Spontaneous high expression of heat-shock proteins in mouse embryonal carcinoma cells and ectoderm from day 8 mouse embryo

When submitted to a heat-shock, mouse embryonal carcinoma (EC) and fibroblast cells show very different behavior. All the EC cells so far analyzed express very high levels of several heat-shock proteins (HSP) in the absence of stress and independent of their origin and culture conditions. In such cells, the 89-kd, 70-kd and 59-kd HSP are the most prominent proteins after actin. In addition, the 89-kd and 59-kd HSP are not stimulated by an arsenite shock in contrast to what is observed with fibroblasts or cells of the parietal yolk sac type. Arsenite induces the synthesis of a 105-kd polypeptide in fibroblasts but not in EC cells. In vitro differentiation of F9 cells induced by retinoic acid and dibutyryl cAMP is accompanied by a decrease in the spontaneous relative abundance of HSP and restores the arsenite-induced synthesis of the 105-kd polypeptide. EC cells are usually believed to be similar to inner cell mass cells of mouse blastocyst. Furthermore, data in the literature together with our own results suggest that the same three HSP are also spontaneously expressed in high amounts in the early mouse embryo.

7SK small nuclear RNA binds to and inhibits the activity of CDK9/cyclin T complexes

The transcription of eukaryotic protein-coding genes involves complex regulation of RNA polymerase (Pol) II activity in response to physiological conditions and developmental cues. One element of this regulation involves phosphorylation of the carboxy-terminal domain (CTD) of the largest polymerase subunit by a transcription elongation factor, P-TEFb, which comprises the kinase CDK9 and cyclin T1 or T2 (ref. 1). Here we report that in human HeLa cells more than half of the P-TEFb is sequestered in larger complexes that also contain 7SK RNA, an abundant, small nuclear RNA (snRNA) of hitherto unknown function. P-TEFb and 7SK associate in a specific and reversible manner. In contrast to the smaller P-TEFb complexes, which have a high kinase activity, the larger 7SK/P-TEFb complexes show very weak kinase activity. Inhibition of cellular transcription by chemical agents or ultraviolet irradiation trigger the complete disruption of the P-TEFb/7SK complex, and enhance CDK9 activity. The transcription-dependent interaction of P-TEFb with 7SK may therefore contribute to an important feedback loop modulating the activity of RNA Pol II.

Transcription-independent RNA polymerase II dephosphorylation by the FCP1 carboxy-terminal domain phosphatase in Xenopus laevis early embryos

The phosphorylation of the RNA polymerase II (RNAP II) carboxy-terminal domain (CTD) plays a key role in mRNA metabolism. The relative ratio of hyperphosphorylated RNAP II to hypophosphorylated RNAP II is determined by a dynamic equilibrium between CTD kinases and CTD phosphatase(s). The CTD is heavily phosphorylated in meiotic Xenopus laevis oocytes. In this report we show that the CTD undergoes fast and massive dephosphorylation upon fertilization. A cDNA was cloned and shown to code for a full-length xFCP1, the Xenopus orthologue of the FCP1 CTD phosphatases in humans and Saccharomyces cerevisiae. Two critical residues in the catalytic site were identified. CTD phosphatase activity was observed in extracts prepared from Xenopus eggs and cells and was shown to be entirely attributable to xFCP1. The CTD dephosphorylation triggered by fertilization was reproduced upon calcium activation of cytostatic factor-arrested egg extracts. Using immunodepleted extracts, we showed that this dephosphorylation is due to xFCP1. Although transcription does not occur at this stage, phosphorylation appears as a highly dynamic process involving the antagonist action of Xp42 mitogen-activated protein kinase and FCP1 phosphatase. This is the first report that free RNAP II is a substrate for FCP1 in vivo, independent from a transcription cycle.

The peptidyl-prolyl isomerase Pin1 interacts with hSpt5 phosphorylated by Cdk9

We identify and characterize several phosphorylated forms of the hSpt5 subunit of the DRB sensitivity-inducing factor (DSIF). A 175-kDa phosphorylated form of hSpt5 is bound to nuclei of interphase HeLa cells. This form is rapidly dephosphorylated when cultured cells are exposed to various drugs belonging to distinct chemical families. All these compounds are known to inhibit the protein kinase Cdk9, which phosphorylates in vitro hSpt5 and Rpb1, the largest subunit of RNA polymerase II. The efficiency to promote the dephosphorylation of both proteins matches their capacity to inhibit purified Cdk9 kinase, suggesting that Cdk9 is the major kinase phosphorylating hSpt5 and Rpb1 in vivo. We show that Cdk9 phosphorylates both the CTR1 and the CTR2 domains of recombinant hSpt5. These domains contain numerous serine-proline and threonine-proline residues similar to those found in the carboxyl-terminal domain (CTD) of Rpb1. The structural homology between hSpt5 CTRs and the Rpb1 CTD is further highlighted by the presence on both proteins of a phosphoepitope recognized by the monoclonal antibody CC-3. Of particular interest, the peptidyl-prolyl isomerase Pin1 interacts with Cdk9-phosphorylated hSpt5. Cdk9 dependent phosphorylation of Rpb1 and hSpt5 followed by Pin1 interaction might thus contribute to the regulation of transcription, pre-mRNA maturation, and the dynamics of these proteins in interphase and mitosis.

Incomplete RNA polymerase II phosphorylation in Xenopus laevis early embryos

Phosphorylation of RNA polymerase II largest subunit on its C-terminal domain (CTD) heptapeptide repeats has been shown to play a key role in the regulation of mRNA synthesis and processing. In many higher metazoans, early embryos do not synthesise mRNAs during the first cell cycles following fertilisation. Transcription resumes and becomes an absolute requirement for development after several cell cycles characteristic of each species. Therefore, CTD phosphorylation has been investigated during early development of the African clawed-frog Xenopus laevis. Fertilisation is shown to trigger an abrupt dephosphorylation of the CTD. Phosphorylation of the CTD resumes concurrently with the mid-blastula transition (MBT). Both are advanced with polyspermy and increased temperatures; they do not occur when replication is impaired with aphidicolin. In Xenopus laevis somatic cells, a set of monoclonal antibodies defined distinct phosphoepitopes on the CTD. Two of them were absent before the MBT indicating that the CTD lacks the phosphorylation at the serine-2 position of the heptapeptide. The possible contribution of RNA polymerase II phosphorylation to the developmental-regulation of maternal mRNA processing in embryos is discussed.

Heat-induced relocalization of protein kinase CK2. Implication of CK2 in the context of cellular stress

Among various other roles described so far, protein kinase CK2 has been involved in cell cycle, proliferation, and development. Here, we show that in response to specific stresses (heat shock or UV irradiation), a pool of the cellular CK2 content relocalizes in a particular nuclear fraction, increasing the activity of the kinase there. Electron microscopic analysis shows that upon heat shock, CK2alpha and CK2beta subunits are both detected in similar speckle structures occurring in the interchromatin space but are differentially targeted inside the nucleolus. This CK2 relocalization process takes place in a time- and dose-dependent manner and is reversible upon recovery at 37 degrees C. Altogether, this work suggests CK2 be involved in the response to physiological stress in higher eukaryotic cells.

Cycloheximide- and puromycin-induced heat resistance: different effects on cytoplasmic and nuclear luciferases

Inhibition of translation can result in cytoprotection against heat shock. The mechanism of this protection has remained elusive so far. Here, the thermoprotective effects of the translation inhibitor cycloheximide (CHX) and puromycin were investigated, using as reporter firefly luciferase localized either in the nucleus or in the cytoplasm. A short preincubation of O23 cells with either translation inhibitor was found to attenuate the heat inactivation of a luciferase directed into the cytoplasm, whereas the heat sensitivity of a nuclear-targeted luciferase remained unaffected. After a long-term CHX pretreatment, both luciferases were more heat resistant. Both the cytoplasmic and the nuclear luciferase are protected against heat-induced inactivation in thermotolerant cells and in cells overexpressing heat shock protein (Hsp)70. CHX incubations further attenuated cytoplasmic luciferase inactivation in thermotolerant and in Hsp70 overexpressing cells, even when Hsp70-mediated protection was saturated. It is concluded that protection by translation inhibition is unlikely due to an increase in the pool of free Hsps normally engaged in translation and released from the nascent polypeptide chains on the ribosomes. Rather, a decrease in nascent chains and thermolabile polypeptides may account for the heat resistance promoted by inhibitors of translation.

Enhanced protein denaturation in indomethacin-treated cells

Indomethacin, a potent anti-inflammatory drug, activates the DNA-binding activity of human heat shock transcription factor 1 (HSF1), but this is insufficient to elevate heat shock gene expression. However, indomethacin pretreatment leads to a complete heat shock response at temperatures that are by themselves insufficient. Here, we showed that the heat-induced loss of enzymatic activity of a nuclear or a cytoplasmic luciferase expressed in murine cells was enhanced when cells had been pretreated with indomethacin. Additionally, in these cells the 70-kDa constitutive heat shock protein exhibited an enhanced aggregation in the presence of indomethacin. Similarly an increase in the aggregation of beta-galactosidase was observed. These data suggest that indomethacin at moderate temperatures accelerates the presence of denatured proteins in the cell, thus lowering the temperature threshold for a heat shock response.

Heat shock protein (Hsp) 40 mutants inhibit Hsp70 in mammalian cells

Heat shock protein (Hsp) 70 and Hsp40 expressed in mammalian cells had been previously shown to cooperate in accelerating the reactivation of heat-denatured firefly luciferase (Michels, A. A., Kanon, B., Konings, A. W. T., Ohtsuka, K., Bensaude, O., and Kampinga, H. H. (1997) J. Biol. Chem. 272, 33283-33289). We now provide further evidence for a functional interaction between Hsp70 and the J-domain of Hsp40 with denatured luciferase resulting in reactivation of heat-denatured luciferase within living mammalian cells. The stimulating effect of Hsp40 on the Hsp70-mediated refolding is lost when the proteins cannot interact as accomplished by their expression in different intracellular compartments. Likewise, the cooperation between Hsp40 and Hsp70 is lost by introduction of a point mutation in the conserved HPD motif of the Hsp40 J-domain or by deletion of the four C-terminal amino acids of Hsp70 (EEVD motif). Most strikingly, co-expression of a truncated protein restricted to the J-domain of Hsp40 had a dominant negative effect on Hsp70-facilitated luciferase reactivation. Taken together, these experiments indicate for the first time that the Hsp70/Hsp40 chaperones functionally interact with a heat-denatured protein within mammalian cells. The dominant negative effect of the Hsp40 J-domain on the activity of Hsp70 demonstrates the importance of J-domain-containing proteins in Hsp70-dependent processes.

Transcription-independent phosphorylation of the RNA polymerase II C-terminal domain (CTD) involves ERK kinases (MEK1/2)

The largest subunit of the mammalian RNA polymerase II possesses a C-terminal domain (CTD) consisting of 52 repeats of the consensus sequence, Tyr(1)-Ser(2)-Pro(3)-Thr(4)-Ser(5)-Pro(6)-Ser(7). Phosphorylation of the CTD is known to play a key role in gene expression. We now show that treatments such as osmotic and oxidative shocks or serum stimulation generate a new type of phosphorylated subunit, the IIm form. This IIm form might be generated in vivo by ERK-type MAP kinase phosphorylation as: (i) ERK1/2 are major CTD kinases found in cell extracts; (ii) the immunoreactivity of the IIm form against a panel of monoclonal antibodies indicates that the CTD is exclusively phosphorylated on Ser-5 in the repeats, like RNA polymerase II phosphorylated in vitro by an ERK1/2; and (iii) the IIm form does not appear when ERK activation is prevented by treating cells with low concentrations of highly specific inhibitors of MEK1/2. Since the IIm subunit is not affected by inhibition of transcription and is not bound to chromatin, it does not participate in transcription.

Regulated phosphorylation of the RNA polymerase II C-terminal domain (CTD)

The largest subunit of RNA polymerase II has an intriguing feature in its carboxyl-terminal domain (CTD) that consists of multiple repeats of an evolutionary conserved motif of seven amino acids. CTD phosphorylation plays a pivotal role in controlling mRNA synthesis and maturation. In exponentially growing cells, the phosphate turnover on the CTD is fast; it is blocked by common inhibitors of transcription, such as 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole and actinomycin D. Transcription-independent changes in CTD phosphorylation are observed at critical developmental stages, such as meiosis and early development.

A short region of its homeodomain is necessary for engrailed nuclear export and secretion

Engrailed homeoprotein, a transcription factor involved in midbrain/hindbrain patterning, primarily localizes to the cell nucleus. However, significant amounts of the protein are also found in the cell cytoplasm or associated with membrane microdomains enriched in cholesterol and glycosphingoglycolipids (Joliot, A., Trembleau, A., Raposo, G., Calvet, S., Volovitch, M. and Prochiantz, A. (1997) Development 124, 1865–1875). This non-nuclear localization, observed in vitro and in vivo, led us to investigate the possibility that Engrailed be transferred between nuclear and non-nuclear compartments. Monkey COS-7 cells expressing chick Engrailed-2 (cEN2) were fused with 3T3 mouse fibroblasts and the passage of cEN2 from COS-7 to 3T3 nuclei was followed in the interspecies heterokaryons. We find that, 10 minutes following cell fusion, cEN2 is detected in the 3T3 nuclei of 80% of the heterokaryons demonstrating rapid cEN2 nuclear export. Export from donor nuclei can be saturated and is strongly reduced after deletion of a 11 amino acid-long (Δ)1 sequence present within a slightly larger domain that extends between helices 2 and 3 of the homeodomain and shows strong similarities with leucine-rich nuclear export signals (NES). This putative NES, when fused with a nuclear reporter protein, allows its nuclear export, demonstrating that it is not only necessary but also sufficient for nuclear export and can therefore be considered as a true nuclear export sequence. In an earlier report (Joliot, A., Maizel, A., Rosenberg, D., Trembleau, A., Dupas, S., Volovitch, M. and Prochiantz, A. (1998) Current Biology 8, 856–863), we demonstrated that the (Δ)1 sequence is necessary for the access of cEN2 to the lumen of a membrane compartment and for its intercellular transfer. The present study thus strongly suggests that the regulation of Engrailed nuclear export could play a role not only in Engrailed transcriptional activity but also in its ability to gain access to a secretory compartment.

The transcriptional inhibitors, actinomycin D and alpha-amanitin, activate the HIV-1 promoter and favor phosphorylation of the RNA polymerase II C-terminal domain

Actinomycin D and alpha-amanitin are commonly used to inhibit transcription. Unexpectedly, however, the transcription of the human immunodeficiency virus (HIV-1) long terminal repeats (LTR) is shown to be activated at the level of elongation, in human and murine cells exposed to these drugs, whereas the Rous sarcoma virus LTR, the human cytomegalovirus immediate early gene (CMV), and the HSP70 promoters are repressed. Activation of the HIV LTR is independent of the NFkappaB and TAR sequences and coincides with an enhanced average phosphorylation of the C-terminal domain (CTD) from the largest subunit of RNA polymerase II. Both the HIV-1 LTR activation and the bulk CTD phosphorylation enhancement are prevented by several CTD kinase inhibitors, including 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole. The efficacies of the various compounds to block CTD phosphorylation and transcription in vivo correlate with their capacities to inhibit the CDK9/PITALRE kinase in vitro. Hence, the positive transcription elongation factor, P-TEFb, is likely to contribute to the average CTD phosphorylation in vivo and to the activation of the HIV-1 LTR induced by actinomycin D.

Highlights of the 1998 Cold Spring Harbor Meeting: Molecular chaperones and the heat shock response

No Abstract Available

Heat shock of HeLa cells inactivates a nuclear protein phosphatase specific for dephosphorylation of the C-terminal domain of RNA polymerase II

Reversible phosphorylation of the C-terminal domain (CTD) of the largest RNA polymerase II (RNAP II) subunit plays a key role in gene expression. Stresses such as heat shock result in marked changes in CTD phosphorylation as well as in major alterations in gene expression. CTD kinases and CTD phosphatase(s) contribute in mediating differential CTD phosphory-lation. We now report that heat shock of HeLa cells at temperatures as mild as 41 degreesC results in a decrease in CTD phosphatase activity in cell extracts. The obser-vation that this CTD phosphatase interacts with the RAP74 subunit of the general transcription factor TFIIF suggests that it corresponds to the previously charac-terized major CTD phosphatase. This conclusion is also supported by the finding that the distribution of the 150 kDa subunit of CTD phosphatase in cells is altered by heat shock. Although CTD phosphatase is found predominantly in low salt extracts in unstressed cells, immunofluorescence microscopy indicates that its intracellular localization is nuclear. The decrease in CTD phosphatase activity correlates with a decrease in amount of 150 kDa phosphatase subunit in the extracts. During heat shock, CTD phosphatase switches to an insoluble form which remains aggregated to the nuclear matrix fraction. In contrast, heat shock did not result in a redistribution of RAP74, indicating that not all nuclear proteins aggregate under these conditions. Accordingly, the heat-inactivation of both the CTD phosphatase and the TFIIH-associated CTD kinase might contribute to the selective synthesis of heat-shock mRNAs.

Phosphorylation of RNA polymerase II C-terminal domain (CTD): a new control for heat shock gene expression?

No Abstract Available

Growth-related changes in phosphorylation of yeast RNA polymerase II

The largest subunit of RNA polymerase II contains a unique C-terminal domain (CTD) consisting of tandem repeats of the consensus heptapeptide sequence Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7. Two forms of the largest subunit can be separated by SDS-polyacrylamide gel electrophoresis. The faster migrating form termed IIA contains little or no phosphate on the CTD, whereas the slower migrating II0 form is multiply phosphorylated. CTD kinases with different phosphoryl acceptor specificities are able to convert IIA to II0 in vitro, and different phosphoisomers have been identified in vivo. In this paper we report the binding specificities of a set of monoclonal antibodies that recognize different phosphoepitopes on the CTD. Monoclonal antibodies like H5 recognize phosphoserine in position 2, whereas monoclonal antibodies like H14 recognize phosphoserine in position 5. The relative abundance of these phosphoepitopes changes when growing yeast enter stationary phase or are heat-shocked. These results indicate that phosphorylation of different CTD phosphoacceptor sites are independently regulated in response to environmental signals.

Hsp70 and Hsp40 chaperone activities in the cytoplasm and the nucleus of mammalian cells

The existence and function of a Hsp40-Hsp70 chaperone machinery in mammalian cells in vivo was investigated. The rate of heat inactivation of firefly luciferase transiently expressed in hamster O23 fibroblasts was analyzed in cells co-transfected with the gene encoding the human Hsp40 (Ohtsuka, K. (1993) Biochem. Biophys. Res. Commun. 197, 235-240), the human inducible Hsp70 (Hunt, C., and Morimoto, R. I. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 6455-6459), or a combination of both. Whereas the expression of human Hsp70 alone in hamster cells was sufficient for the protection of firefly luciferase during heat shock, expression of the human Hsp40 alone was not. Rather, this led to a small but significant increase in the heat sensitivity of luciferase. The expression of the human Hsp40 only led to heat protection when the human Hsp70 was expressed as well. Under such conditions the rate of luciferase reactivation from the heat-inactivated state was increased, but the rate of inactivation during heat shock was not affected. Using constructs that direct firefly luciferase either to the cytoplasm or to the nucleus (Michels, A. A., Nguyen, V.-T., Konings, A. W. T., Kampinga, H. H., and Bensaude, O. (1995) Eur. J. Biochem. 234, 382-389), it was demonstrated that these chaperone functions are found in both compartments. Our data provide the first evidence on how the Hsp40/Hsp70 chaperone complex acts as heat protector in mammalian cells in vivo.

Nuclear translocation and carboxyl-terminal domain phosphorylation of RNA polymerase II delineate the two phases of zygotic gene activation in mammalian embryos

In mammalian embryos, zygotic gene transcription initiates after a limited number of cell divisions through a two-step process termed the zygotic gene activation (ZGA). Here we report that RNA polymerase II undergoes major changes in mouse and rabbit preimplantation embryos during the ZGA. In transcriptionally inactive unfertilized oocytes, the RNA polymerase II largest subunit is predominantly hyperphosphorylated on its carboxy-terminal domain (CTD). The CTD is markedly dephosphorylated several hours after fertilization, before the onset of a period characterized by a weak transcriptional activity. The largest subunit of RNA polymerase II then lacks immunological and drug-sensitivity characteristics related to its phosphorylation by the TFIIH-associated kinase and gradually translocates into the nuclei independently of DNA replication and mitosis. A phosphorylation pattern of the largest subunit, close to that observed in somatic cells, is established in both mouse and rabbit embryos at the stage when transcription becomes a requirement for further development (respectively at the 2- and 8/16-cell stage). As these events occurred in the presence of actinomycin D, the nuclear translocation of RNA polymerase II and the phosphorylation of the CTD might be major determinants of ZGA.

Phosphorylation of the RNA polymerase II largest subunit during Xenopus laevis oocyte maturation

Xenopus laevis oogenesis is characterized by an active transcription which ceases abruptly upon maturation. To survey changes in the characteristics of the transcriptional machinery which might contribute to this transcriptional arrest, the phosphorylation status of the RNA polymerase II largest subunit (RPB1 subunit) was analyzed during oocyte maturation. We found that the RPB1 subunit accumulates in large quantities from previtellogenic early diplotene oocytes up to fully grown oocytes. The C-terminal domain (CTD) of the RPB1 subunit was essentially hypophosphorylated in growing oocytes from Dumont stage IV to stage VI. Upon maturation, the proportion of hyperphosphorylated RPB1 subunits increased dramatically and abruptly. The hyperphosphorylated RPB1 subunits were dephosphorylated within 1 h after fertilization or heat shock of the matured oocytes. Extracts from metaphase II-arrested oocytes showed a much stronger CTD kinase activity than extracts from prophase stage VI oocytes. Most of this kinase activity was attributed to the activated Xp42 mitogen-activated protein (MAP) kinase, a MAP kinase of the ERK type. Making use of artificial maturation of the stage VI oocyte through microinjection of a recombinant stable cyclin B1, we observed a parallel activation of Xp42 MAP kinase and phosphorylation of RPB1. Both events required protein synthesis, which demonstrated that activation of p34(cdc2)off kinase was insufficient to phosphorylate RPB1 ex vivo and was consistent with a contribution of the Xp42 MAP kinase to RPB1 subunit phosphorylation. These results further support the possibility that the largest RNA polymerase II subunit is a substrate of the ERK-type MAP kinases during oocyte maturation, as previously proposed during stress or growth factor stimulation of mammalian cells.

Heat-shock inactivation of the TFIIH-associated kinase and change in the phosphorylation sites on the C-terminal domain of RNA polymerase II

The C-terminal domain (CTD) of the RNA polymerase II largest subunit (RPB1) plays a central role in transcription. The CTD is unphosphorylated when the polymerase assembles into a preinitiation complex of transcription and becomes heavily phosphorylated during promoter clearance and entry into elongation of transcription. A kinase associated to the general transcription factor TFIIH, in the preinitiation complex, phosphorylates the CTD. The TFIIH-associated CTD kinase activity was found to decrease in extracts from heat-shocked HeLa cells compared to unstressed cells. This loss of activity correlated with a decreased solubility of the TFIIH factor. The TFIIH-kinase impairment during heat-shock was accompanied by the disappearance of a particular phosphoepitope (CC-3) on the RPB1 subunit. The CC-3 epitope was localized on the C-terminal end of the CTD and generated in vitro when the RPB1 subunit was phosphorylated by the TFIIH-associated kinase but not by another CTD kinase such as MAP kinase. In apparent discrepancy, the overall RPB1 subunit phosphorylation increased during heat-shock. The decreased activity in vivo of the TFIIH kinase might be compensated by a stress-activated CTD kinase such as MAP kinase. These results also suggest that heat-shock gene transcription may have a weak requirement for TFIIH kinase activity.

The nuclear matrix protein p255 is a highly phosphorylated form of RNA polymerase II largest subunit which associates with spliceosomes

The monoclonal antibody CC-3 recognizes a phosphodependent epitope on a 255 kDa nuclear matrix protein (p255) recently shown to associate with splicing complexes as part of the [U4/U6.U5] tri-snRNP particle [Chabot et al. (1995) Nucleic Acids Res. 23, 3206-3213]. In mouse and Drosophila cultured cells the electrophoretic mobility of p255, faster in the latter species, was identical to that of the hyperphosphorylated form of RNA polymerase II largest subunit (IIo). The CC-3 immunoreactivity of p255 was abolished by 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, which is known to cause the dephosphorylation of the C-terminal domain of subunit IIo by inhibiting the TFIIH-associated kinase. The identity of p255 was confirmed by showing that CC-3-immunoprecipitated p255 was recognized by POL3/3 and 8WG16, two antibodies specific to RNA polymerase II largest subunit. Lastly, the recovery of RNA polymerase II largest subunit from HeLa splicing mixtures was compromised by EDTA, which prevents the interaction of p255 with splicing complexes and inhibits splicing. Our results indicate that p255 represents a highly phosphorylated form of RNA polymerase II largest subunit physically associated with spliceosomes and possibly involved in coupling transcription to RNA processing.

In vivo degradation of RNA polymerase II largest subunit triggered by alpha-amanitin

Alpha-Amanitin is a well-known specific inhibitor of RNA polymerase II (RNAPII) in vitro and in vivo. It is a cyclic octapeptide which binds with high affinity to the largest subunit of RNAPII, RPB1. We have found that in murine fibroblasts exposure to alpha-amanitin triggered degradation of the RPB1 subunit, while other RNAPII subunits, RPB5 and RPB8, remained almost unaffected. Transcriptional inhibition in alpha-amanitin-treated cells was slow and closely followed the disappearance of RPB1. The degradation rate of RPB1 was alpha-amanitin dose dependent and was not a consequence of transcriptional arrest. Alpha-Amanitin-promoted degradation of RPB1 was prevented in cells exposed to actinomycin D, another transcriptional inhibitor. Epitope-tagged recombinant human RPB1 subunits were expressed in mouse fibroblasts. In cells exposed to alpha-amanitin the wild-type recombinant subunit was degraded like the endogenous protein, but a mutated alpha-amanitin-resistant subunit remained unaffected. Hence, alpha-amanitin did not activate a proteolytic system, but instead its binding to mRPB1 likely represented a signal for degradation. Thus, in contrast to other inhibitors, such as actinomycin D or 5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole, which reversibly act on transcription, inhibition by alpha-amanitin cannot be but an irreversible process because of the destruction of RNAPII.

Ccl1, a cyclin associated with protein kinase Kin28, controls the phosphorylation of RNA polymerase II largest subunit and mRNA transcription

The Kin28 protein kinase interacts physically and genetically with cyclin Ccl1. Kin28 has been reported recently to be involved in the in vivo phosphorylation of the largest subunit of RNA polymerase II (Rpb1) in Saccharomyces cerevisiae. Now, we show that in a strain harboring a conditional ccl1-ts mutation, the C-terminal domain (CTD) of the Rpb1 subunit is under-phosphorylated at restrictive temperature. The transcription of a set of genes, chosen at random, is severely affected in a kin28-ts mutant shifted at restrictive temperature. Here, we report that the same set of genes requires a functional CCL1 gene product to be transcribed. These findings, added to previously published data, establishes that Kin28p is a cyclin-dependent kinase (CDK) with Ccl1p as a companion, both of them being necessary for general transcription and CTD phosphorylation.

Heat-shock induced protein modifications and modulation of enzyme activities

Upon heat stress, the cell physiology is profoundly altered. The extent of the alterations depends on the severity of the stress and may lead to cell death. The heat shock response is an array of metabolic changes characterized by the impairment of major cellular functions and by an adaptative reprogramming of the cell metabolism. The enhanced synthesis of the HSPs is a spectacular manifestation of this reprogramming. Numerous post translational modifications of proteins occur in response to heat stress and can be related to altered cellular functions. Some proteins are heat-denatured and temporarily inactivated. Heat-denaturation is reversible, chaperones may contribute to the repair. The extent of heat-denaturation depends on the cell metabolism: (a) it is attenuated in thermotolerant cells or in cells overexpressing the appropriate chaperones (b) it is enhanced in energy-deprived cells. Covalent modifications may also rapidly alter protein function. Changes in protein glycosylation, methylation, acetylation, farnesylation, ubiquitination have been found to occur during stress. But protein phosphorylation is the most studied modification. Several protein kinase cascades are activated, among which the various mitogen activated protein kinase (MAP kinase) cascades which are also triggered by a wide range of stimuli. As a possible consequence, stress modifies the phosphorylation status and the activity of components from the transcriptional and translational apparatuses. The same kinases also target key enzymes of the cellular metabolism. Protein denaturation results in constitutive hsp titration, this titration is a signal to trigger the heat-shock gene transcription and to activate some of the protein kinase cascades.

Thermostability of a nuclear-targeted luciferase expressed in mammalian cells. Destabilizing influence of the intranuclear microenvironment

Protein denaturation and aggregation are most likely the cause for the noxious effects of heat shock. There are some indications that the nucleus is one of the most sensitive cellular compartments. To test the possibility that the intranuclear microenvironment might be detrimental to the heat stability of proteins, we compared the in situ thermal stability of a reporter protein localized in the nucleus or in the cytoplasm. A recombinant firefly (Photynus pyralis) luciferase carrying a point mutation in the C-terminal domain remains in the cytoplasm (cyt-luciferase). A nuclear localization sequence was fused to the N-terminal domain of cyt-luciferase; the resulting nuc-luciferase was efficiently targeted to the cell nucleus. In both cases, decreased luciferase activity and solubility were found in lysates from heat-shocked cells. These characteristics were taken as an indication of thermal denaturation in situ. The heat-inactivated luciferases were partially reactivated during recovery after stress, indicating the capacity of both the cytoplasmic and nuclear compartments to reassemble proteins from an aggregated state. Although both the nuc- and the cyt-luciferases were heat inactivated at similar rates in vitro, nuc-luciferase was more susceptible to thermal denaturation in situ compared to cyt-luciferase. This observation suggests that the microenvironment of an intracellular compartment may modulate the thermal stability of proteins. The local concentration might be one element of this microenvironment affecting the heat-stability of proteins. In cells made thermotolerant by a priming shock, the thermal inactivation of the recombinant luciferases occurred at a slower rate during a second challenging stress. However, this decreased thermal sensitivity was less pronounced for the nuc-luciferase (threefold) than for the cyt-luciferase (sevenfold). The nuclear luciferase might become a useful tool to investigate the action of molecular chaperones in the nucleus.

Phosphorylation state of the RNA polymerase II C-terminal domain (CTD) in heat-shocked cells. Possible involvement of the stress-activated mitogen-activated protein (MAP) kinases

RNA polymerase (RNAP) II is a multisubunit enzyme composed of several different subunits. Phosphorylation of the C-terminal domain (CTD) of the largest subunit is tightly regulated. In quiescent or in exponentially growing cells, both the unphosphorylated (IIa) and the multiphosphorylated (IIo) subunits of RNAP II are found in equivalent amounts as the result of the equilibrated antagonist action of protein kinases and phosphatases. In Drosophila and mammalian cells, heat shock markedly modifies the phosphorylation of the RNAP II CTD. Mild heat shocks result in dephosphorylation of the RNAP II CTD. This dephosphorylation is blocked in the presence of actinomycin D, as the CTD dephosphorylation observed in the presence of protein kinase inhibitors. Thus, heat shock might inactivate CTD kinases which are operative at normal growth temperatures, as some protein kinase inhibitors do. In contrast, severe heat shocks are found to increase the amount of phosphorylated subunit independently of the transcriptional activity of the cells. Mild and severe heat shocks activate protein kinases, which then phosphorylate, in vitro and in vivo, the CTD fused to beta-galactosidase. Most of the heat-shock-activated CTD kinases present in cytosolic lysates co-purify with the activated mitogen-activated protein (MAP) kinases, p42mapk and p44mapk. The weak CTD kinase activation occurring upon mild heat shock might be insufficient to compensate for the heat inactivation of the already existing CTD kinases. However, under severe stress, the MAP kinases are strongly heat activated and might prevail over the phosphatases. A survey of different cells and different heat-shock conditions shows that the RNAP II CTD hyperphosphorylation rates follow the extent of MAP kinase activation. These observations lead to the proposal that the RNAP II CTD might be an in vivo target for the activated p42mapk and p44mapk MAP kinases.

The KIN28 gene is required both for RNA polymerase II mediated transcription and phosphorylation of the Rpb1p CTD

Kin28p, associated with cyclin Ccl1p, is a putative cyclin-dependent kinase (CDK) of the p34cdc2 family in Saccharomyces cerevisiae. Search for mutations co-lethal (syn mutations) with a kin28 thermosensitive mutation (kin28-ts3) has uncovered genetic interactions between gene KIN28 and genes RAD3, SIN4, STI1 and CDC37. The genetic interaction between KIN28 and the CDC37 cell division cycle gene suggests that a connection exists between the activity of CDK-Kin28p and cell-cycle progression. Both RAD3 and SIN4 gene products are implicated in the RNA polymerase II transcription process. Here we show that RNA polymerase II transcription is drastically reduced in a kin28-ts mutant, at restrictive temperature. This impairment correlates with a markedly decreased phosphorylation of the C-terminal domain (CTD) of the largest subunit of RNA polymerase II (Rpb1p). Thus, the Kin28 gene product is required in vivo for RNA polymerase II phosphorylation and transcriptional activity as recently suggested by experiments using an in vitro reconstituted system.

Stathmin is a major substrate for mitogen-activated protein kinase during heat shock and chemical stress in HeLa cells

Stathmin is a ubiquitous, highly conserved 19-kDa cytoplasmic protein whose expression and phosphorylation are regulated in relation to cell proliferation, differentiation or activation, in many biological systems. In this report, we show that stathmin undergoes major phosphorylation in HeLa cells submitted to heat or chemical stress. Heat-shock-induced stathmin phosphorylation was very rapid, as maximal incorporation of phosphate was observed at 5 min. Phosphorylation of stathmin might, therefore, occur as a very early step in the intracellular response to heat shock. The sites of phosphorylation of stathmin involved during the stress response were identified as mostly Ser25 and, to a lesser extent, Ser38. These sites are both followed by a proline residue, and known to be good substrates in vitro for mitogen-activated protein kinase (MAP-kinase) and p34cdc2 kinase, respectively. In lysates from heat-shocked cells, an increased stathmin-kinase activity, distinct from the histone-H1-kinase activity, was found to phosphorylate stathmin mostly on Ser25, the main site for MAP-kinase in vitro. This stathmin-kinase coeluted quantitatively with the stress-activated MAP-kinase from an FPLC MonoQ column. Furthermore, a stathmin kinase activity was precipitated from lysates of heat-shocked HeLa cells by an anti-(MAP-kinase) serum. Together, these results indicate that the phosphorylation of stathmin by MAP-kinase is likely to be a significant component of the signalling array controlling the cellular response to stress, and they further underline the general involvement of stathmin in intracellular signalling.

Enhanced phosphorylation of the C-terminal domain of RNA polymerase II upon serum stimulation of quiescent cells: possible involvement of MAP kinases

The largest subunit of RNA polymerase (RNAP) II contains at it C-terminus an unusual domain comprising tandem repeats of the consensus sequence Tyr-Ser-Pro-Thr-Ser-Pro-Ser. This C-terminal domain (CTD) can undergo phosphorylation at multiple sites giving rise to a form of the enzyme designated RNAP IIO. The unphosphorylated form is designated RNAP IIA. The largest subunits of RNAPs IIO and IIA are designated IIo and IIa, respectively. In quiescent NIH 3T3 fibroblasts, subunits IIo and IIa are present in comparable amounts. Upon serum stimulation, the amount of subunit IIo increases markedly and remains elevated for several hours. The increase of subunit IIo also occurs in transcription-inhibited cells and, therefore, is not a consequence of serum-activated transcription. This observation suggests that serum stimulation activates a CTD kinase and/or inhibits a CTD phosphatase. This hypothesis is supported by the finding that serum stimulates phosphorylation of a beta-galactosidase-CTD fusion protein expressed in these cells. Furthermore, an enhanced CTD kinase activity was discovered in lysates from serum-stimulated fibroblasts and was found to copurify with MAP kinases on a Mono Q column and to bind to anti-MAP kinase antibodies. The idea that MAP kinases phosphorylate the CTD in vivo is supported by the observation that subunit IIa, but not subunit IIb which lacks the CTD, is phosphorylated at multiple sites by purified MAP kinase. Consequently, the MAP kinases are a new class of CTD kinases which appear to be involved in the phosphorylation of RNAP II following serum stimulation. This phosphorylation may contribute to the transcriptional activation of serum-stimulated genes.

Inhibitors of transcription such as 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole and isoquinoline sulfonamide derivatives (H-8 and H-7) promote dephosphorylation of the carboxyl-terminal domain of RNA polymerase II largest subunit

The RNA polymerase IIO and IIA differ by the extent of phosphorylation in the carboxyl-terminal domain (CTD) of the largest subunit. It has been proposed that the IIA form of RNA polymerase II interacts with the promoter to form a stable preinitiation complex whereas the IIO form would be generated upon entry into initiation of transcription. Phosphorylation of the CTD might be required to release the interaction between the polymerase and the promoter binding factors. In this paper, we show that in the presence of actinomycin D, the phosphorylated IIO form accumulates. In contrast, the dephosphorylated IIA form accumulates while the amount of phosphorylated IIo form decreases in cells treated with CTD-kinase inhibitors such as the nucleoside analog, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole or the isoquinoline sulfonamide derivatives H-7* or H-8. These changes are fast and suggest a very rapid phosphate turnover on the CTD. Transcription is inhibited in intact cells by drug concentrations that are effective in altering CTD phosphorylation, although no causal relationship is established yet. These effects do not concern other cellular functions such as protein synthesis. Thus isoquinoline sulfonamide derivatives might be helpful to further dissect the role of CTD phosphorylation in transcription.

Phosphorylation of the RNA polymerase II largest subunit during heat shock and inhibition of transcription in HeLa cells

The phosphorylation of the C-terminal domain (CTD) of the largest subunit of eukaryotic RNA polymerase II has been investigated in HeLa cells exposed to heat shock. In control cells, the phosphorylated subunit, IIo, and the dephosphorylated subunit, IIa, were found in similar amounts. During heat shock, however, the phosphorylated subunit, IIo, accumulated, whereas the amount of IIa subunit decreased. Since phosphorylation of the CTD had been suggested to play a role in the initiation of transcription and since heat shock was known to perturb gene expression at the level of transcription, the phosphorylation state of RNA polymerase II was examined in cells that had been treated with various inhibitors of transcription. Under normal growth temperature, actinomycin D (over 0.1 microgram/ml) and okadaic acid, a phosphatase inhibitor, were found to inhibit polymerase dephosphorylation. Whereas 5,6-dichlorobenzimidazole riboside (DRB), N-(2-[Methylamino]ethyl)-5-isoquinolinesulfonamide (H-8), and actinomycin D (over 5 micrograms/ml) were found to inhibit polymerase phosphorylation. Actinomycin D concentrations, which inhibited the dephosphorylation process, were lower than those required to inhibit the phosphorylation process. In contrast, during heat shock or exposure to sodium arsenite, a chemical inducer of the heat-shock response, the phosphorylated subunit, IIo, accumulated even in the presence of inhibitors of transcription such as DRB, H-8, and actinomycin D. These experiments demonstrated the existence of a heat-shock-induced CTD-phosphorylation process that might contribute to the regulation of transcription during stress.

Increased thermal aggregation of proteins in ATP-depleted mammalian cells

In an attempt to understand the influence of the intracellular environment on protein stability, the thermal denaturation of various reporter proteins was examined within cultured mammalian cells. Loss of solubility and of enzymatic activities were taken as indicators of thermal denaturation. Photinus pyralis luciferase, Escherichia coli beta-galactosidase, the 70-kDa constitutive heat-shock proteins and the 68-kDa dsRNA-dependent protein kinase are found mostly in the supernatant fractions of centrifuged lysates from control unshocked mammalian cells. However, when cells are lysed after heat shock, a proportion of the reporter molecules is found to be aggregated to the nuclear pellets. This insolubilization does not affect all cellular proteins; many of them remain unaffected by heat shock. The heat-induced insolubilization of all four reporter proteins is markedly enhanced when the intracellular ATP concentration is drastically decreased after inhibition of both oxidative phosphorylation and glycolysis. Although ATP molecules bind to luciferase and protect it from thermal inactivation in vitro, the consequences of strong ATP depletion on luciferase thermal stability within the cells are found to be much greater than expected from in vitro data. The 70-kDa constitutive heat-shock proteins and the 68-kDa protein kinase are ATP-binding proteins but ATP depletion also considerably increases the aggregation of beta-galactosidase to the nuclear pellets, although this enzyme is not known to be an ATP-binding molecule. Insolubilization of all four reporter proteins occurs in ATP-depleted cells even at normal growing temperatures (37 degrees C). Protein denaturation may be enhanced either by the aggregation and disappearance of the intracellular 'free' chaperones or by the trapping of unfolded protein molecules on chaperones; the chaperone/unfolded protein complexes could not dissociate in the absence of ATP. Enhanced protein denaturation due to ATP depletion is proposed to account for the greater heat sensitivity of ATP-depleted cells and for the ability of mitochondrial uncouplers to trigger a heat-shock response in some cells.

MAP kinase activation during heat shock in quiescent and exponentially growing mammalian cells

In numerous cases of signal transduction, the mitogen-activated protein kinases (MAP kinases) or extracellular regulated kinases (ERKs) are found to be activated by phosphorylations which result in electrophoretic mobility changes. Activities of MAP kinases in cytosolic extracts can also be monitored by the capacity of such extracts to phosphorylate myelin basic protein. These two assays were used to demonstrate that MAP kinases were rapidly activated during heat shock of both quiescent and exponentially growing mammalian (hamster, rat, mouse and human) cells. Thus, the MAP kinase cascade is likely to also ensure heat-shock signal transduction and contribute to the regulation of the complex array of metabolic changes designated as the heat-shock response.

Decreased stress inducibility of the HSP68 protein in a rat hepatoma variant clone

Analysis of the stress response of closely related rat hepatoma clones revealed that the major inducible heat-shock protein 68 (HSP68) was only slightly inducible upon stress in the glucocorticoid-resistant, dedifferentiated clone-2 cells, but strongly activated in the differentiated, glucocorticoid-sensitive Faza 967 cells from which clone 2 was derived. The decreased inducibility of HSP68 in clone-2 cells was not the consequence of altered kinetics of protein synthesis recovery, was not correlated with the deficient inducibility of other major heat-shock proteins and had no effect on the heat sensitivity of the cells. This deficiency was observed after treatment with mild and strong heat and various chemicals. The results of nuclear run-on experiments suggested that the impairment of HSP68 mRNA induction most likely occurs at the transcriptional level and is probably specific for the corresponding gene. In Faza 967 and clone-2 cells, stress activated comparable levels of heat-shock-factor binding to the heat-shock element, and the expression of a reporter gene under the control of murine HSP70.1 promoter was strongly stimulated in both cells. Therefore, our results raise the possibility that the deficient stress inducibility of HSP68 is due to some specific regulation of the endogeneous HSP68 gene, rather than to a deficiency of the heat-shock factor or mutation of the corresponding gene.

Binding activity of glucocorticoid receptors after heat shock

The response of glucocorticoid receptors (GR) to heat was measured by the change in ligand binding activity both in control cells and in cells made tolerant to heat by a prior mild heat exposure. The study was prompted by earlier data showing that one of the heat shock proteins (HSP90) is an essential component of the GR complex and that treatment of mammalian cells with hydrocortisone induces resistance to heat damage. The GR rapidly loses binding activity after commencement of heating. There is a 50% loss of activity after 4 min at 45 degrees C, 8 min at 44 degrees C, or 17 min at 43 degrees C. The reduction in binding is due mainly to a reduction in affinity of binding to the ligand. The ability to bind glucocorticoid recovers quickly after heat treatment. Activity returns to levels 60-80% of normal by 2 h after a heat treatment that initially reduces binding to less than 20% of normal. However, complete restoration of binding activity takes approximately 3 days. The recovery of binding activity does not require protein synthesis. Pretreatment of cells with hydrocortisone, using conditions that induce heat resistance, reduces the activity to 10-20% of control, but residual receptors display a heat sensitivity similar to that of control cells. There was evidence for a limited degree of protection of GR from heat damage in thermotolerant cells.

Heat shock increases turnover of 90 kDa heat shock protein phosphate groups in HeLa cells

The 90 kDa heat shock protein (hsp90) is a major phosphoprotein which associates various other cellular polypeptides such as actin, calmodulin, steroid hormone receptors and certain protein-kinases. Little is known about the function of hsp90 in recovery from stress. In this report, we describe a dramatic increase in the rate of both phosphate uptake and dephosphorylation of hsp90 in HeLa cells submitted to acute stresses. This increased turnover of hsp90 phosphate groups might reflect a greater protein binding activity of hsp90 in stressed cells.

Denaturation of proteins during heat shock. In vivo recovery of solubility and activity of reporter enzymes

Using beta-galactosidase and luciferase as reporter enzymes, we have previously shown that enzymatic inactivation occurring during a heat shock is concomitant with protein insolubilization (Nguyen, V. T., Morange, M., and Bensaude, O. (1989) J. Biol. Chem. 264, 10487-10492). In this paper, we observe that pretreatment of cells with D2O and glycerol, compounds known to stabilize protein structure, leads to a parallel decrease of protein inactivation and insolubilization, suggesting that these two phenomena result most probably from heat-induced protein denaturation. We found that heat shock-promoted inactivation and insolubilization are not irreversible processes, since even in the absence of protein synthesis, beta-galactosidase solubility and luciferase solubility and activity are recovered in vivo after a heat treatment. Cognate heat shock proteins might be involved in this renaturation process.

Heat-shock-induced denaturation of proteins. Characterization of the insolubilization of the interferon-induced p68 kinase

Heat-shock stress causes inactivation and aggregation of various cellular proteins which become further insoluble. Previous studies have shown that the interferon-induced p68 kinase activity was greatly reduced in extracts of heat-shocked HeLa cells, and that the loss of activity was due to a decreased solubility of the enzyme. Here we show that the p68 kinase which is normally evenly distributed in the cytoplasm, aggregates as a thick ring around the nucleus in heat-shocked cells. The 70-kDa constitutive heat-shock proteins are major insolubilized proteins during stress and we find them to colocalize with the p68 kinase after stress. Treatments of cells with drugs which disrupt the cytoskeleton, such as colcemid and cytochalasin E, do not hinder the enzyme insolubilization during heat-shock. On the contrary, heat-protectors such as glycerol and deuterium oxide (D2O) keep the p68 kinase under a soluble and active form during heat-shock stress. Similarly, an attenuation of the insolubilization of this enzyme is observed in cells rendered thermo-tolerant by a previous heat-shock, suggesting that heat-shock proteins may also contribute to the protection. During the recovery period at normal temperature after heat-shock, resolubilization occurs and most of the enzyme is again recovered under an active soluble form.