The mRNAs for cyclin A, c-myc and ribosomal proteins L4 and S6 are associated with cytoskeletal-bound polysomes in HepG2 cells

Proteomic dissection of rice cytoskeleton reveals the dominance of microtubule and microfilament proteins, and novel components in the cytoskeleton-bound polysome

The plant cytoskeleton persistently undergoes remodeling to achieve its roles in supporting cell division, differentiation, cell expansion and organelle transport. However, the links between cell metabolism and cytoskeletal networks, particularly how the proteinaceous components execute such processes remain poorly understood. We investigated the cytoskeletal proteome landscape of rice to gain better understanding of such events. Proteins were extracted from highly enriched cytoskeletal fraction of four-week-old rice seedlings, and the purity of the fraction was stringently monitored. A total of 2577 non-redundant proteins were identified using both gel-based and gel-free approaches, which constitutes the most comprehensive dataset, thus far, for plant cytoskeleton. The data set includes both microtubule and microfilament-associated proteins and their binding proteins comprising hypothetical as well as novel cytoskeletal proteins. Further, various in-silico analyses were performed, and the proteins were functionally classified on the basis of their gene ontology. The catalogued proteins were validated through their sequence analysis. Extensive comparative analysis of our dataset with the non-redundant set of cytoskeletal proteins across plant species affirms unique as well as overlapping candidates. Together, these findings unveil new insights of how cytoskeletons undergo dynamic remodeling in rice to drive seedling development processes in rapidly changing in planta environment.

RNA-dependent association with myosin IIA promotes F-actin-guided trafficking of the ELAV-like protein HuR to polysomes

The role of the mRNA-binding protein human antigen R (HuR) in stabilization and translation of AU-rich elements (ARE) containing mRNAs is well established. However, the trafficking of HuR and bound mRNA cargo, which comprises a fundamental requirement for the aforementioned HuR functions is only poorly understood. By administering different cytoskeletal inhibitors, we found that the protein kinase Cδ (PKCδ)-triggered accumulation of cytoplasmic HuR by Angiotensin II (AngII) is an actin-myosin driven process functionally relevant for stabilization of ARE-bearing mRNAs. Furthermore, we show that the AngII-induced recruitment of HuR and its bound mRNA from ribonucleoprotein particles to free and cytoskeleton bound polysomes strongly depended on an intact actomyosin cytoskeleton. In addition, HuR allocation to free and cytoskeletal bound polysomes is highly sensitive toward RNase and PPtase and structurally depends on serine 318 (S318) located within the C-terminal RNA recognition motif (RRM3). Conversely, the trafficking of the phosphomimetic HuRS318D, mimicking HuR phosphorylation at S318 by the PKCδ remained PPtase resistant. Co-immunoprecipitation experiments with truncated HuR proteins revealed that the stimulus-induced association of HuR with myosin IIA is strictly RNA dependent and mediated via the RRM3. Our data implicate a microfilament dependent transport of HuR, which is relevant for stimulus-induced targeting of ARE-bearing mRNAs from translational inactive ribonucleoprotein particles to polysomes.

Competition between the signal sequence and a 3'UTR localisation signal during redirection of beta-globin mRNA to the endoplasmic reticulum: implications for biotechnology

Secretion of an intracellular protein from a cell factory requires as a first step the redirection of the mRNA for synthesis of the protein on the endoplasmic reticulum. The feasibility of retargeting a mRNA coding for an intracellular protein to the endoplasmic reticulum was investigated using Ltk- fibroblasts stably transfected with gene constructs in which rabbit beta-globin coding region and 5'UTR was linked to albumin signal sequence and different 3'untranslated regions. Globin transcripts with the native globin 3'untranslated region or with the 3'untranslated region of c-myc are present in free/cytoskeletal-bound polysomes. The addition of the signal sequence from rat albumin redirects both these globin transcripts to membrane-bound polysomes but the presence of the c-myc 3'UTR reduces the extent of redirection. Globin transcripts with both the signal sequence and 3'untranslated region from the albumin gene are efficiently redirected to membrane-bound polysomes. The results suggest competition between 5' and 3' localising signals. The addition of the signal sequence does not destabilise the mRNA nor affect translational efficiency. It is concluded that it is possible to retarget an mRNA to the endoplasmic reticulum while maintaining stability and translational capacity. This has important implications for the development of vectors to promote secretion of intracellular proteins from cell factories.

HuR regulates beta-tubulin isotype expression in ovarian cancer

The supply of oxygen and nutrients to solid tumors is inefficient because cancer tissues have an inadequate number of microvessels, thus inducing the selective growth of the most aggressive cancer cells. This explains why many of the factors underlying a poor prognosis are induced in hypoxic/hypoglycemic conditions. Among these factors, a prominent role in several solid tumors is played by the class III beta-tubulin gene (TUBB3). The study described here reveals that glucose deprivation enhances TUBB3 expression at both the gene and protein levels in A2780 ovarian cancer cells. In silico analysis of TUBB3 mRNA sequence predicted a putative binding site for the RNA-binding protein Hu antigen (HuR) in the 3' flanking untranslated region. A hypoglycemic-dependent engagement of this site was shown using RNA pull-down and ribonucleoimmunoprecipitation techniques. Thereafter, HuR gene silencing revealed that TUBB3 translation is HuR dependent in hypoglycemia because HuR silencing inhibited the entry of TUBB3 mRNA into cytoskeletal and free polysomes. Finally, the clinical value of this finding was assessed in a clinical cohort of 46 ovarian cancer patients in whom it was found that HuR cytoplasmic staining was associated with high levels of TUBB3 and poor survival.

Posttranslational modification of the AU-rich element binding protein HuR by protein kinase Cdelta elicits angiotensin II-induced stabilization and nuclear export of cyclooxygenase 2 mRNA

The mRNA stabilizing factor HuR is involved in the posttranscriptional regulation of many genes, including that coding for cyclooxygenase 2 (COX-2). Employing RNA interference technology and actinomycin D experiments, we demonstrate that in human mesangial cells (hMC) the amplification of cytokine-induced COX-2 by angiotensin II (AngII) occurs via a HuR-mediated increase of mRNA stability. Using COX-2 promoter constructs with different portions of the 3' untranslated region of COX-2, we found that the increase in COX-2 mRNA stability is attributable to a distal class III type of AU-rich element (ARE). Likewise, the RNA immunoprecipitation assay showed AngII-induced binding of HuR to this ARE. Using the RNA pulldown assay, we demonstrate that the AngII-caused HuR assembly with COX-2 mRNA is found in free and cytoskeleton-bound polysomes indicative of an active RNP complex. Mechanistically, the increased HuR binding to COX-2-ARE by AngII is accompanied by increased nucleocytoplasmic HuR shuttling and depends on protein kinase Cdelta (PKCdelta), which physically interacts with nuclear HuR, thereby promoting its phosphorylation. Mapping of phosphorylation sites identified serines 221 and 318 as critical target sites for PKCdelta-triggered HuR phosphorylation and AngII-induced HuR export to the cytoplasm. Posttranslational modification of HuR by PKCdelta represents an important novel mode of HuR activation implied in renal COX-2 regulation.

Identification of AUF1 as a rapamycin-responsive binding protein to the 5'-terminal oligopyrimidine element of mRNAs

In vertebrates, mRNAs containing a 5'-terminal oligopyrimidine (TOP) motif are coordinately post-transcriptionally regulated. Binding of specific proteins to this element has been proposed to downregulate expression of TOP mRNAs at the level of translational initiation. We previously reported that rapamycin induces binding activity to the TOP element of ribosomal protein (r-protein) L32 mRNA. In this study, we adapt DEAE-cellulose/oligo dT-cellulose tandem column chromatography to purify TOP element-binding proteins from bovine submaxillary lymph nodes (SLN). We also show by northwestern blot analysis that two proteins of molecular weight 47kDa (47BP) and 43kDa (43BP) specifically bind to a (32)P-labeled riboprobe containing TOP regulatory element of the r-protein L32. Microsequencing of the purified 47BP revealed an internal sequence of 15 amino acids identical to the consensus sequence of the 2x RBD-Gly family. Western blot analysis of the cytoplasm fractions using an AUF1 antibody revealed that these two proteins are p45 AUF1 and p42 AUF1. Increases of the four isoforms of AUF1 protein were observed in 100,000g supernatant fractions of rapamycin-administered rat SLN. Furthermore, decreases of p45 AUF1 and p42 and/or p40 AUF1 were observed in the polysomal fractions of BJAB cells in which translation of TOP mRNAs was selectively suppressed by rapamycin treatment. Taken together, these results suggest that AUF1 is a TOP mRNA-binding protein that may participate in the translational suppression of TOP mRNAs resulting from rapamycin treatment.

The level of synthesis and secretion of Gaussia princeps luciferase in transfected CHO cells is heavily dependent on the choice of signal peptide

There is a great demand for the improvement of mammalian cell production systems such that they can compete economically with their prokaryotic counterparts. Of a number of parameters that need to be explored to accomplish this we have tested the effects of different signal peptides on the synthesis and secretion of Gaussia princeps luciferase in mammalian cells. A series of plasmids were transfected into CHO cells where the coding region for the marine luciferase was fused to the signal peptide coding regions derived from different sources. Both cell extracts and medium samples were analysed for luciferase activity. When the native Gaussia luciferase signal sequence in the vector was substituted by that from human interleukin-2 or albumin then the amount of active recombinant protein produced was substantially reduced, both in transiently and stably transfected cells. Western blotting showed that enzyme activity and protein levels mirrored one another. The major decrease in luciferase activity was shown not to be a result of decreased mRNA levels, indicating the involvement of a post-transcriptional event. When the coding region of human endostatin was fused to that of the Gaussia luciferase signal peptide then an elevated level of secreted endostatin was observed compared to when that of the albumin signal peptide was used. Stable transfection of HepG2 cells with the different signal peptide constructs gave essentially the same results as seen in CHO cells. The overall results indicate that the choice of signal peptide can be imperative to ensure an optimal synthesis and secretion of a recombinant protein in a mammalian cell culture system.

The 3'-untranslated region directs ribosomal protein-encoding mRNAs to specific cytoplasmic regions

mRNA localization is a conserved post-transcriptional process crucial for a variety of systems. We have analyzed the subcellular distribution of mRNAs encoding human cytosolic and mitochondrial ribosomal proteins. Biochemical fractionation experiments showed that the transcripts for cytosolic ribosomal proteins associate preferentially with the cytoskeleton via actin microfilaments. Transfection in HeLa cells of a GFP reporter construct containing the cytosolic ribosomal protein L4 3'-UTR showed that the 3'-UTR is necessary for the association of the transcript to the cytoskeleton. Using confocal analysis we demonstrate that the chimeric transcript is specifically associated with the perinuclear cytoskeleton. We also show that mRNA for mitochondrial ribosomal protein S12 is asymmetrically distributed in the cytoplasm. In fact, this transcript was localized mainly in the proximity of mitochondria, and the localization was 3'-UTR-dependent. In summary, ribosomal protein mRNAs constitute a new class of localized transcripts that share a common localization mechanism.

Actin Cytoskeleton Organization and Posttranscriptional Regulation of Endothelial Nitric Oxide Synthase During Cell Growth

Posttranscriptional regulation of endothelial nitric oxide synthase (eNOS) expression is an important mechanism by which endothelial cells respond to various physiological and pathophysiological stimuli. Previously, we showed that eNOS expression was dramatically altered by the state of cell growth and that the mechanism responsible for this regulation was entirely posttranscriptional, occurring via changes in eNOS mRNA stability. The present study identifies a role for actin cytoskeleton organization in the posttranscriptional regulation of eNOS during cell growth and examines the relationship between the state of actin polymerization and eNOS expression. We identified monomeric actin (globular [G]-actin) as the major component of a 51-kDa ribonucleoprotein that binds to the eNOS mRNA 3' untranslated region in UV-crosslinking analysis. Binding activity of the ribonucleoprotein complex correlated with the relative concentration of G-actin versus filamentous actin (F-actin). ENOS transcripts colocalized with cytoplasmic G-actin in cells subjected to fluorescence in situ hybridization and G-actin fluorescence staining. In subcellular fractionation studies, eNOS transcripts were enriched in the free polysomal fraction of nonproliferating cells and enriched in the cell matrix-associated polysomal fraction of proliferating cells. Furthermore, an inverse relationship between the concentration of G-actin and eNOS expression was observed in endothelial cells subjected to pharmacological alteration of their cytoskeleton; lower G/F-actin ratios correlated with increased eNOS expression. Our findings provide some insight into how endothelial cells may use the dynamic organization of the actin cytoskeleton to regulate expression of an enzyme that is crucial to vascular homeostasis.

Distinct regions in the 3' untranslated region are responsible for targeting and stabilizing utrophin transcripts in skeletal muscle cells

In this study, we have sought to determine whether utrophin transcripts are targeted to a distinct subcellular compartment in skeletal muscle cells, and have examined the role of the 3' untranslated region (UTR) in regulating the stability and localization of utrophin transcripts. Our results show that utrophin transcripts associate preferentially with cytoskeleton-bound polysomes via actin microfilaments. Because this association is not evident in myoblasts, our findings also indicate that the localization of utrophin transcripts with cytoskeleton-bound polysomes is under developmental influences. Transfection of LacZ reporter constructs containing the utrophin 3'UTR showed that this region is critical for targeting chimeric mRNAs to cytoskeleton-bound polysomes and controlling transcript stability. Deletion studies resulted in the identification of distinct regions within the 3'UTR responsible for targeting and stabilizing utrophin mRNAs. Together, these results illustrate the contribution of posttranscriptional events in the regulation of utrophin in skeletal muscle. Accordingly, these findings provide novel targets, in addition to transcriptional events, for which pharmacological interventions may be envisaged to ultimately increase the endogenous levels of utrophin in skeletal muscle fibers from Duchenne muscular dystrophy (DMD) patients.

mRNA localization by a 145-nucleotide region of the c-fos 3'--untranslated region. Links to translation but not stability

The presence of a localization signal in the 3'-untranslated region of c-fos mRNA was investigated by in situ hybridization and cell fractionation techniques. Cells were transfected with chimeric gene constructs in which the beta-globin coding region was used as a reporter and linked to either its own 3'-untranslated region, the c-fos 3'-untranslated region, or the c-fos 3'-untranslated region containing different deletions. Replacement of the endogenous beta-globin 3'-untranslated region by that from c-fos caused a redistribution of the transcripts so that they were recovered in cytoskeletal-bound polysomes and seen localized in the perinuclear cytoplasm. Deletion of the AU-rich instability region did not affect transcript localization, but removal of a distinct 145-nucleotide region of the 3'-untranslated region abolished it. The prevention of transcript translation by desferrioxamine led to a marked loss of transcript localization, independent of mRNA instability. The data show that the 3'-untranslated region of c-fos mRNA, as c-myc, contains a localization signal, which targets the mRNA to the perinuclear cytoskeleton. We propose that this is important to ensure efficient nuclear import of these key regulatory proteins. mRNA localization by the fos 3'-untranslated region is independent of mRNA instability, and the two are determined by different regulatory elements.

Influence of metallothionein-1 localization on its function

Metallothioneins (MTs) have a major role to play in metal metabolism, and may also protect DNA against oxidative damage. MT protein has been found localized in the nucleus during S-phase. The mRNA encoding the MT-1 isoform has a perinuclear localization, and is associated with the cytoskeleton; this targeting, due to signals within the 3'-untranslated region (3'-UTR), facilitates nuclear localization of MT-1 during S-phase [Levadoux, Mahon, Beattie, Wallace and Hesketh (1999) J. Biol. Chem. 274, 34961-34966]. Using cells transfected with MT gene constructs differing in their 3'-UTRs, the role of MT protein in the nucleus has been studied. Chinese hamster ovary cells were transfected with either the full MT gene (MTMT cells) or with the MT 5'-UTR and coding region linked to the 3'-UTR of glutathione peroxidase (MTGSH cells). Cell survival following exposure to oxidative stress and chemical agents was higher in cells expressing the native MT gene than in cells where MT localization was disrupted, or in untransfected cells. Also, MTMT cells showed less DNA damage than MTGSH cells in response to either hydrogen peroxide or mutagen. After exposure to UV light or mutagen, MTMT cells showed less apoptosis than MTGSH cells, as assessed by DNA fragmentation and flow cytometry. The data indicate that the perinuclear localization of MT mRNA is important for the function of MT in a protective role against DNA damage and apoptosis induced by external stress.

Regulatory signals in messenger RNA: determinants of nutrient–gene interaction and metabolic compartmentation

Nutrition has marked influences on gene expression and an understanding of the interaction between nutrients and gene expression is important in order to provide a basis for determining the nutritional requirements on an individual basis. The effects of nutrition can be exerted at many stages between transcription of the genetic sequence and production of a functional protein. This review focuses on the role of post-transcriptional control, particularly mRNA stability, translation and localization, in the interactions of nutrients with gene expression. The effects of both macronutrients and micronutrients on regulation of gene expression by post-transcriptional mechanisms are presented and the post-transcriptional regulation of specific genes of nutritional relevance (glucose transporters, transferrin, selenoenzymes, metallothionein, lipoproteins) is described in detail. The function of the regulatory signals in the untranslated regions of the mRNA is highlighted in relation to control of mRNA stability, translation and localization and the importance of these mRNA regions to regulation by nutrients is illustrated by reference to specific examples. The localization of mRNA by signals in the untranslated regions and its function in the spatial organization of protein synthesis is described; the potential of such mechanisms to play a key part in nutrient channelling and metabolic compartmentation is discussed. It is concluded that nutrients can influence gene expression through control of the regulatory signals in these untranslated regions and that the post-transcriptional regulation of gene expression by these mechanisms may influence nutritional requirements. It is emphasized that in studies of nutritional control of gene expression it is important not to focus only on regulation through gene promoters but also to consider the possibility of post-transcriptional control.

Ribosomal protein L9 is the product of GRC5, a homolog of the putative tumor suppressor QM in S. cerevisiae

Genes encoding members of the highly conserved QM family have been identified in eukaryotic organisms from yeast to man. Results of previous studies have suggested roles for QM in control of cell growth and proliferation, perhaps as a tumor suppressor, and in energy metabolism. We identified recessive lethal alleles of the Saccharomyces cerevisiae QM homolog GRC5 that increased GCN4 expression when present in multiple copies. These alleles encode truncated forms of the yeast QM protein Grc5p. Using a functional epitope-tagged GRC5 allele, we localized Grc5p to a 60S fraction that contained the large ribosomal subunit. Two-dimensional gel analysis of highly purified yeast ribosomes indicated that Grc5p corresponds to 60S ribosomal protein L9. This identification is consistent with the predicted physical characteristics of eukaryotic QM proteins, the highly biased codon usage of GRC5, and the presence of putative Rap1p-binding sites in the 5' sequences of the yeast GRC5 gene.

The 3' untranslated region plays a role in the targeting of metallothionein-I mRNA to the perinuclear cytoplasm and cytoskeletal-bound polysomes

The mechanism of localisation of metallothionein-I (MT-I) mRNA was studied in transfected cells by in situ hybridisation and cell fractionation. Hepatoma cells were transfected with the 5'-untranslated region and coding region of the beta-globin gene alone or linked to either the beta-globin 3'-untranslated region (3'-UTR) or the MT-I 3'-UTR. The wild-type beta-globin mRNA and the beta-globin mRNA lacking its native 3'-UTR were present in free and cytoskeletal-bound polysomes to a similar extent and showed no localisation. Chimaeric globin-metallothionein transcripts were significantly enriched in cytoskeletal-bound polysomes and were localised in the perinuclear cytoplasm. Chimaeric globin-metallothionein and wild-type globin transcripts were of similar stability. Chinese Hamster Ovary cells were transfected with constructs in which the MT-I 5'-untranslated region and coding sequences were linked to either the endogenous 3'-UTR or the glutathione peroxidase 3'-UTR. Wild-type MT-I transcripts were localised in the perinuclear cytoplasm but the chimaeric MT-I-glutathione peroxidase transcripts showed no distinct localisation. The results indicate that the 3'-UTR of MT-I mRNA contains a localisation signal which promotes both the association of the mRNA with the cytoskeleton and its perinuclear localisation.

Association of influenza virus NP and M1 proteins with cellular cytoskeletal elements in influenza virus-infected cells

We have investigated the association of the influenza virus matrix (M1) and nucleoprotein (NP) with the host cell cytoskeletal elements in influenza virus-infected MDCK and MDBK cells. At 6.5 h postinfection, the newly synthesized M1 was Triton X-100 (TX-100) extractable but became resistant to TX-100 extraction during the chase with a t1/2 of 20 min. NP, on the other hand, acquired TX-100 resistance immediately after synthesis. Significant fractions of both M1 and NP remained resistant to differential detergent (Triton X-114, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate [CHAPS], octylglucoside) extraction, suggesting that M1 and NP were interacting with the cytoskeletal elements. However, the high-molecular-weight form of the viral transmembrane protein hemagglutinin (HA), which had undergone complex glycosylation, also became resistant to TX-100 extraction but was sensitive to octylglucoside detergent extraction, indicating that HA, unlike M1 or NP, was interacting with TX-100-insoluble lipids and not with cytoskeletal elements. Morphological analysis with cytoskeletal disrupting agents demonstrated that M1 and NP were associated with microfilaments in virus-infected cells. However, M1, expressed alone in MDCK or HeLa cells from cloned cDNA or coexpressed with NP, did not become resistant to TX-100 extraction even after a long chase. NP, on the other hand, became TX-100 insoluble as in the virus-infected cells. M1 also did not acquire TX-100 insolubility in ts 56 (a temperature-sensitive mutant with a defect in NP protein)-infected cells at the nonpermissive temperature. Furthermore, early in the infectious cycle in WSN-infected cells, M1 acquired TX-100 resistance very slowly after a long chase and did not acquire TX-100 resistance at all when chased in the presence of cycloheximide. On the other hand, late in the infectious cycle, M1 acquired TX-100 resistance when chased in either the presence or absence of cycloheximide. Taken together, these results demonstrate that M1 and NP interact with host microfilaments in virus-infected cells and that M1 requires other viral proteins or subviral components (possibly viral ribonucleoprotein) for interaction with host cytoskeletal components. The implication of these results for viral morphogenesis is discussed.

The compartmentalization of protein synthesis: importance of cytoskeleton and role in mRNA targeting

Following the synthesis of mRNA molecules in eukaryotic cells, the transcripts are processed in the nucleus and subsequently transported through the nuclear membrane into the cytoplasm before being sequestered into polysomes where the information contained in the RNA molecule is translated into an amino acid sequence. Recent evidence suggests that an association of mRNAs with the cytoskeleton might be important in targeting mechanisms and, furthermore, in the transport of mRNA from the nucleus to its correct location in the cytoplasm. Until recently, polysomes have been considered to exist in two classes, namely free or membrane-bound. There is now compelling evidence, however, that ribosomes, in addition to being associated with endoplasmic reticulum membranes, also are associated with components of the cytoskeleton. Thus, a large number of morphological and biochemical studies have shown that mRNA, polysomes and translational factors are associated with cytoskeletal structures. Although the actual nature and significance of the interaction between components of the translational apparatus and the cytoskeleton is not yet understood in detail, it would seem evident that such interactions are important in both the spatial organization and control of protein synthesis. Recent work has shown that a subcellular fraction, enriched in cytoskeletal components, contains polysomes and these (cytoskeletal-bound) polysomes have been shown to contain specific mRNA species. Thus, a population of cytoskeletal-bound polysomes may provide a specialized mechanism for the sorting, targeting and topographical segregation of mRNAs. In this review, current knowledge of the subcellular compartmentalization of mRNAs is discussed.

Directional budding of human immunodeficiency virus from monocytes

Time-lapse cinematography revealed that activated human immunodeficiency virus (HIV)-infected monocytes crawl along surfaces, putting forward a leading pseudopod. Scanning electron micrographs showed monocyte pseudopods associated with spherical structures the size of HIV virions, and transmission electron micrographs revealed HIV virions budding from pseudopods. Filamentous actin (F-actin) was localized by electron microscopy in the pseudopod by heavy meromyosin decoration. Colocalization of F-actin and p24 viral antigen by light microscopy immunofluorescence indicated that F-actin and virus were present on the same pseudopod. These observations indicate that monocytes produce virus from a leading pseudopod. We suggest that HIV secretion at the leading edges of donor monocytes/macrophages may be an efficient way for HIV to infect target cells.

A localisation signal in the 3′ untranslated region of c-myc mRNA targets c-myc mRNA and beta-globin reporter sequences to the perinuclear cytoplasm and cytoskeletal-bound polysomes

There is increasing evidence that in mammalian cells some mRNAs are localised to specific parts of the cytoplasm and a proportion of mRNAs and polyribosomes are associated with the cytoskeleton. It has been shown previously that c-myc mRNA is present in the perinuclear cytoplasm and associated with the cytoskeleton, and that this localisation is dependent upon the 3′ untranslated region of the mRNA. The present studies show that in transfected fibroblasts the c-myc 3′ untranslated region is able to localise beta-globin reporter sequences to the perinuclear cytoplasm. Studies with constructs containing deletions within the 3′ untranslated region identify the region between bases 194 and 280 as critical for localisation. Transfection of cells with constructs in which this region is linked to beta-globin sequences showed that it was sufficient to localise the chimaeric transcripts to the perinuclear cytoplasm and to cytoskeletal-bound polyribosomes. Transfection with constructs containing a mutated AUUUA sequence within the 194–280 base region showed that this conserved AUUUA is required for targeting of both c-myc mRNA and a chimaeric transcript of beta-globin transcripts linked to the c-myc 3′ untranslated region. The region between bases 194 and 280 did not induce instability of beta-globin transcripts and the AUUUA mutation had little effect upon mRNA stability. We propose that this 86 nt region of the 3′ untranslated region contains a localisation signal to target c-myc mRNA so that it is retained on cytoskeletal-bound polysomes in the perinuclear cytoplasm; a conserved AUUUA sequence appears to be a critical part of this signal.

Evidence for differences in the post-transcriptional regulation of rat metallothionein isoforms

The expression of metallothionein (MT)-1 and -2 mRNAs in rat liver following administration of Cd or Cu was investigated using specific oligonucleotides. The specificity was confirmed using a competitive prehybridization assay. Cd injection caused a biphasic induction of both isoform mRNAs, whereas Cu induced a sustained, monophasic response. Analysis of polyribosomal RNA showed that, after both Cd and Cu treatments, the recruitment of MT-1 mRNA into polyribosomes paralleled the increase in transcription, but the increase of polyribosomal MT-2 mRNA was less than that of total MT-2 mRNA. This indicates that not all the MT-2 mRNA induced was translated, suggesting that there is translational control of MT-2 mRNA expression, but not of MT-1 mRNA. This hypothesis was supported by the observation that, after Cu treatment, the induction of MT-1 protein was induced to the same extent as MT-1 mRNA, whereas the total MT protein (MT-1 + MT-2) was increased far less (7-fold) than MT-2 mRNA (30-fold).

Localisation of metallothionein isoform mRNAs in rat hepatoma (H4) cells

The localisation of metallothionein isoform mRNAs in rat hepatoma (H4) cells was investigated using two approaches, namely Northern hybridisation of total RNA extracted from free, cytoskeletal-bound and membrane-bound polysomes isolated by a sequential detergent/salt extraction procedure and in situ hybridisation. The cytoskeletal-bound polysomes were enriched in metallothionein-I (MT-I) and c-myc mRNAs but showed a significantly lower enrichment in MT-II mRNA. These findings indicate that the MT-I mRNA is localised to the cytoskeleton during translation. In situ hybridisation using a biotin-labelled oligonucleotide probe revealed a predominantly perinuclear localisation for the MT-I mRNA.