPase activating protein.

The functions of a number of proteins in the cell require both nuclear import and nuclear export, a process referred to as nucleocytoplasmic shuttling. Nuclear transport of some shuttling proteins relies on separate signals for import and export, and bidirectional transport depends on interactions with both import and export receptors. Examples of this type of shuttling protein include the proteins p53 and NFAT.1316 Other shuttling proteins, such as the hnRNA A1 protein that assembles into RNP complexes, contain a single transport signal that mediates both import and export.2 Nucleocytoplasmic shuttling is also a property of many, if not all, steroid hormone receptors. This endows the cell with the ability to regulate transcription by controlling the distribution of steroid hormone receptors. The pathway for nuclear import of steroid hormone receptors, which has been studied extensively with GR, is initiated in the cytoplasm by ligand binding. This induces a conformational change that releases chaperones and exposes the NLS, which is recognized by the nuclear import receptor importin-β. The pathway for nuclear export of steroid hormone receptors has, until recently, remained obscure. Steroid hormone receptors do not contain a leucine-rich NES, and there is clear evidence that nuclear export of these proteins is not mediated by the export receptor Crm1. This includes the finding that steroid hormone receptor export is insensitive to Leptomycin B, a compound that specifically inhibits Crm1 function and blocks leucine-rich NES-dependent export.17 As discussed below, our laboratory found that nuclear export of GR is mediated by the Ca2+ -binding protein CRT. This export pathway appears to be used by other members of the nuclear receptor (NR) superfamily.

Purification of CRT Using an Export Assay

Because multiple receptors and pathways are used for nuclear import,9 we reasoned that cells should contain export receptors in addition to Crm1. To test this hypothesis, our laboratory developed an assay that reconstitutes the nuclear export of the NES-containing protein PKI.18 The PKI export assay is carried out in digitonin-permeabilized cells, the most widely used model system for analysis of nuclear transport in vitro.14,18,19 PKI is loaded into the nuclei of permeabilized cells during the import phase, and cytosol is added to stimulate nuclear export during the export phase. The cytosol-dependence of the export phase allowed us to make several observations that were consistent with the presence of multiple export factors.18 The key observation was that quantitative depletion of Crm1 from cytosol by treatment with Phenyl-Sepharose resulted in only partial depletion of total export activity, as measured in the PKI export assay.

We devised a purification scheme that involved ammonium sulfate precipitation, ion exchange, and gel filtration chromatography steps, using PKI export as the assay. This resulted in purification of a single protein, with an apparent molecular weight of ˜60 kDa on silver-stained gels, that was sufficient to stimulate PKI export in permeabilized cells. Mass spectrometry identified the protein as CRT. Definitive evidence that the export activity could be attributed to CRT, and not to a minor contaminant, was obtained by showing that recombinant CRT was sufficient to promote export in PKI assay. The mechanism of CRT-dependent PKI export, like that of Crm1, requires Ran-GTP.20 Mutations in the NES that inhibit recognition by Crm1 also inhibit recognition by CRT. Although the CRT and Crm1 export pathways for NES proteins display clear functional similarities, the proteins are unrelated at the sequence level.

Subcellular Distribution of CRT

CRT contains an amino terminal signal sequence and a carboxyl terminal KDEL retention signal, hallmarks of an ER protein. Immunofluorescence microscopy clearly shows that CRT is localized to the ER, the organelle from which the protein was first isolated.21 Nevertheless, multiple laboratories have reported finding CRT in locations outside of the ER, including the nucleus. The apparent localization of CRT to the nucleus seemed consistent with previous data indicating that CRT could suppress transcriptional activity of steroid hormone receptors. Unfortunately, the localization of endogenous CRT by immunofluorescence is technically difficult because the high concentration of CRT in the ER obscures detection of the non-ER pool of the protein.2224

We chose instead to analyze the distribution of CRT in HeLa cells by classical sub-cellular fractionation and immunoblotting, using well-established marker proteins to define the compartments. We found that CRT is present in the microsomal compartment, coincident with ER marker proteins, and in the soluble compartment, coincident with soluble marker proteins. 20 Proteinase K digestion was used to show that the microsomal pool of CRT is contained within vesicles and susceptible to digestion only in the presence of detergent, and that the soluble pool of CRT is degraded by proteinase K even in the absence of detergent. This provides clear evidence that CRT is found in both ER and non-ER compartments. These data also explain how our purification scheme, which started with a detergent-free, high-speed extract devoid of organelles, resulted in the isolation of CRT. The biosynthetic pathway that generates the non-ER, cytosolic form of CRT is under investigation in our laboratory.

CRT Is the Export Receptor for GR

Several years prior to our isolation of CRT as an export factor, CRT was shown to interact with GR and other members of the NR superfamily.2527 Transfection of CRT inhibited the transactivation mediated by NRs, and recombinant CRT inhibited NR binding to DNA response elements in gel shift assays. The latter result provided a potential molecular explanation for the inhibitory effect of CRT on transcription. Nonetheless, our results showing that CRT mediates nuclear export of NES-containing proteins led us to consider whether CRT might also function as an export receptor for NRs. We viewed this as an attractive hypothesis because nuclear export would inhibit the transactivation by relocating NRs to the cytoplasm, where NRs reassemble into multi-subunit complexes containing heat shock proteins including hsp90.28 It should be noted that even in the presence of their respective ligand, NRs including GR are actively shuttling between the nucleus and cytoplasm, indicating that cells have robust mechanism for nuclear export for these proteins. As mentioned above, the absence of a leucine-rich NES in NRs and the insensitivity of NR export to Leptomycin B appeared to rule out the Crm1 pathway.

The system we initially chose for examining whether CRT mediates GR export was the digitonin-permeabilized cell assay. By using a green fluorescent protein fusion (GFP) of GR, we were able to show that addition of recombinant CRT is sufficient to stimulate GR export from the nucleus.20 This result was corroborated in vivo using CRT-deficient cells isolated from embryos of CRT knockout mice.29 The in vivo assay involves ligand addition and subsequent withdrawal to allow for GR-GFP export (Fig. 8.1A). The crt-/- cells were found to be deficient for GR export, and the transport defect was corrected by back-transfection of CRT ( Fig. 8.1B). We also found that recombinant CRT could potently stimulate GR export when microinjected into the hamster cell line BHK.20 These experiments established, for the first time, that CRT mediates the nuclear to cytoplasmic localization of GR in a pathway that is independent of Crm1.

Figure 1. CRT mediates the nuclear export of GR in vivo.

Figure 1

CRT mediates the nuclear export of GR in vivo. (A) In vivo assay for GR export. Import is induced by the addition of Dex. Following Dex withdrawal, cells are incubated and imaged at the indicated time-points. (B) Nuclear export of GR is impaired in the (more...)

Identification of the Export Signal in GR

We set out to characterize the signal within GR that is recognized by CRT. For this analysis, we constructed a GFP reporter that would reveal the export activity of sequences from GR in a fluorescence microscopy assay.30 The GFP reporter contained the ligand binding domain of GR that facilitates dexamethasone (Dex)-inducible nuclear import. Because previous work suggested the DBD was a strong candidate as an export signal, we transplanted the 69 residues that include the GR DBD to the GFP reporter and tested it in the assay. The GFP reporter is mostly nuclear up to six hours after removal of ligand ( Fig. 8.2, No DBD). This contrasts with the distribution of the GFP reporter that contains the GR DBD, which undergoes nuclear export and is clearly cytoplasmic by four hours ( Fig. 8.2, GR-DBD (418–486)).

Figure 2. Two adjacent phenylalanines in the GR DBD are both required for a functional NES.

Figure 2

Two adjacent phenylalanines in the GR DBD are both required for a functional NES. The GFP reporter alone (no DBD) remains nuclear during the course of the experiment, while including the GR DBD (WT DBD) in the GFP reporter confers export. The FF⇒AA (more...)

The DBD of GR, like that of all other NR superfamily members, contains two zinc-binding loops and makes sequence-specific contacts with its corresponding DNA response element.31,32 We predicted that these zinc-binding loops of GR might be important for CRT-recognition, since they are critical for DBD structure. Surprisingly, cysteine-to-alanine point mutations that are known to disrupt the structure of either the first or second zinc-binding loop caused only a modest reduction in export activity.30 We analyzed the effects of alanine mutations within the 15 amino acid region between the two zinc-binding loops, a region identified in peptide-binding experiments as a CRT-binding site.25,26 While several of these mutations reduced the export activity of the DBD, the most striking defect in export was caused by mutating two adjacent phenylalanines.20,30 We mutated each phenylalanine individually to determine if both are required for NES function. Both single F⇒A point mutations (F444A and F445A) led to a major reduction in export activity that appeared similar to the double mutant (FF⇒AA), suggesting that both phenylalanines are important for nuclear export (Fig. 8.2). These particular phenylalanines are invariant residues in the DBD of all NR superfamily members, and are near the middle of the DNA recognition helix.

The DBD Is Necessary for Export

To determine if the DBD is necessary for GR export, we used a heterokaryon shuttling assay that scores export from a donor nucleus and import into an acceptor nucleus (Fig. 8.3).30 This type of assay has been used to demonstrate that a variety of proteins, which appear to be constitutively nuclear, actually undergo nucleocytoplasmic shuttling.33,34 For example, GR appears to be constitutively nuclear in the presence of its ligand, however, the heterokaryon shuttling assay reveals that GR undergoes constant movement between the nucleus and cytoplasm.33 In the assay, cells expressing GR-GFP are treated with ligand to induce nuclear import, and then fused with cells labeled with a red fluorescent dye. The appearance of GR-GFP in the nuclei of multi-nucleate cells that contain the red dye demonstrates that the reporter has undergone nuclear export from the donor nuclei and nuclear import into acceptor nuclei (Fig. 8.3). The FF⇒AA mutation that abolishes the export activity in the context of the isolated DBD, also abolishes the export of full-length GR, expressed as a fusion to GFP (Fig. 8.3). Thus, the DBD of GR, which is sufficient for nuclear export, is also required for nuclear export.

Figure 3. The DBD export signal is necessary for GR shuttling in vivo.

Figure 3

The DBD export signal is necessary for GR shuttling in vivo. Interspecies heterokaryon shuttling assays were performed with Cos cells transfected with full-length GR fused to GFP (FITC) and NIH 3T3 cells labeled with the dye CellTracker CMTMR (Rhodamine). (more...)

Our data indicated that both an intact DBD and the protein CRT are necessary for nuclear export of GR. This export pathway requires a physical interaction between these proteins, based on the following observations. First, CRT binds directly to the DBD, and the FF⇒AA mutation that inhibits nuclear export also inhibits the binding of CRT.30 Second, CRT introduced by microinjection can promote nuclear export of GR that contains a functional DBD, but not of GR that contains mutations in the DBD.30 Third, the presence of excess DBD is sufficient to competitively-inhibit CRT-dependent export, but the competition occurs only if the competing DBD can be recognized by CRT.30 Our finding that DBD-dependent export is saturable in vivo provides evidence that NRs compete for a limiting component for nuclear export. CRT is one of the rate limiting components for this export pathway.

Regulating GR Export

While some aspects of CRT-mediated recognition of the export signal in GR have now been established, the regulation of this export pathway is largely unexplored. By analogy with nuclear import pathways, the CRT-dependent export pathway for GR and other NRs could be regulated at several different levels. First, the accessibility of the DBD as the export signal is a potential point of regulation. Since the DBD will not be exposed when the NR is directly bound to DNA, accessibility of the DBD to CRT will be determined by the rate of NR dissociation from DNA. Second, the assembly of NRs into large, macromolecular transcription complexes, or partitioning into the insoluble nuclear matrix, or both, might result in nuclear retention. Since nuclear retention could be dominant over nuclear export, controlling the release from nuclear retention would be another potential point of regulation. Third, it is possible that CRT itself might be subject to positive or negative regulation. Positive regulation could be achieved by raising the concentration of the cytosolic pool of CRT, or by covalent modification of CRT by phosphorylation,35 or by increasing the activity of CRT through Ca2+-binding. Fourth, CRT could be regulated by the Ran GTPase, which we have shown is a necessary component of export complexes that contain CRT and proteins with a leucine-rich NES.20

We have performed experiments that address two of the potential regulatory mechanisms described above. These are regulation by the Ran GTPase, and regulation by Ca2+ binding. In contrast to the critical role that Ran plays in leucine-rich NES export for both the Crm1 and CRT export pathways, Ran does not appear to be an essential factor for CRT-dependent GR export. This was tested by examining GR export in permeabilized cells under conditions where CRT was rate-limiting and Ran was present in excess. In these assays, wild-type Ran does not stimulate CRT-dependent export of GR, and mutant forms of Ran that interfere with its GTPase cycle neither stimulate nor inhibit CRT-dependent export of GR.36

We examined whether Ca2+ is important for CRT-dependent export of GR by two experimental approaches. In the first approach, Ca2+ was stripped from CRT using the chelator EGTA, and the Ca2+-free CRT was tested for GR export in a permeabilized cell assay. Significantly, this revealed that Ca2+ is necessary for CRT-dependent GR export. Binding assays performed in parallel confirmed that Ca2+ removal from CRT inhibits binding to the DBD. In the second approach, C-terminal deletion mutants of CRT that lack the high capacity, low affinity Ca2+ -binding sites were tested in the GR export and DBD binding assays. The results from these experiments indicated that these Ca2+ -binding sites are not essential for CRT export activity. Rather, the low affinity, high capacity Ca2+ -binding sites appear to regulate the activity of CRT, since Ca2+ -binding to these sites is necessary in the context of full-length CRT. Ca2+ -binding induces a change in the structure of CRT from an extended conformation to a more compact conformation, the latter of which is active for DBD binding and nuclear export. 36 It has been shown previously that Ca2+ binding to CRT is important for its chaperone functions as well.37

Common Pathways for NR Transport

A general mechanism is thought to account for the nuclear import of virtually all NRs. This mechanism involves the assembly of the NR into a cytoplasmic complex with several factors including hsp90, which maintain the NR in a conformation that is competent for ligand-binding.28 Ligand binding initiates a series of events including dissociation of hsp90, exposure of the NLS, recognition by the nuclear import machinery, and import into the nucleus. These molecular events are best understood for GR, in part because this NR shows efficient relocalization to the cytoplasm when ligand is removed from the system. That NRs such as the estrogen receptor (ER) are predominantly nuclear in the absence of ligand may be a consequence of nuclear retention or ligand-independent regulation of nuclear import.

Our studies suggest that a general mechanism may also account for nuclear export of most NRs. After finding that GR export is mediated by its DBD, we tested the DBDs of nine additional NRs for nuclear export activity. The motivation for these experiments derived from the fact that the sequence and structure of the DBD is highly conserved among NRs (Fig. 8.4). The additional DBDs tested were three other steroid receptors (AR, ER, and progesterone receptor [PR]); four non-steroid receptors (RAR, RXR, thyroid hormone receptor [TR], and vitamin D receptor [VDR]), and two orphan receptors (liver X receptor [LXR] and RevErb). Each of the ten DBDs is sufficient for nuclear export activity, indicating that NR DBDs define a new type of export signal.30 The DBDs are structurally similar and appear to use a common export pathway since the DBDs from two different NRs, GR and VDR, compete for nuclear export in vitro and in vivo.

Figure 4. Alignment of DBDs from NRs used in this study and the percent identity to the DBD of human GR.

Figure 4

Alignment of DBDs from NRs used in this study and the percent identity to the DBD of human GR. Highly conserved residues (bold) including the cysteines that coordinate zinc binding (green) and the pair of phenylalanines that are present in the DNA recognition (more...)

Why Do Nuclear Receptors Undergo Export?

Nucleocytoplasmic shuttling of NRs should be taken into account when considering the function, regulation, and activity of these transcription factors. Nuclear export can be viewed as an absolute mechanism for turning off transcription since it removes the NR from its primary site of action. Nuclear export has, in fact, been found to be an evolutionarily conserved mechanism for regulating activity of multiple transcription factors.38 Some well-studied examples include the Ca2+ -regulated trafficking of NF-AT in mammals and the phosphate-regulated trafficking of PHO4 in yeast. In addition, since NRs can also regulate the activity of a variety of co-activators and repressors, nuclear export of NRs is an important pathway that impacts on the activity of proteins outside of the NR superfamily. GR is known to negatively regulate the transcription factor NF-κB by transcriptional interference.39,40 Transcriptional interference involves the sequestration of shared co-factors by steroid receptors, and this usually occurs within the nucleus. The GR-mediated regulation of the NF-κB pathway is critical for the anti-inflammatory effects of glucocorticoids.

Nucleocytoplasmic shuttling may also be linked to the turnover of NRs. Artificially accelerating nuclear export increases ubiquitin-dependent degradation of GR.41 A similar observation regarding turnover has been made with the tumor suppressor p53. Blocking Crm1-dependent nuclear export with Leptomycin B was found to inhibit p53 turnover, resulting in the accumulation of p53 in the nucleus.42 Likewise, it has been shown that blocking nuclear export of IκB-α prevents its turnover, since its degradation occurs in the cytoplasm.43

Finally, nucleocytoplasmic shuttling of NR superfamily members is important for their non-genomic activities, which involve ligand-induced signaling events in the cytoplasm.44,45 Non-genomic effects of ligands are manifest through NRs within minutes of addition, and include direct interactions with cytoplasmic components of the Src/Map Kinase and phosphatidylinositol-3-OH (PI-3) kinase signaling pathways.4648 NRs identified in this type of signaling include ER and TR, receptors that appear mostly nuclear in the presence or absence of ligand. Nucleocytoplasmic shuttling ensures that a sufficient supply of these NRs is available in the cytoplasm to participate in signaling pathways.

Concluding Remarks

The identification of CRT as a receptor for nuclear export of leucine-rich NES-containing proteins and previous links between CRT and NR activity have provided an unanticipated entrée into analyzing NR function. Recent progress in this area has included identification of the DBD as an export signal, and demonstration that the DBD is necessary and sufficient for nuclear export of both GR and AR.20,30,49 These observations provide a framework for future experiments that should address how CRT physically contacts the DBD, how these proteins translocate through the NPC, and how these interactions may be regulated by conditions that influence growth and development.

References

1.
Gorlich D, Kutay U. Transport between the cell nucleus and the cytoplasm. Annu Rev Cell Dev Biol. 1999;15:607–660. [PubMed: 10611974]
2.
Nakielny S, Dreyfuss G. Transport of proteins and RNAs in and out of the nucleus. Cell. 1999;99:677–690. [PubMed: 10619422]
3.
Stoffler D, Fahrenkrog B, Aebi U. The nuclear pore complex: from molecular architecture to functional dynamics. Curr Opin Cell Biol. 1999;11:391–401. [PubMed: 10395558]
4.
Wente SR. Gatekeepers of the nucleus. Science. 2000;288:1374–1377. [PubMed: 10827939]
5.
Vasu SK, Forbes DJ. Nuclear pores and nuclear assembly. Curr Opin Cell Biol. 2001;13:363–375. [PubMed: 11343909]
6.
Fischer U, Huber J, Boelens WC. et al. The HIV-1 Rev activation domain is a nuclear export signal that accesses an export pathway used by specific cellular RNAs. Cell. 1995;82:475–83. [PubMed: 7543368]
7.
Wen W, Meinkoth JL, Tsien RY. et al. Identification of a signal for rapid export of proteins from the nucleus. Cell. 1995;82:463–473. [PubMed: 7634336]
8.
Fornerod M, van Deursen J, van Baal S. et al. The human homologue of yeast CRM1 is in a dynamic subcomplex with CAN/Nup214 and a novel nuclear pore component Nup88. EMBO J. 1997;16:807–816. [PMC free article: PMC1169681] [PubMed: 9049309]
9.
Pemberton LF, Blobel G, Rosenblum JS. Transport routes through the nuclear pore complex. Curr Opin Cell Biol. 1998;10:392–399. [PubMed: 9640541]
10.
Fornerod M, Ohno M, Yoshida M. et al. CRM1 is an export receptor for leucine-rich nuclear export signals. Cell. 1997;90:1051–60. [PubMed: 9323133]
11.
Stade K, Ford CS, Guthrie C. et al. Exportin 1 (Crm1p) is an essential nuclear export factor. Cell. 1997;90:1041–1050. [PubMed: 9323132]
12.
Steggerda SM, Paschal BP. Regulation of nuclear import and export by the GTPase Ran. Int Rev Cytol. 2002;217:41–91. [PubMed: 12019565]
13.
Klemm JD, Beals CR, Crabtree GR. Rapid targeting of nuclear proteins to the cytoplasm. Curr Biol. 1997;7:638–644. [PubMed: 9285717]
14.
Kehlenbach RH, Dickmanns A, Gerace L. Nucleocytoplasmic shuttling factors including Ran and Crm1 mediate nuclear export of NFAT in vitro. J Cell Biol. 1998;141:863–874. [PMC free article: PMC2132762] [PubMed: 9585406]
15.
Roth J, Dobbelstein M, Freedman DA. et al. Nucleo-cytoplasmic shuttling of the hdm2 oncoprotein regulates the levels of the p53 protein via a pathway used by the human immunodeficiency virus rev protein. EMBO J. 1998;17:554–564. [PMC free article: PMC1170405] [PubMed: 9430646]
16.
Stommel JM, Marchenko ND, Jimenez GS. et al. A leucine-rich nuclear export signal in the p53 tetramerization domain: regulation of subcellular localization and p53 activity by NES masking. EMBO J. 1999;18:1660–1672. [PMC free article: PMC1171253] [PubMed: 10075936]
17.
Wolff B, Sanglier JJ, Wang Y. Leptomycin B is an inhibitor of nuclear export: inhibition of nucleo-cytoplasmic translocation of the human immunodeficiency virus type 1 (HIV-1) Rev protein and Rev-dependent mRNA. Chem Biol. 1997;4:139–147. [PubMed: 9190288]
18.
Holaska JM, Paschal BM. A cytosolic activity distinct from Crm1 mediates nuclear export of protein kinase inhibitor in permeabilized cells. Proc Natl Acad Sci USA. 1998;95:14739–14744. [PMC free article: PMC24519] [PubMed: 9843959]
19.
Adam SA, Sterne-Marr RE, Gerace L. Nuclear protein import in permeabilized mammalian cells requires soluble cytoplasmic factors. J Cell Biol. 1990;111:807–816. [PMC free article: PMC2116268] [PubMed: 2391365]
20.
Holaska JM, Black BE, Love DC. et al. Calreticulin is a receptor for nuclear export. J Cell Biol. 2001;152:127–140. [PMC free article: PMC2193655] [PubMed: 11149926]
21.
Ostwald TJ, MacLennan DH. Isolation of a high affinity calcium-binding protein from sarcoplasmic reticulum. J Biol Chem. 1974;249:974–979. [PubMed: 4272851]
22.
Michalak M, Burns K, Andrin C. et al. Endoplasmic reticulum form of calreticulin modulates glucocorticoid-sensitive gene expression. J Biol Chem. 1996;271:29436–29445. [PubMed: 8910610]
23.
Jethmalani SM, Henle KJ, Gazitt Y. et al. Intracellular distribution of heat-induced stress glycoproteins. J Cell Biochem. 1997;66:98–111. [PubMed: 9215532]
24.
Roderick HL, Campbell AK, Llewellyn DH. Nuclear localisation of calreticulin in vivo is enhanced by its interaction with glucocorticoid receptors. FEBS Lett. 1997;405:181–185. [PubMed: 9089287]
25.
Burns K, Duggan B, Atkinson EA. et al. Modulation of gene expression by calreticulin binding to the glucocorticoid receptor. Nature. 1994;367:476–480. [PubMed: 8107808]
26.
Dedhar S, Rennie PS, Shago M. et al. Inhibition of nuclear hormone receptor activity by calreticulin. Nature. 1994;367:480–483. [PubMed: 8107809]
27.
Wheeler DG, Horsford J, Michalak M. et al. Calreticulin inhibits vitamin D3 signal transduction. Nucleic Acids Res. 1995;l23:3268–3274. [PMC free article: PMC307187] [PubMed: 7667104]
28.
Buchner J. Hsp90 & Co.-a holding for folding. Trends Biochem Sci. 1999;24:136–141. [PubMed: 10322418]
29.
Mesaeli N, Nakamura K, Zvaritch E. et al. Calreticulin is essential for cardiac development. J Cell Biol. 1999;144:857–868. [PMC free article: PMC2148186] [PubMed: 10085286]
30.
Black BE, Holaska JM, Rastinejad F. et al. DNA binding domains in diverse nuclear receptors function as nuclear export signals. Curr Biol. 2001;11:1749–1758. [PubMed: 11719216]
31.
Evans RM. The steroid and thyroid hormone receptor superfamily. Science. 1988;240:889–895. [PubMed: 3283939]
32.
Luisi BF, Xu WX, Otwinowski Z. et al. Crystallographic analysis of the interaction of the glucocorticoid receptor with DNA. Nature. 1991;352:497–505. [PubMed: 1865905]
33.
Madan AP, DeFranco DB. Bidirectional transport of glucocorticoid receptors across the nuclear envelope. Proc Natl Acad Sci USA. 1993;90:3588–3592. [PMC free article: PMC46346] [PubMed: 8386376]
34.
Michael WM, Choi M, Dreyfuss G. A nuclear export signal in hnRNP A1: a signal-mediated, temperature-dependent nuclear protein export pathway. Cell. 1995;83:415–422. [PubMed: 8521471]
35.
Singh NK, Atreya CD, Nakhasi HL. Identification of calreticulin as a rubella virus RNA binding protein. Proc Natl Acad Sci USA. 1994;91:12770–12774. [PMC free article: PMC45521] [PubMed: 7809119]
36.
Holaska JM, Black BE, Rastinejad FR. et al. Ca2+-dependent nuclear export mediated by calreticulin. Mol Cell Biol. 2002;22:6286–6297. [PMC free article: PMC133999] [PubMed: 12167720]
37.
Vassilakos A, Michalak M, Lehrman MA. et al. Oligosaccharide binding characteristics of the molecular chaperones calnexin and calreticulin. Biochemistry. 1998;37:3480–3490. [PubMed: 9521669]
38.
Komeili A, O'Shea EK. Nuclear transport and transcription. Curr Opin Cell Biol. 2000;12:355–360. [PubMed: 10801461]
39.
Gottlicher M, Heck S, Herrlich P. Transcriptional cross-talk, the second mode of steroid hormone action. J Mol Med. 1998;76:480–489. [PubMed: 9660166]
40.
Karin M, Chang L. AP-1/glucocorticoid receptor crosstalk taken to a higher level. J Endocrinol. 2001;169:447–451. [PubMed: 11375114]
41.
Liu J, DeFranco DB. Protracted nuclear export of glucocorticoid receptor limits its turnover and does not require the exportin 1/CRM1-directed nuclear export pathway. Mol Endocrinol. 2000;14:40–51. [PubMed: 10628746]
42.
Freedman DA, Levine AJ. Nuclear export is required for degradation of endogenous p53 by MDM2 and human papillomavirus E6. Mol Cell Biol. 1998;18:7288–7293. [PMC free article: PMC109310] [PubMed: 9819415]
43.
Rodriguez MS, Thompson J, Hay RT. et al. Nuclear retention of IkB-a protects it from signal-induced degradation and inhibits nuclear factor kB transcriptional activation. J Biol Chem. 1999;274:9108–9115. [PubMed: 10085161]
44.
Falkenstein E, Tillmann HC, Christ M. et al. Multiple actions of steroid hormones-a focus on rapid, nongenomic effects. Pharmacol Rev. 2000;52:513–555. [PubMed: 11121509]
45.
Manolagas SC, Kousteni S. Perspective: nonreproductive sites of action of reproductive hormones. Endo. 2001;142:2200–2204. [PubMed: 11356663]
46.
Migliaccio A, Castoria G, Di Domenico M. et al. Steroid-induced androgen receptor-oestradiol receptor b-Src complex triggers prostate cancer cell progression. EMBO J. 2000;19:5406–5417. [PMC free article: PMC314017] [PubMed: 11032808]
47.
Simoncini T, Hafezi-Moghadam A, Brazil DP. et al. Interaction of oestrogen receptor with the regulatory subunit of phosphatidylinositol-3-OH kinase. Nature. 2000;407:538–541. [PMC free article: PMC2670482] [PubMed: 11029009]
48.
Kousteni S, Bellido T, Plotkin LI. et al. Nongenotropic, sex-nonspecific signaling through the estrogen or androgen receptors: dissociation from transcriptional activity. Cell. 2001;104:719–730. [PubMed: 11257226]
49.
DeFranco DB. DNA-binding domains find a surprising partner. Curr Biol. 2001;11:R1036–R1037. [PubMed: 11747843]
Copyright © 2000-2013, Landes Bioscience.
Bookshelf ID: NBK6543

Views

  • PubReader
  • Print View
  • Cite this Page

Related information

  • PMC
    PubMed Central citations
  • PubMed
    Links to PubMed

Recent Activity

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...