Integrated Stress Response in Neuronal Pathology and in Health

Neurodegeneration involves progressive pathological loss of a specific population of neurons, glial activation, and dysfunction of myelinating oligodendrocytes leading to cognitive impairment and altered movement, breathing, and senses. Neuronal degeneration is a hallmark of aging, stroke, drug abuse, toxic chemical exposure, viral infection, chronic inflammation, and a variety of neurological diseases. Accumulation of intra- and extracellular protein aggregates is a common characteristic of cell pathologies. Excessive production of reactive oxygen species and nitric oxide, induction of endoplasmic reticulum stress, and accumulation of misfolded protein aggregates have been shown to trigger a defensive mechanism called integrated stress response (ISR). Activation of ISR is important for synaptic plasticity in learning and memory formation. However, sustaining of ISR may lead to the development of neuronal pathologies and altered patterns in behavior and perception.

Effect of HIV-1 TAT Peptide Fusion on 5' mRNA Cap Analogs Cell Membrane Permeability and Translation Inhibition

The development of targeted anticancer drugs has been one of the most challenging goals of current research. Eukaryotic translation initiation factor 4E (eIF4E) is an oncogene that stimulates mRNA translation via binding to the 5' endcap structure. It is well documented that eIF4E is overexpressed in many cancers including breast, prostate, head and neck, and stomach malignancies and leads to oncogenic transformation and metastasis. One approach to block eIF4E function in cancer cells is based on the disruption of the interaction between eIF4E and the 5' mRNA cap structure using cap analog inhibitors. Since analogs are cell-impermeable due to their anionic nature, we used a cell penetrating peptide (CPP) for delivery of model cap analogs into cancer cells. The human immunodeficiency virus I (HIV-1) transactivator of transcription derived peptide (TAT) was conjugated with the analogs m⁷GMP and m⁷GpppG using click chemistry methodology. We observed that both conjugates (m⁷GMP-TAT and m⁷GpppG-TAT), contrary to TAT alone, did not translocate through the artificial phospholipid membrane of giant unilamellar vesicles. This suggests that passive transport is not the mechanism by which translocation of cap analogs occurs. In contrast, synthesized fluorescently labeled m⁷GpppG-TAT translocated into the human breast adenocarcinoma cancer cell line MCF-7. Furthermore, we demonstrated that m⁷GMP-TAT and m⁷GpppG-TAT inhibited cap-dependent translation up to 30% both in vivo and in vitro while simultaneously not affecting cell growth and viability. These results demonstrate the usefulness of cell penetration peptides as carriers for the internalization of cap analogs.

Carbonyl-protein content increases in brain and blood of female rats after chronic oxycodone treatment

BACKGROUND

Opioids are the most effective drugs commonly prescribed to treat pain. Due to their addictive nature, opioid pain relievers are now second to marijuana, ahead of cocaine with respect to dependence. Ours and other studies suggest potential toxic effects of chronic opioid administration leading to neuronal degeneration. It has been suggested that protein carbonylation may represent a sensitive biomarker of cellular degeneration. To evaluate whether prolonged oxycodone administration is associated with accumulation of protein aggregates that may contribute to neuronal degeneration we measured protein carbonylation levels in brain and also in blood plasma of rats after 30-days of 15 mg/kg daily oxycodone administration.

RESULTS

We observed a significant increase in the level of carbonylated proteins in rat brain cortex after 30-days of oxycodone treatment compare to that in water treated animals. Also, oxycodone treated rats demonstrated accumulation of insoluble carbonyl-protein aggregates in blood plasma.

CONCLUSIONS

Our data suggests that tests detecting insoluble carbonyl-protein aggregates in blood may serve as an inexpensive and minimally invasive method to monitor neuronal degeneration in patients with a history of chronic opioid use. Such methods could be used to detect toxic side effects of other medications and monitor progression of aging and neurodegenerative diseases.

Chronic oxycodone induces axonal degeneration in rat brain

BACKGROUND

Chronic opioid therapy for non-malignant pain conditions has significantly increased over the last 15 years. Recently, the correlation between opioid analgesics and alternations in brain structure, such as leukoencephalopathy, axon demyelination, and white matter lesions, has been demonstrated in patients with a history of long-term use of prescription opioids. The exact mechanisms underlying the neurotoxic effect of opioids on the central nervous system are still not fully understood. We investigated the effect of chronic opioids using an animal model in which female rats were orally gavaged with 15 mg/kg of oxycodone every 24 h for 30 days. In addition we tested oxycodone, morphine and DAMGO in breast adenocarcinoma MCF7 cells, which are known to express the μ-opioid receptor.

RESULTS

We observed several changes in the white matter of animals treated with oxycodone: deformation of axonal tracks, reduction in size of axonal fascicles, loss of myelin basic protein and accumulation of amyloid precursor protein beta (β-APP), suggesting axonal damages by chronic oxycodone. Moreover, we demonstrated activation of pro-apoptotic machinery amid suppression of anti-apoptotic signaling in axonal tracks that correlated with activation of biomarkers of the integrated stress response (ISR) in these structures after oxycodone exposure. Using MCF7 cells, we observed induction of the ISR and pro-apoptotic signaling after opioid treatment. We showed that the ISR inhibitor, ISRIB, suppresses opioid-induced Bax and CHOP expression in MCF7 cells.

CONCLUSIONS

Altogether, our data suggest that chronic opioid administration may cause neuronal degeneration by activation of the integrated stress response leading to induction of apoptotic signaling in neurons and also by promoting demyelination in CNS.

Inhibition of Mitogen-activated Protein Kinase (MAPK)-interacting Kinase (MNK) Preferentially Affects Translation of mRNAs Containing Both a 5'-Terminal Cap and Hairpin

The MAPK-interacting kinases 1 and 2 (MNK1 and MNK2) are activated by extracellular signal-regulated kinases 1 and 2 (ERK1/2) or p38 in response to cellular stress and extracellular stimuli that include growth factors, cytokines, and hormones. Modulation of MNK activity affects translation of mRNAs involved in the cell cycle, cancer progression, and cell survival. However, the mechanism by which MNK selectively affects translation of these mRNAs is not understood. MNK binds eukaryotic translation initiation factor 4G (eIF4G) and phosphorylates the cap-binding protein eIF4E. Using a cell-free translation system from rabbit reticulocytes programmed with mRNAs containing different 5'-ends, we show that an MNK inhibitor, CGP57380, affects translation of only those mRNAs that contain both a cap and a hairpin in the 5'-UTR. Similarly, a C-terminal fragment of human eIF4G-1, eIF4G(1357-1600), which prevents binding of MNK to intact eIF4G, reduces eIF4E phosphorylation and inhibits translation of only capped and hairpin-containing mRNAs. Analysis of proteins bound to m(7)GTP-Sepharose reveals that both CGP and eIF4G(1357-1600) decrease binding of eIF4E to eIF4G. These data suggest that MNK stimulates translation only of mRNAs containing both a cap and 5'-terminal RNA duplex via eIF4E phosphorylation, thereby enhancing the coupled cap-binding and RNA-unwinding activities of eIF4F.

Chronic oxycodone induces integrated stress response in rat brain

BACKGROUND

Oxycodone is an opioid that is prescribed to treat multiple types of pain, especially when other opioids are ineffective. Unfortunately, similar to other opioids, repetitive oxycodone administration has the potential to lead to development of analgesic tolerance, withdrawal, and addiction. Studies demonstrate that chronic opioid exposure, including oxycodone, alters gene expression profiles and that these changes contribute to opioid-induced analgesic effect, tolerance and dependence. However, very little is known about opioids altering the translational machinery of the central nervous system. Considering that opioids induce clinically significant levels of hypoxia, increase intracellular Ca(2+) levels, and induce the production of nitric oxide and extracellular glutamate transmission, we hypothesize that opioids also trigger a defensive mechanism called the integrated stress response (ISR). The key event in the ISR activation, regardless of the trigger, is phosphorylation of translation initiation factor 2 alpha (eIF2α), which modulates expression and translational activation of specific mRNAs important for adaptation to stress. To test this hypothesis, we used an animal model in which female rats were orally gavaged with 15 mg/kg of oxycodone every 24 h for 30 days.

RESULTS

We demonstrated increased levels of hsp70 and BiP expression as well as phosphorylation of eIF2α in various rat brain areas after oxycodone administration. Polysomal analysis indicated oxycodone-induced translational stimulation of ATF4 and PDGFRα mRNAs, which have previously been shown to depend on the eIF2α kinase activation. Moreover, using breast adenocarcinoma MCF7 cells, which are known to express the μ-opioid receptor, we observed induction of the ISR pathway after one 24-h treatment with oxycodone.

CONCLUSIONS

The combined in vivo and in vitro data suggest that prolonged opioid treatment induces the integrated stress response in the central nervous system; it modulates translational machinery in favor of specific mRNA and this may contribute to the drug-induced changes in neuronal plasticity.

Mnk mediates integrin α6β4-dependent eIF4E phosphorylation and translation of VEGF mRNA

It was previously shown that integrin α6β4 contributes to translation of cancer-related mRNAs such as VEGF via initiation factor eIF4E. In this study, we found that integrin α6β4 regulates the activity of eIF4E through the Ser/Thr kinase Mnk. Although a role for Mnk in various aspects of cancer progression has been established, a link between integrin and Mnk activity has not. Here we show that Mnk1 is a downstream effector of integrin α6β4 and mediates the α6β4 signaling, important for translational control. Integrin α6β4 signals through MEK and p38 MAPK to increase phosphorylation of Mnk1 and eIF4E. Inhibition of Mnk1 activity by CGP57380 or downregulation by shRNA blocks α6β4-dependent translation of VEGF mRNA. Our studies suggest that Mnk1 could be a therapeutic target in cancers where the integrin α6β4 level is high.

A C. elegans eIF4E-family member upregulates translation at elevated temperatures of mRNAs encoding MSH-5 and other meiotic crossover proteins

Caenorhabditis elegans expresses five family members of the translation initiation factor eIF4E whose individual physiological roles are only partially understood. We report a specific role for IFE-2 in a conserved temperature-sensitive meiotic process. ife-2 deletion mutants have severe temperature-sensitive chromosome-segregation defects. Mutant germ cells contain the normal six bivalents at diakinesis at 20 degrees C but 12 univalents at 25 degrees C, indicating a defect in crossover formation. Analysis of chromosome pairing in ife-2 mutants at the permissive and restrictive temperatures reveals no defects. The presence of RAD-51-marked early recombination intermediates and 12 well condensed univalents indicate that IFE-2 is not essential for formation of meiotic double-strand breaks or their repair through homologous recombination but is required for crossover formation. However, RAD-51 foci in ife-2 mutants persist into inappropriately late stages of meiotic prophase at 25 degrees C, similar to mutants defective in MSH-4/HIM-14 and MSH-5, which stabilize a critical intermediate in crossover formation. In wild-type worms, mRNAs for msh-4/him-14 and msh-5 shift from free messenger ribonucleoproteins to polysomes at 25 degrees C but not in ife-2 mutants, suggesting that IFE-2 translationally upregulates synthesis of MSH-4/HIM-14 and MSH-5 at elevated temperatures to stabilize Holliday junctions. This is confirmed by an IFE-2-dependent increase in MSH-5 protein levels.

Kinetic mechanism for assembly of the m7GpppG.eIF4E.eIF4G complex

Interaction of the mRNA cap with the translational machinery is a critical and early step in the initiation of protein synthesis. To better understand this process, we determined kinetic constants for the interaction of m(7)GpppG with human eIF4E by stopped-flow fluorescence quenching in the presence of a 90-amino acid fragment of human eIF4G that contains the eIF4E-binding domain (eIF4G(557-646)). The values obtained, k(on) = 179 x 10(6) m(-1) s(-1) and k(off) = 79 s(-1), were the same as reported previously in the absence of an eIF4G-derived peptide. We also used surface plasmon resonance to determine kinetic constants for the binding of eIF4E to eIF4G(557-646), both in the presence and absence of m(7)GpppG. The results indicated that eIF4G(557-646) binds eIF4E and eIF4E.m(7)GpppG at the same rate, with k(on) = 3 x 10(6) m(-1) s(-1) and k(off) = 0.01 s(-1). Our data represent the first full kinetic description of the interaction of eIF4E with its two specific ligands. The results demonstrate that the formation of the m(7)GpppG.eIF4E.eIF4G(557-646) complex obeys a sequential, random kinetic mechanism and that there is no preferential pathway for its formation. Thus, even though eIF4G(557-646) binds eIF4E tightly, it does not increase the affinity of eIF4E for m(7)GpppG, as has been claimed in several previous publications. We did, in fact, observe increased binding to m(7)GTP-Sepharose in the presence of eIF4G(557-646), but only with recombinant eIF4E that was prepared from inclusion bodies.

Translational dysregulation by Pateamine A

Pateamine A inhibits translation by preventing proper translational initiation complex formation. In the December issue of Chemistry & Biology, Bordeleau et al. demonstrated that the effects of Patemine A on translation are mediated through the interaction between the RNA helicase eIF4A and mRNA .

Translation initiation factor eIF4G-1 binds to eIF3 through the eIF3e subunit

eIF3 in mammals is the largest translation initiation factor ( approximately 800 kDa) and is composed of 13 nonidentical subunits designated eIF3a-m. The role of mammalian eIF3 in assembly of the 48 S complex occurs through high affinity binding to eIF4G. Interactions of eIF4G with eIF4E, eIF4A, eIF3, poly(A)-binding protein, and Mnk1/2 have been mapped to discrete domains on eIF4G, and conversely, the eIF4G-binding sites on all but one of these ligands have been determined. The only eIF4G ligand for which this has not been determined is eIF3. In this study, we have sought to identify the mammalian eIF3 subunit(s) that directly interact(s) with eIF4G. Established procedures for detecting protein-protein interactions gave ambiguous results. However, binding of partially proteolyzed HeLa eIF3 to the eIF3-binding domain of human eIF4G-1, followed by high throughput analysis of mass spectrometric data with a novel peptide matching algorithm, identified a single subunit, eIF3e (p48/Int-6). In addition, recombinant FLAG-eIF3e specifically competed with HeLa eIF3 for binding to eIF4G in vitro. Adding FLAG-eIF3e to a cell-free translation system (i) inhibited protein synthesis, (ii) caused a shift of mRNA from heavy to light polysomes, (iii) inhibited cap-dependent translation more severely than translation dependent on the HCV or CSFV internal ribosome entry sites, which do not require eIF4G, and (iv) caused a dramatic loss of eIF4G and eIF2alpha from complexes sedimenting at approximately 40 S. These data suggest a specific, direct, and functional interaction of eIF3e with eIF4G during the process of cap-dependent translation initiation, although they do not rule out participation of other eIF3 subunits.

Mechanism and regulation of translation in C. elegans

C. elegans represents a favorable system to study the extraordinarily complicated process of eukaryotic protein synthesis, which involves over 100 RNAs and over 200 polypeptides just for the core machinery. Initial research in protein synthesis relied on fractionated mammalian and plant systems, but in the mid-1970s, the powerful genetics of Saccharomyces cerevisiae began to yield new insights for translation in all eukaryotes. C. elegans has many features of higher eukaryotes that are not shared by yeast. This allows protein synthesis researchers to combine biochemistry, cell biology, developmental biology, genetics, and genomics to study regulation of gene expression at the translational level. Most components of the core translational machinery have been identified in C. elegans, including rRNAs, 5S RNA, tRNAs, ribosomal proteins, and aminoacyl tRNA synthetases. C. elegans has amino acid sequence homologs for 56 of the known initiation, elongation, and release factor polypeptides, but few of these have been isolated, functionally identified, or studied at the biochemical level. Similarly, C. elegans has homologs for 22 components of the major signal transduction pathways implicated in control of protein synthesis. The translational efficiency of individual mRNAs relies on cis-regulatory elements that include either a 7-methylguanosine- or 2,2,7-trimethylguanosine-containing cap, the 5'-terminal spliced leader, sequence elements in the 3'-untranslated regions, and the 3'-terminal poly(A) tract. Several key developmental pathways in C. elegans are predominantly governed by translational mechanisms. Some evidence has been presented that well described regulatory mechanisms in other organisms, including covalent modification of translation factors, sequestration of translation factors, and mRNA-specific changes in poly(A) length, also occur in C. elegans. The most interesting unexplored questions may involve changes in the translation of individual mRNAs during development, in response to physiological changes, or after genetic manipulations. Given the highly developed state of C. elegans genomics, it can be expected that future application of computational tools, including data visualization, will help detect new instances of translational control.

Sindbis virus translation is inhibited by a PKR/RNase L-independent effector induced by alpha/beta interferon priming of dendritic cells

The tropism of Sindbis virus (SB) for cells of the dendritic cell (DC) lineage and the virulence of SB in vivo are largely determined by the efficacy of alpha/beta interferon (IFN-alpha/beta)-mediated antiviral responses. These responses are essentially intact in the absence of PKR and/or RNase L (K. D. Ryman, L. J. White, R. E. Johnston, and W. B. Klimstra, Viral Immunol. 15:53-76, 2002). In the present studies, we investigated the nature of antiviral effects and identity of antiviral effectors primed by IFN-alpha/beta treatment of bone marrow-derived DCs (BMDCs) generated from mice deficient in PKR and RNase L (TD). IFN-alpha/beta priming exerted significant antiviral activity at very early stages of SB replication and most likely inhibited the initial translation of infecting genomes. The early effect targeted cap-dependent translation as protein synthesis from an SB-like and a simple RNA were inhibited by interferon treatment, but an encephalomyocarditis virus internal ribosome entry site-driven element exhibited no inhibition. Phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 was defective after virus infection of TD cells, suggesting other mechanisms of translation inhibition. To identify components of these alternative antiviral pathway(s), we have compared global gene regulation in BMDCs derived from normal 129 Sv/Ev, IFNAR1-/-, and TD mice following infection with SB or treatment with IFN-alpha/beta. Candidate effectors of alternative antiviral pathways were those genes induced by virus infection or IFN-alpha/beta treatment in 129 Sv/Ev and TD-derived BMDC but not in virus-infected or IFN-alpha/beta-treated IFNAR1-/- cells. Statistical analyses of gene array data identified 44 genes that met these criteria which are discussed.

Translation of a small subset of Caenorhabditis elegans mRNAs is dependent on a specific eukaryotic translation initiation factor 4E isoform

The mRNA cap-binding protein eukaryotic translation initiation factor 4E (eIF4E) participates in protein synthesis initiation, translational repression of specific mRNAs, and nucleocytoplasmic shuttling. Multiple isoforms of eIF4E are expressed in a variety of organisms, but their specific roles are poorly understood. We investigated one Caenorhabditis elegans isoform, IFE-4, which has homologues in plants and mammals. IFE-4::green fluorescent protein (GFP) was expressed in pharyngeal and tail neurons, body wall muscle, spermatheca, and vulva. Knockout of ife-4 by RNA interference (RNAi) or a null mutation produced a pleiotropic phenotype that included egg-laying defects. Sedimentation analysis demonstrated that IFE-4, but not IFE-1, was present in 48S initiation complexes, indicating that it participates in protein synthesis initiation. mRNAs affected by ife-4 knockout were determined by DNA microarray analysis of polysomal distribution. Polysome shifts, in the absence of total mRNA changes, were observed for only 33 of the 18,967 C. elegans mRNAs tested, of which a disproportionate number were related to egg laying and were expressed in neurons and/or muscle. Translational regulation was confirmed by reduced levels of DAF-12, EGL-15, and KIN-29. The functions of these proteins can explain some phenotypes observed in ife-4 knockout mutants. These results indicate that translation of a limited subset of mRNAs is dependent on a specific isoform of eIF4E.

Phosphorylation of Mnk1 by caspase-activated Pak2/gamma-PAK inhibits phosphorylation and interaction of eIF4G with Mnk

The mitogen-activated protein kinase-interacting kinase 1 (Mnk1) is phosphorylated by caspase-cleaved protein kinase Pak2/gamma-PAK but not by Cdc42-activated Pak2. Phosphorylation of Mnk1 is rapid, reaching 1 mol/mol within 15 min of incubation with Pak2. A kinetic analysis of the phosphorylation of Mnk1 by Pak2 yields a K(m) of 0.6 microm and a V(max) of 14.9 pmol of (32)P/min/microg of Pak2. Two-dimensional tryptic phosphopeptide mapping of Mnk1 phosphorylated by Pak2 yields two distinct phosphopeptides. Analysis of the phosphopeptides by automated microsequencing and manual Edman degradation identified the sites in Mnk1 as Thr(22) and Ser(27). Mnk1, activated by phosphorylation with Erk2, phosphorylates the eukaryotic initiation factor (eIF) 4E and the eIF4G components of eIF4F. Phosphorylation of Mnk1 by Pak2 does not activate Mnk1, as measured with either eIF4E or eIF4F as substrate. Phosphorylation of Erk2-activated Mnk1 by Pak2 has no effect on phosphorylation of eIF4E but reduces phosphorylation of eIF4G by Mnk1 by up to 50%. Phosphorylation of Mnk1 by Pak2 inhibits binding of eIF4G peptides containing the Mnk1 binding site by up to 80%. When 293T cells are subjected to apoptotic induction by hydrogen peroxide, Mnk1 is phosphorylated at both Thr(22) and Ser(27). These results indicate a role for Pak2 in the down-regulation of translation initiation in apoptosis by phosphorylation of Mnk1.

Characterization of the two eIF4A-binding sites on human eIF4G-1

Eukaryotic translation initiation factor 4G-1 (eIF4G) plays a critical role in the recruitment of mRNA to the 43 S preinitiation complex. eIF4G has two binding sites for the RNA helicase eIF4A, one in the central domain and one in the COOH-terminal domain. Recombinant eIF4G fragments that contained each of these sites separately bound eIF4A with a 1:1 stoichiometry, but fragments containing both sites bound eIF4A with a 1:2 stoichiometry. eIF3 did not interfere with eIF4A binding to the central site. Interestingly, at the same concentration of free eIF4A, more eIF4A was bound to an eIF4G fragment containing both eIF4A sites than the sum of binding to fragments containing the single sites, indicating cooperative binding. Binding of eIF4A to an immobilized fragment of eIF4G containing the COOH-terminal site was competed by a soluble eIF4G fragment containing the central site, indicating that a single eIF4A molecule cannot bind simultaneously to both sites. The association rate constant, dissociation rate constant, and dissociation equilibrium constant for each site were determined by surface plasmon resonance and found to be, respectively, 1.2 x 10(5) m(-1) s(-1), 2.1 x 10(-3) s(-1), and 17 nm for the central site and 5.1 x 10(3) m(-1) s(-1), 1.7 x 10(-3) s(-1), and 330 nm for the COOH-terminal site.

Mutually cooperative binding of eukaryotic translation initiation factor (eIF) 3 and eIF4A to human eIF4G-1

Eukaryotic translation initiation factor 4G-1 (eIF4G) plays a critical role in the recruitment of mRNA to the 43 S preinitiation complex. The central region of eIF4G binds the ATP-dependent RNA helicase eIF4A, the 40 S binding factor eIF3, and RNA. In the present work, we have further characterized the binding properties of the central region of human eIF4G. Both titration and competition experiments were consistent with a 1:1 stoichiometry for eIF3 binding. Surface plasmon resonance studies showed that three recombinant eIF4G fragments corresponding to amino acids 642-1560, 613-1078, and 975-1078 bound eIF3 with similar kinetics. A dissociation equilibrium constant of approximately 42 nm was derived from an association rate constant of 3.9 x 10(4) m(-1) s(-1) and dissociation rate constant of 1.5 x 10(-3) s(-1). Thus, the eIF3-binding region is included within amino acid residues 975-1078. This region does not overlap with the RNA-binding site, which suggests that eIF3 binds eIF4G directly and not through an RNA bridge, or the central eIF4A-binding site. Surprisingly, the binding of eIF3 and eIF4A to the central region was mutually cooperative; eIF3 binding to eIF4G increased 4-fold in the presence of eIF4A, and conversely, eIF4A binding to the central (but not COOH-terminal) region of eIF4G increased 2.4-fold in the presence of eIF3.

Interaction of the universal mRNA-binding protein, p50, with actin: a possible link between mRNA and microfilaments

We have shown previously that p50 is the most abundant protein associated with a variety of eukaryotic mRNAs and exhibits about 98% amino acid sequence identity to mammalian Y-box binding transcription factors. The dual function of p50 in the cell as a regulator of both transcription and translation has been suggested. To gain insight into the role of p50 in these processes, we performed the yeast two-hybrid screen to identify p50 molecular partners. Here we report the identification of actin as a p50-interacting protein. Coimmunoprecipitation of p50 and actin from HeLa extracts as well as in vitro binding studies indicate specificity and a high affinity for the interaction between p50 and actin. Interestingly, p50 binding to actin is affected by mRNA; binding was observed at a low p50/mRNA ratio and was greatly reduced at higher ratios. Since the p50/mRNA ratio appears to be important for mRNA translatability, we speculate that p50 can regulate the attachment of mRNA to the actin network depending on its translational activity. Using immunofluorescence, we show that p50 binds to actin filaments in permeabilized cells and causes actin fibers to bundle in vitro. Together, these findings support the view that p50 may play an important role in mRNA transport, anchoring, and localization on actin filaments in the cell.

Light microscopic analysis of ligand-induced actin filament suprastructures

In this study, we describe a simple light microscopic assay which allows to rapidly determine the ligand-induced organization of actin filaments into specific suprastructures, such as web-like arrangements or bundles. The validity of this assay is demonstrated by accompanying low shear (falling ball) viscometry. While the visually identified webs demonstrated viscosity values significantly higher than the F-actin control, the bundles were characterized by viscosities distinctly lower than that of the control. In addition, we show that at least in some cases, the type of actin suprastructure formed depends on the molar ratio between the ligand and actin filaments. The assay should be useful to assess the conditions under which a particular ligand leads to a specific actin filament organization, to determine quickly the biological activity of recombinant proteins or isolated actin-binding domains, and to define new F-actin ligands.

[Evaluation of health status of agricultural machine operators and agriculture students]

To lower morbidity with transitory disablement, to better primary prophylaxis and health preservation for machine operators and professional training students, a system of therapeutic and sanitary measures is to be put into practice. Those measures include improved organization of medical care, better hygienic conditions, safe work and healthy lifestyle of the examinees.

[Phosphorylation of proteins of mRNP-particles of rabbit reticulocytes and their possible role in translation regulation]

Protein kinase activity was found associated with free mRNP particles of rabbit reticulocytes. This enzyme was able to transfer phosphate group(s) from ATP to at least nine proteins of mRNP. Rabbit reticulocytes postmitochondrial supernatant proves the existence of protein phosphatase activity specifically dephosphorylating some mRNP proteins. The role of mRNP proteins phosphorylation--dephosphorylation in translation process is discussed.